Interesting Back Spacing for 130GTX. Is this far off spec considered normal, and does it even matter?


Chris White
 

Sorry for the repost and deletion of my previous topic.  I cleaned it up a bit after learning a little bit about what to expect.

The following is my experience dialing in the Field Flattener. 

I've had a paucity of clear nights since I purchased my 130GTX so it has taken me a while to get spacing and tilt dialed in, but I finally have succeed in getting it pretty darn good.  I initially started out at 103.9mm of spacing (per AP spec) which accounts for a 3mm thick filter in the optical path.  Stars looked pretty bad.  I was able to get a slight improvement by adding 2mm of spacing, which a friend told me that fixed his spacing issues.  Then I saw Roland post on here that one would actually want less spacing than specified for a very large chip.  (I'm using a 36mm x 24mm IMX455 chip camera which has 3.76um pixels which is capable of revealing the warts of any system.)

So, I started looking at spacing a couple of mm below spec and stars looked much worse.  So I decided to throw out all of my assumptions with respect to spacing specification and just started adding spacing one night using the fact that stars were pointing inward from the corners as my guide.  To my surprise, every time I added another mm of spacing, stars looked incrementally better.  All the way out to 111.5mm of spacing!  This shocked me, as its 7.6mm longer in spacing than specified! 

Last night I was able to dial in the tilt that I had in the system which required a 100micron adjustment on two of the three screws on my Gerd CTU.  Stars now look very good to my eyes, and I'm a pixel peeper.  I've included the aberration inspector below so you can see.  Stars look very round to the corners.  They look tight and in focus.  There is a tiny amount of flaring, maybe... but its hard to tell.  I'll need to make an integration to really see if this is an issue.  But overall, stars look pretty good, and this is Bin1. 

So a question for AP is, is it normal to see a 130GTX so far out of spec?  Is this caused by the scope or the flattener being a little out of spec?  I have not tested the QTCC yet with this spacing, but do you think this could be a problem when it comes to reducing the scope?  Stars look pretty good to me with the field flattener, but could they be even better? Moshen suggested that I platesolve to confirm actual focal length, and it was 875mm, which is pretty much spot on to spec.  So why the large backspacing discrepancy?  (I've triple checked my measurements from the sensor to the shoulder of the flattener (where the threads start emerging from the flattener.  111.5mm for sure. )


The AA below is from a 600 second Luminance filter shot on a 900GTO.  There might be a little bit of flaring on some of the brighter stars, but hard to tell with a single sub.  Also included is the CCDI maps.  Looks pretty good, although there is still a little bit of remaining tilt that I might be able to improve on... although not sure if my eyes can even see this as an issue.  I'll need to do an integration to see if it is meaningful. 

Here is the original fit file from camera in case anyone is interested in a closer look at the file:  https://www.dropbox.com/sh/o5xsd38vs3zenpl/AAAjocgM74BXPWeqc4wXnXHYa?dl=0

 
Thanks in advance for any input. 


Dale Ghent
 

The 35FF is a 1.1x flattener. My 130GTX+35FF works out to a focal length of 871.5mm, which gets me a focal ratio of 6.703. Yours is not so different from this, at f/6.730.

From what I understand, backfocus distance is influenced by the focal ratio, so the manufacturing and figuring variances across different combinations of OTA and flattener. This means that the stated backfocus distance (80.8mm) is more of a guideline than something you have to strictly measure to.

The 111.5mm you state isn't too far off from the 102.9mm distance of the 35FF/QTCC sans extension. Are you confusing this with the 80.8mm backfocus distance when using the spacer?

On Jun 18, 2021, at 09:01, Chris White <chris.white@whitephotogallery.com> wrote:

Sorry for the repost and deletion of my previous topic. I cleaned it up a bit after learning a little bit about what to expect.

The following is my experience dialing in the Field Flattener.

I've had a paucity of clear nights since I purchased my 130GTX so it has taken me a while to get spacing and tilt dialed in, but I finally have succeed in getting it pretty darn good. I initially started out at 103.9mm of spacing (per AP spec) which accounts for a 3mm thick filter in the optical path. Stars looked pretty bad. I was able to get a slight improvement by adding 2mm of spacing, which a friend told me that fixed his spacing issues. Then I saw Roland post on here that one would actually want less spacing than specified for a very large chip. (I'm using a 36mm x 24mm IMX455 chip camera which has 3.76um pixels which is capable of revealing the warts of any system.)

So, I started looking at spacing a couple of mm below spec and stars looked much worse. So I decided to throw out all of my assumptions with respect to spacing specification and just started adding spacing one night using the fact that stars were pointing inward from the corners as my guide. To my surprise, every time I added another mm of spacing, stars looked incrementally better. All the way out to 111.5mm of spacing! This shocked me, as its 7.6mm longer in spacing than specified!

Last night I was able to dial in the tilt that I had in the system which required a 100micron adjustment on two of the three screws on my Gerd CTU. Stars now look very good to my eyes, and I'm a pixel peeper. I've included the aberration inspector below so you can see. Stars look very round to the corners. They look tight and in focus. There is a tiny amount of flaring, maybe... but its hard to tell. I'll need to make an integration to really see if this is an issue. But overall, stars look pretty good, and this is Bin1.

So a question for AP is, is it normal to see a 130GTX so far out of spec? Is this caused by the scope or the flattener being a little out of spec? I have not tested the QTCC yet with this spacing, but do you think this could be a problem when it comes to reducing the scope? Stars look pretty good to me with the field flattener, but could they be even better? Moshen suggested that I platesolve to confirm actual focal length, and it was 875mm, which is pretty much spot on to spec. So why the large backspacing discrepancy? (I've triple checked my measurements from the sensor to the shoulder of the flattener (where the threads start emerging from the flattener. 111.5mm for sure. )

The AA below is from a 600 second Luminance filter shot on a 900GTO. There might be a little bit of flaring on some of the brighter stars, but hard to tell with a single sub. Also included is the CCDI maps. Looks pretty good, although there is still a little bit of remaining tilt that I might be able to improve on... although not sure if my eyes can even see this as an issue. I'll need to do an integration to see if it is meaningful.

Here is the original fit file from camera in case anyone is interested in a closer look at the file: https://www.dropbox.com/sh/o5xsd38vs3zenpl/AAAjocgM74BXPWeqc4wXnXHYa?dl=0

Thanks in advance for any input.

<AA-2.JPG><CCDI-1.JPG><CCDI-2.JPG>


Chris White
 
Edited

On Fri, Jun 18, 2021 at 10:27 AM, Dale Ghent wrote:
The 111.5mm you state isn't too far off from the 102.9mm distance of the 35FF/QTCC sans extension. Are you confusing this with the 80.8mm backfocus distance when using the spacer?
Dale,

No, I'm not using the spacer and have been going off the 102.9mm.  I guess I am assuming that the 111.5 IS actually quite far off from 102.9mm.  From what I have read from other users with this Scope and FF they have been using 102.1mm to 104.9mm for optimal back spacing with a full frame chip. I guess I'm just surprised to be so far over the spec to get it dialed in, and just trying to understand if this is a normal data point for this scope/FF.  I'm quite pleased with the result and hope to get similar performance with the reducer, although I know the image circle will be smaller and it's likely that corner stars wont be as good. 

I remember from my terrestrial imaging days that cameras, lenses and field flatteners all had some variability with manufacturing and sometimes this variability compounded and other times it canceled out with respect to potential issues.  I imagine that the same sort of thing applies here and that combining different copies of the same gear might produce different results for spacing.  If that's the case, than I will need to spend time with the QTCC to find optimal spacing as it will likely be different than my Field Flattener.  If, however this variability largely lies within the OTA itself... it might not matter. 

I'm happy to do all the testing and verify, but I also like to understand this stuff as well, hence my query.


Roland Christen
 


I guess I'm just surprised to be so far over the spec to get it dialed in, and just trying to understand if this is a normal data point for this scope/FF. 
I will repeat again what I wrote in another post:

The GTX focal length is quite exact, but the field flattener acts like a very weak Barlow, so it increases the focal length of the main optics by 5 or 6%. The farther out the image sits, the more Barlow effect you get.

The reason we make field flatteners this way (with weak Barlow) is to be able to stuff more field into a smaller area. The outer pencil rays diverge slightly instead of converging, so there is less vignetting from limited size adapters and filters. It also allows the flattener optics to be farther from the focal plane so that there is room for filter wheels, off-axis guiders, rotators, and the large distance to the chip that most CCDs had in the past.

Of course now things have changed with ultra-short filter wheels, off-axis units and shutterless CMOS cameras. Where in the past we needed 100+mm of back focus to accomodate all the peripherals, now most of the flatteners coming from overseas will give you barely 2" or so. This greatly reduces the need for large flattener optics and the need for the light beam to diverge to get past the restrictive filter sizes.

Rolando



-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-ug.groups.io
Sent: Fri, Jun 18, 2021 9:41 am
Subject: Re: [ap-ug] Interesting Back Spacing for 130GTX. Is this far off spec considered normal, and does it even matter?

[Edited Message Follows]
On Fri, Jun 18, 2021 at 10:27 AM, Dale Ghent wrote:
The 111.5mm you state isn't too far off from the 102.9mm distance of the 35FF/QTCC sans extension. Are you confusing this with the 80.8mm backfocus distance when using the spacer?
Dale,

No, I'm not using the spacer and have been going off the 102.9mm.  I guess I am assuming that the 111.5 IS actually quite far off from 102.9mm.  From what I have read from other users with this Scope and FF they have been using 102.1mm to 104.9mm for optimal back spacing with a full frame chip. I guess I'm just surprised to be so far over the spec to get it dialed in, and just trying to understand if this is a normal data point for this scope/FF.  I'm quite pleased with the result and hope to get similar performance with the reducer, although I know the image circle will be smaller and it's likely that corner stars wont be as good. 

I remember from my terrestrial imaging days that cameras, lenses and field flatteners all had some variability with manufacturing and sometimes this variability compounded and other times it canceled out with respect to potential issues.  I imagine that the same sort of thing applies here and that combining different copies of the same gear might produce different results for spacing.  If that's the case, than I will need to spend time with the QTCC to find optimal spacing as it will likely be different than my Field Flattener.  If, however this variability largely lies within the OTA itself... it might not matter. 


I'm happy to do all the testing and verify, but I also like to understand this stuff as well, hence my query.


Chris White
 
Edited

On Fri, Jun 18, 2021 at 11:33 AM, Roland Christen wrote:
I will repeat again what I wrote in another post:
Roland,

I appreciate your help, but this does not address my questions, at least not in a way that I understand.  Spec for the GTX+FF is for 102.9mm.  I'm at 111.5mm to get what I believe is optimal.  Is this normal?  Would I expect the QTCC to be the same 111.5mm on my scope?

EDIT-  For clarity it strikes me that you think I am asking about the plate solved focal length being over spec by a tiny bit.  That is NOT what this thread is about.  It is specifically about the backspacing being over spec.  See my direct questions in the paragraph above. Thanks!!!

Thank you very much,


Roland Christen
 

My not being there to verify your measurements, I cannot really say.

Rolando

-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-ug.groups.io
Sent: Fri, Jun 18, 2021 10:52 am
Subject: Re: [ap-ug] Interesting Back Spacing for 130GTX. Is this far off spec considered normal, and does it even matter?

On Fri, Jun 18, 2021 at 11:33 AM, Roland Christen wrote:
I will repeat again what I wrote in another post:
Roland,

I appreciate your help, but this does not address my questions, at least not in a way that I understand.  Spec for the GTX+FF is for 102.9mm.  I'm at 111.5mm to get what I believe is optimal.  Is this normal?  Would I expect the QTCC to be the same 111.5mm on my scope?

Thank you very much,


Chris White
 

On Fri, Jun 18, 2021 at 01:12 PM, Roland Christen wrote:
My not being there to verify your measurements, I cannot really say.
 
Rolando

Roland,

Here's the next best thing.  I actually measured from the Flattener to the Camera side of the filter wheel, and then added 12.5mm for the sensor setback to arrive at 111.5mm.  I've set my caliper at 111.5mm and held that up against the image train to illustrate exactly the two points from which I am measuring.  If this satisfies you that I am measuring correctly, then you can also evaluate the camera fits available at the link in my first post in this thread.  

Thank you for your patience with me, I hope i am not wasting your time.  I am just trying to understand if it is normal that my spacing is so long for achieving round stars in the corners.  And also if I would expect to see the same spacing when using the QTCC.  Thank you again.

 


Roland Christen
 

Remember, there are also filters and cover glasses in the optical train, which change the raw back focus distance.

Rolando



-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-ug.groups.io
Sent: Fri, Jun 18, 2021 12:52 pm
Subject: Re: [ap-ug] Interesting Back Spacing for 130GTX. Is this far off spec considered normal, and does it even matter?

On Fri, Jun 18, 2021 at 01:12 PM, Roland Christen wrote:
My not being there to verify your measurements, I cannot really say.
 
Rolando

Roland,

Here's the next best thing.  I actually measured from the Flattener to the Camera side of the filter wheel, and then added 12.5mm for the sensor setback to arrive at 111.5mm.  I've set my caliper at 111.5mm and held that up against the image train to illustrate exactly the two points from which I am measuring.  If this satisfies you that I am measuring correctly, then you can also evaluate the camera fits available at the link in my first post in this thread.  

Thank you for your patience with me, I hope i am not wasting your time.  I am just trying to understand if it is normal that my spacing is so long for achieving round stars in the corners.  And also if I would expect to see the same spacing when using the QTCC.  Thank you again.

 


ROBERT WYNNE
 

I think there is likely some tolerance issues at play here amongst all added components and the scope. I can't imagine holding exact tolerances to the 3rd decimal or even +- 1mm across all platforms. Even the coefficient of expansion on any given day may account for deviation off stated nominal specifications especially in aluminum. -Best, Robert

On 06/18/2021 7:27 AM Dale Ghent <daleg@elemental.org> wrote:


The 35FF is a 1.1x flattener. My 130GTX+35FF works out to a focal length of 871.5mm, which gets me a focal ratio of 6.703. Yours is not so different from this, at f/6.730.

From what I understand, backfocus distance is influenced by the focal ratio, so the manufacturing and figuring variances across different combinations of OTA and flattener. This means that the stated backfocus distance (80.8mm) is more of a guideline than something you have to strictly measure to.

The 111.5mm you state isn't too far off from the 102.9mm distance of the 35FF/QTCC sans extension. Are you confusing this with the 80.8mm backfocus distance when using the spacer?



On Jun 18, 2021, at 09:01, Chris White <chris.white@whitephotogallery.com> wrote:

Sorry for the repost and deletion of my previous topic. I cleaned it up a bit after learning a little bit about what to expect.

The following is my experience dialing in the Field Flattener.

I've had a paucity of clear nights since I purchased my 130GTX so it has taken me a while to get spacing and tilt dialed in, but I finally have succeed in getting it pretty darn good. I initially started out at 103.9mm of spacing (per AP spec) which accounts for a 3mm thick filter in the optical path. Stars looked pretty bad. I was able to get a slight improvement by adding 2mm of spacing, which a friend told me that fixed his spacing issues. Then I saw Roland post on here that one would actually want less spacing than specified for a very large chip. (I'm using a 36mm x 24mm IMX455 chip camera which has 3.76um pixels which is capable of revealing the warts of any system.)

So, I started looking at spacing a couple of mm below spec and stars looked much worse. So I decided to throw out all of my assumptions with respect to spacing specification and just started adding spacing one night using the fact that stars were pointing inward from the corners as my guide. To my surprise, every time I added another mm of spacing, stars looked incrementally better. All the way out to 111.5mm of spacing! This shocked me, as its 7.6mm longer in spacing than specified!

Last night I was able to dial in the tilt that I had in the system which required a 100micron adjustment on two of the three screws on my Gerd CTU. Stars now look very good to my eyes, and I'm a pixel peeper. I've included the aberration inspector below so you can see. Stars look very round to the corners. They look tight and in focus. There is a tiny amount of flaring, maybe... but its hard to tell. I'll need to make an integration to really see if this is an issue. But overall, stars look pretty good, and this is Bin1.

So a question for AP is, is it normal to see a 130GTX so far out of spec? Is this caused by the scope or the flattener being a little out of spec? I have not tested the QTCC yet with this spacing, but do you think this could be a problem when it comes to reducing the scope? Stars look pretty good to me with the field flattener, but could they be even better? Moshen suggested that I platesolve to confirm actual focal length, and it was 875mm, which is pretty much spot on to spec. So why the large backspacing discrepancy? (I've triple checked my measurements from the sensor to the shoulder of the flattener (where the threads start emerging from the flattener. 111.5mm for sure. )

The AA below is from a 600 second Luminance filter shot on a 900GTO. There might be a little bit of flaring on some of the brighter stars, but hard to tell with a single sub. Also included is the CCDI maps. Looks pretty good, although there is still a little bit of remaining tilt that I might be able to improve on... although not sure if my eyes can even see this as an issue. I'll need to do an integration to see if it is meaningful.

Here is the original fit file from camera in case anyone is interested in a closer look at the file: https://www.dropbox.com/sh/o5xsd38vs3zenpl/AAAjocgM74BXPWeqc4wXnXHYa?dl=0

Thanks in advance for any input.

<AA-2.JPG><CCDI-1.JPG><CCDI-2.JPG>



Chris White
 

I've accounted for that.  Please see my original post in this thread.

If this is a waste of your time, I apologize.  But respectfully, a filter is not going to change backspacing requirements to the tune of adding 7.6mm, especially when you said in another thread that one would want LESS back spacing than specified for large sensors.  In my original post I stated that I had a 3mm thick filter so the specified backspacing of 102.9mm would become 103.9mm. I'm at 111.5mm, which is 7.6mm over spec when accounting for the filter. 

My questions are still unanswered. 
Is it normal to see a 130GTX [backspacing] so far out of spec?  Is this caused by the scope or the flattener being a little out of spec?
Would I expect the QTCC to be the same 111.5mm on my scope?


Roland Christen
 

If you want to obsess about it, then remove the field flattener and take an image without it. Then do a plate solve or whatever allows the focal length of the main lens to be measured. You will see that the lens has the requisite focal length that was advertised.

Rolando

-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-ug.groups.io
Sent: Fri, Jun 18, 2021 1:08 pm
Subject: Re: [ap-ug] Interesting Back Spacing for 130GTX. Is this far off spec considered normal, and does it even matter?

I've accounted for that.  Please see my original post in this thread.

If this is a waste of your time, I apologize.  But respectfully, a filter is not going to change backspacing requirements to the tune of adding 7.6mm, especially when you said in another thread that one would want LESS back spacing than specified for large sensors.  In my original post I stated that I had a 3mm thick filter so the specified backspacing of 102.9mm would become 103.9mm. I'm at 111.5mm, which is 7.6mm over spec when accounting for the filter. 

My questions are still unanswered. 
Is it normal to see a 130GTX [backspacing] so far out of spec?  Is this caused by the scope or the flattener being a little out of spec?
Would I expect the QTCC to be the same 111.5mm on my scope?


Chris White
 

On Fri, Jun 18, 2021 at 02:25 PM, Roland Christen wrote:
If you want to obsess about it, then remove the field flattener and take an image without it. Then do a plate solve or whatever allows the focal length of the main lens to be measured. You will see that the lens has the requisite focal length that was advertised.
Roland,

I have no idea why you are stating this as a reply.  No where did I say that gear was not performing as advertised.  In my ORIGINAL post I stated that I was at 875mm FL which IS what the spec calls for with the flattener.  I'm asking about backspacing, trying to understand why I needed so much additional backspacing over spec to get perfect stars.  Thats all.  I'm not sure why you are getting defensive and repeatedly ignoring my questions.  Have you even read my post?  Your replies indicate not.

I am curious, just trying to understand.  I am not an optical genius, I am just interested in understanding why 111.5mm is optimal.  Forget it.  I see that for some reason you are offended by my questions.  I've received the message loud and clear, and I will move on.    Good bye. 


Seb@stro
 

Hi Chris,

You might find what you're looking for in the two previous threads/posts below. Roland explains how field curvature is affected by spacing with FF and FR. 



CS!
Sébastien


Roland Christen
 

I apologize for sounding defensive, and in anything that I wrote in my posts.

I tried to explain how the field flattener works and that small changes in positioning will increase the back spacing (weak Barlow effect). I cannot fully explain what you are seeing as far as spacing requirements of your camera. At this time I don't have your particular equipment, but hope to have it in the near future. If it looks like the recommended back spacing for that camera is wrong on our website, we will update it.

In optics, some variability is inevitable. The main lens focal length is very closely fixed due to very tight tolerance on glass index and the grinding/polishing tooling. Measured focal length is generally less than +- 1mm. For field flatteners the element radii are extreme and very slight difference in curvature of any of the elements has a much larger effect than that of the main lens. Even so, the tooling is very tightly controlled.

These 130 field flatteners are designed with edge rays diverging in order to fit as much light as possible thru the limited filter apertures. It may not be well known to astro-imagers that normal converging light rays are heavily vignetted when passing thru limited aperture filters (50mm diameter filters will not pass all the light into the corners of a 35mm full aperture chip if the light rays are converging). Because of this diverging geometry, any additional glass, such as filters and cover glasses, have an outsize effect on the field curvature. Therefore it will require somewhat longer distance (and slightly larger Barlow effect) in order to compensate for this additional glass compared to a glass-free optical path.

Roland Christen

-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-ug.groups.io
Sent: Fri, Jun 18, 2021 1:34 pm
Subject: Re: [ap-ug] Interesting Back Spacing for 130GTX. Is this far off spec considered normal, and does it even matter?

On Fri, Jun 18, 2021 at 02:25 PM, Roland Christen wrote:
If you want to obsess about it, then remove the field flattener and take an image without it. Then do a plate solve or whatever allows the focal length of the main lens to be measured. You will see that the lens has the requisite focal length that was advertised.
Roland,

I have no idea why you are stating this as a reply.  No where did I say that gear was not performing as advertised.  In my ORIGINAL post I stated that I was at 875mm FL which IS what the spec calls for with the flattener.  I'm asking about backspacing, trying to understand why I needed so much additional backspacing over spec to get perfect stars.  Thats all.  I'm not sure why you are getting defensive and repeatedly ignoring my questions.  Have you even read my post?  Your replies indicate not.

I am curious, just trying to understand.  I am not an optical genius, I am just interested in understanding why 111.5mm is optimal.  Forget it.  I see that for some reason you are offended by my questions.  I've received the message loud and clear, and I will move on.    Good bye. 


ROBERT WYNNE
 

Tolerance stack up or down when it comes to ultra precise instrumentation generally requires some knowledge of the end user that not everything in life is perfect and requires some adjustment. That said Astro-Physics comes pretty darn close to perfection for my money. Frankly, controlling the length of a 1000mm OTA along with placement of figuring an ultra precise lens required to attain a focal point coupled with minimum airy disk time after time is a tribute to the A-P team and its founder. A difference of .3" should not be that great a concern if it can be easily adjusted. So far from the earliest literature to present  I have yet to read this is an easy hobby to pursue. Though I would like a FF for my 155 when they come available if possible. -Best, Robert

On 06/20/2021 1:02 PM Roland Christen via groups.io <chris1011@...> wrote:
 
 
I apologize for sounding defensive, and in anything that I wrote in my posts.
 
I tried to explain how the field flattener works and that small changes in positioning will increase the back spacing (weak Barlow effect). I cannot fully explain what you are seeing as far as spacing requirements of your camera. At this time I don't have your particular equipment, but hope to have it in the near future. If it looks like the recommended back spacing for that camera is wrong on our website, we will update it.
 
In optics, some variability is inevitable. The main lens focal length is very closely fixed due to very tight tolerance on glass index and the grinding/polishing tooling. Measured focal length is generally less than +- 1mm. For field flatteners the element radii are extreme and very slight difference in curvature of any of the elements has a much larger effect than that of the main lens. Even so, the tooling is very tightly controlled.
 
These 130 field flatteners are designed with edge rays diverging in order to fit as much light as possible thru the limited filter apertures. It may not be well known to astro-imagers that normal converging light rays are heavily vignetted when passing thru limited aperture filters (50mm diameter filters will not pass all the light into the corners of a 35mm full aperture chip if the light rays are converging). Because of this diverging geometry, any additional glass, such as filters and cover glasses, have an outsize effect on the field curvature. Therefore it will require somewhat longer distance (and slightly larger Barlow effect) in order to compensate for this additional glass compared to a glass-free optical path.
 
Roland Christen
 
-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-ug.groups.io
Sent: Fri, Jun 18, 2021 1:34 pm
Subject: Re: [ap-ug] Interesting Back Spacing for 130GTX. Is this far off spec considered normal, and does it even matter?

On Fri, Jun 18, 2021 at 02:25 PM, Roland Christen wrote:
If you want to obsess about it, then remove the field flattener and take an image without it. Then do a plate solve or whatever allows the focal length of the main lens to be measured. You will see that the lens has the requisite focal length that was advertised.
Roland,

I have no idea why you are stating this as a reply.  No where did I say that gear was not performing as advertised.  In my ORIGINAL post I stated that I was at 875mm FL which IS what the spec calls for with the flattener.  I'm asking about backspacing, trying to understand why I needed so much additional backspacing over spec to get perfect stars.  Thats all.  I'm not sure why you are getting defensive and repeatedly ignoring my questions.  Have you even read my post?  Your replies indicate not.

I am curious, just trying to understand.  I am not an optical genius, I am just interested in understanding why 111.5mm is optimal.  Forget it.  I see that for some reason you are offended by my questions.  I've received the message loud and clear, and I will move on.    Good bye. 


Chris White
 
Edited

On Sun, Jun 20, 2021 at 04:02 PM, Roland Christen wrote:
Because of this diverging geometry, any additional glass, such as filters and cover glasses, have an outsize effect on the field curvature. Therefore it will require somewhat longer distance (and slightly larger Barlow effect) in order to compensate for this additional glass compared to a glass-free optical path.
Roland,

Thank you for your thoughtful reply.  Although all of it is very interesting to me, the information that I copied from your post is exactly the kind of thing I was hoping for, and this makes a lot of sense to me as to why so much additional spacing would be required.  I had no idea that this kind of relationship could exist, and indeed I am using 50mm filters that are 22mm away from the sensor and this is the narrowest aperture in my light path.

To be honest, I am stunned by how good my stars look across this large chip and with 3.76um pixels.   Tight round stars to the corners with tiny pixels is truly remarkable. The GTX and field flattener is truly amazing optically.  I have the QTCC installed now and hope to have testing completed soon. 

On a side note, I take it from your post that you are considering an IMX455 chip?  I have generally not been very pleased with CMOS offerings, and been much happier with my QSI683 and 6120, but these new back illuminated sony chips are truly amazing.  The small pixels are great for widefield and they bin very nicely for longer focal length.  The data looks very good and processing is a dream.  I think you'll be pleased. 

Thank you again,


M Hambrick
 

Hi Chris

In the photo you posted of your imaging train I recognize the flattener, camera, filter wheel, off-axis guider, and what looks like a 3/4" extension attached to the back of the flattener. What is that device behind the extension (under the 30 mm mark on the calipers) ? Is that your tilt adjustment ?

Mike


Fredd Drevon
 

I look forward to your testing with the Quad TTC. I have the same equipment and cannot get round stars in the corners. I’m not sure that matters that much because this would be for mostly nebulae imaging. It could become more of an issue with mosaics where corners get closer to the centre. I have also the flattener but haven’t tested it with the 130GTX and asi 6200. I will be moving to a dark site (in Spain) with good seeing and haven’t completely decided yet it if makes more sense to go with Quad or flattener (the usual speed vs resolution debate) : resolution of 1.31 arc/sec for Quad vs 0.89 with flattener. My current thinking is to keep to Quad for nebulae and switch to flattener if I do galaxies. Does that make sense ?


Chris White
 

On Mon, Jun 21, 2021 at 11:52 AM, M Hambrick wrote:
Hi Chris

In the photo you posted of your imaging train I recognize the flattener, camera, filter wheel, off-axis guider, and what looks like a 3/4" extension attached to the back of the flattener. What is that device behind the extension (under the 30 mm mark on the calipers) ? Is that your tilt adjustment ?

Mike

Mike,

Yes, that is the Gerd M68 CTU.  Even though the ZWO comes with a tilt plate, the Gernd is MUCH more precise.  One full rotation of the screw (accessed on the edge) is 200microns, so its really easy to get very precise adjustments for tilt. 

In this image is:  (3) 1mm AP Spacer Rings > Precise Parts Adapter > Gerd CTU > M68 Moonlite Spacer Ring > OAG > FW > Camera = 111.5mm total. 


Chris White
 

On Mon, Jun 21, 2021 at 12:03 PM, Fredd Drevon wrote:
I look forward to your testing with the Quad TTC. I have the same equipment and cannot get round stars in the corners. I’m not sure that matters that much because this would be for mostly nebulae imaging. It could become more of an issue with mosaics where corners get closer to the centre. I have also the flattener but haven’t tested it with the 130GTX and asi 6200. I will be moving to a dark site (in Spain) with good seeing and haven’t completely decided yet it if makes more sense to go with Quad or flattener (the usual speed vs resolution debate) : resolution of 1.31 arc/sec for Quad vs 0.89 with flattener. My current thinking is to keep to Quad for nebulae and switch to flattener if I do galaxies. Does that make sense ?
Fredd,

I'm not finished testing yet, but I did have an hour last night before clouds came.  I was able to get spacing dialed where I am 90% sure is optimal.  Spacing is 106.1mm.  I have a small amount of tilt to work out which did show up in my CCDI plot with the flattener even though stars were round to my eyes. 

I am pretty sure that the QTCC will not produce perfect round stars with 3.76um pixels.  However, I am REALLY close to getting mostly perfect round stars when I bin2 with the QTCC.  I'd be more than happy with this as the goal here would be the large field of view not high resolution, and would be great for mosaics.  The system Entendue would be extremely high as well, which would be great for me as I get very few clear nights.  I will post the results when I have them.

As far as your thoughts on when to use the QTCC and when to use the FF.  That's my thought exactly.  Use the reducer for nebula season and the Flattener for galaxy season or if I want a higher resolution with tighter framing.  I also have the BARADV which I might test for galaxies if I want to get a little more sampling than 0.89"/px, but to be honest I dont think my seeing would be able to support this.  Most likely I would use this for some high res video imaging of planets and moon.   I also have an Edge 925, and it works very well with the 6200 at f10, but to be honest I like the idea of being lazy and only using one scope all year round.