Stowaway, 92TCC and corner stars


Henry Kwok
 

Hello,

Questions about trying to get good corner stars with my Stowaway + 92TCC.

I have been trying to get good corner stars with my Stowaway + 92TCC + APS-C size QHY268m camera. No matter what I tried the corner stars are ever slightly malformed. The same camera / FW / OAG are used interchangably with my AP130GT + 67PF flattener with no problem, via two different custom adaptors to get the required back focus distances for the two set ups. I can tweak the corner star shape a bit by manually fine tuning the focus while watching the corners, but it is not going to be good enough for auto focus. I am not sure if I am expecting too much of the 92TCC, as it is supposed to be able to cover an APS-C size sensor. I don't know if there are other things to try, or whether I should just buy a the 92FF and accept a slower speed.

Just a bit of sanity check:
- The back focus of 64.1 mm for 92TCC, where is it measured from?  With the custom adaptor I designed and had made, I asked for a effective length of 8.1 mm, measured from the very end of the 92TCC (i.e. the female 2.7" thread portion of the adaptor which goes onto the 92TCC does not contribute to the effective length). With that this gives me 64.1-8.1=56 mm, which is the amount taken up by my camera+OAG+FW.

- Is the 92TCC designed to give pinpoint corner stars using a APS-size sensor with pixel size of only 3.75 micron? Or am I asking too much of it? Would a 92FF mitigate the corner problem?

- The fact that the star shape can be tweaked with focus, does it suggest that the problem may be to do with spacing? or is it tilt (given the AP130GT with 67PF is slower it may not manifest).  How do I isolate the problem?

Corner star shape bothers me, even though it is probably not important for most compositions, and I am keen to get to the bottom of it.

Thank you                                                                                                                      

 


Bill Long
 

Timely post.

There is a really, really good write up here about tilt and spacing: Fixing the dreaded tilt and backspacing error in optical systems with objective analysis. - AstroBin

A fellow AP fan Chris White wrote it up, and its a great read and should help answer your questions.


From: main@ap-ug.groups.io <main@ap-ug.groups.io> on behalf of Henry Kwok <henry.ck.kwok@...>
Sent: Friday, July 29, 2022 8:48 PM
To: main@ap-ug.groups.io <main@ap-ug.groups.io>
Subject: [ap-ug] Stowaway, 92TCC and corner stars
 
Hello,

Questions about trying to get good corner stars with my Stowaway + 92TCC.

I have been trying to get good corner stars with my Stowaway + 92TCC + APS-C size QHY268m camera. No matter what I tried the corner stars are ever slightly malformed. The same camera / FW / OAG are used interchangably with my AP130GT + 67PF flattener with no problem, via two different custom adaptors to get the required back focus distances for the two set ups. I can tweak the corner star shape a bit by manually fine tuning the focus while watching the corners, but it is not going to be good enough for auto focus. I am not sure if I am expecting too much of the 92TCC, as it is supposed to be able to cover an APS-C size sensor. I don't know if there are other things to try, or whether I should just buy a the 92FF and accept a slower speed.

Just a bit of sanity check:
- The back focus of 64.1 mm for 92TCC, where is it measured from?  With the custom adaptor I designed and had made, I asked for a effective length of 8.1 mm, measured from the very end of the 92TCC (i.e. the female 2.7" thread portion of the adaptor which goes onto the 92TCC does not contribute to the effective length). With that this gives me 64.1-8.1=56 mm, which is the amount taken up by my camera+OAG+FW.

- Is the 92TCC designed to give pinpoint corner stars using a APS-size sensor with pixel size of only 3.75 micron? Or am I asking too much of it? Would a 92FF mitigate the corner problem?

- The fact that the star shape can be tweaked with focus, does it suggest that the problem may be to do with spacing? or is it tilt (given the AP130GT with 67PF is slower it may not manifest).  How do I isolate the problem?

Corner star shape bothers me, even though it is probably not important for most compositions, and I am keen to get to the bottom of it.

Thank you                                                                                                                      

 


Roland Christen
 


- The fact that the star shape can be tweaked with focus, does it suggest that the problem may be to do with spacing? or is it tilt
Could be slight tilt in your camera or could also be spacing. If the stars in the corner get better when you focus inward, then it suggests adding 1mm of spacing. It means you have inward curvature and not quite enough spacing. Remember also to add 1mm of back distance for the filter thickness if you use filters in your optical train.

Roland Christen
Astro-Physics Inc.


-----Original Message-----
From: Henry Kwok <henry.ck.kwok@...>
To: main@ap-ug.groups.io
Sent: Fri, Jul 29, 2022 10:48 pm
Subject: [ap-ug] Stowaway, 92TCC and corner stars

Hello,

Questions about trying to get good corner stars with my Stowaway + 92TCC.

I have been trying to get good corner stars with my Stowaway + 92TCC + APS-C size QHY268m camera. No matter what I tried the corner stars are ever slightly malformed. The same camera / FW / OAG are used interchangably with my AP130GT + 67PF flattener with no problem, via two different custom adaptors to get the required back focus distances for the two set ups. I can tweak the corner star shape a bit by manually fine tuning the focus while watching the corners, but it is not going to be good enough for auto focus. I am not sure if I am expecting too much of the 92TCC, as it is supposed to be able to cover an APS-C size sensor. I don't know if there are other things to try, or whether I should just buy a the 92FF and accept a slower speed.

Just a bit of sanity check:
- The back focus of 64.1 mm for 92TCC, where is it measured from?  With the custom adaptor I designed and had made, I asked for a effective length of 8.1 mm, measured from the very end of the 92TCC (i.e. the female 2.7" thread portion of the adaptor which goes onto the 92TCC does not contribute to the effective length). With that this gives me 64.1-8.1=56 mm, which is the amount taken up by my camera+OAG+FW.

- Is the 92TCC designed to give pinpoint corner stars using a APS-size sensor with pixel size of only 3.75 micron? Or am I asking too much of it? Would a 92FF mitigate the corner problem?

- The fact that the star shape can be tweaked with focus, does it suggest that the problem may be to do with spacing? or is it tilt (given the AP130GT with 67PF is slower it may not manifest).  How do I isolate the problem?

Corner star shape bothers me, even though it is probably not important for most compositions, and I am keen to get to the bottom of it.

Thank you                                                                                                                      

 


Henry Kwok
 

Thanks for the link to the article, Bill. Very useful. I have used ASTAP on the images with my AP130 and my AP92. the AP130 has a little aberration and the HFD at the corners are slightly different such that the "box" is not perfectly square. However, with the AP92 the stars on one side have vastly different HFD than the other side, such that the box looks like a trapezoid. I guess this means I have tilt with my AP92, as spacing should not have a gradient like that. Question though is, where is the tilt coming from? Before investing in a tilt adjuster, wouldn't it be better to find out where the tilt is introduced? The imaging chain (CMOS + FW + OAG) is solidly bolted together, and there is little tilt when using it with the AP130, so presumably the tilt lies with the custom adaptor or the AP92/TCC92 itself? Is there a easy way to isolate? Some kind of rotation test??


Bill Long
 

That usually suggests tilt is the dominant (but not necessarily the only) source of error. Both of these two issues need to be tackled and solved. That is the beauty of the article Chris wrote, is that he is sharing methods to help solve both of them and understand both of them as you move through the process.

The source of the tilt can be a challenge to identify. The solution can be many different elements of change. The approach to use tilt and spacing adjustment tools can mitigate hard to find problems (i.e. a poorly machined adapter) but square goods through the imaging train is ideal -- but not necessarily required. 

Isolation of specific adapters is, as you described, is a not fun and very tedious method of moving isoalted parts around to try and find the one that is problematic. That is a way to tackle the problem, but there are products you can use that can solve even those underlying issues (by compensating for them).

In the end -- it is very much your call in terms of what you want to do/spend to solve the issues. I think its important to call out, for the sake of this being an AP forum -- the scope is not the problem nor is the corrector.

From: main@ap-ug.groups.io <main@ap-ug.groups.io> on behalf of Henry Kwok <henry.ck.kwok@...>
Sent: Friday, July 29, 2022 10:32 PM
To: main@ap-ug.groups.io <main@ap-ug.groups.io>
Subject: Re: [ap-ug] Stowaway, 92TCC and corner stars
 
Thanks for the link to the article, Bill. Very useful. I have used ASTAP on the images with my AP130 and my AP92. the AP130 has a little aberration and the HFD at the corners are slightly different such that the "box" is not perfectly square. However, with the AP92 the stars on one side have vastly different HFD than the other side, such that the box looks like a trapezoid. I guess this means I have tilt with my AP92, as spacing should not have a gradient like that. Question though is, where is the tilt coming from? Before investing in a tilt adjuster, wouldn't it be better to find out where the tilt is introduced? The imaging chain (CMOS + FW + OAG) is solidly bolted together, and there is little tilt when using it with the AP130, so presumably the tilt lies with the custom adaptor or the AP92/TCC92 itself? Is there a easy way to isolate? Some kind of rotation test??


 

>>> so presumably the tilt lies with the custom adaptor or the AP92/TCC92 itself?

it's also possible it's within the camera/sensor. There's a reason ZWO includes a tilt plate with their cameras (i know you are using QHY, but i think they all have potential to suffer from this)

Brian


Bill Long
 

In either case it is not within the scope nor the corrector themselves. 

From: main@ap-ug.groups.io <main@ap-ug.groups.io> on behalf of Brian Valente <bvalente@...>
Sent: Friday, July 29, 2022 11:02 PM
To: main@ap-ug.groups.io <main@ap-ug.groups.io>
Subject: Re: [ap-ug] Stowaway, 92TCC and corner stars
 
>>> so presumably the tilt lies with the custom adaptor or the AP92/TCC92 itself?

it's also possible it's within the camera/sensor. There's a reason ZWO includes a tilt plate with their cameras (i know you are using QHY, but i think they all have potential to suffer from this)

Brian


Henry Kwok
 

Yeah... I am sure the scope and the 92TCC are not to blame. So it is most likely the custom adaptor which is made locally by a machinist and I am not sure what his precision is.

I think I will invest in both a tilt adjuster and also make a new custom adaptor. Can someone please verify what distance to specify with Precisionparts, if the camera + EFW + OAG take up 56 mm? Is it 8.1 mm, which is what I used with the current adaptor?


Bill Long
 

The plotted distance from the sensor to the corrector by Roland is 64.1mm. 

From: main@ap-ug.groups.io <main@ap-ug.groups.io> on behalf of Henry Kwok <henry.ck.kwok@...>
Sent: Friday, July 29, 2022 11:22 PM
To: main@ap-ug.groups.io <main@ap-ug.groups.io>
Subject: Re: [ap-ug] Stowaway, 92TCC and corner stars
 
Yeah... I am sure the scope and the 92TCC are not to blame. So it is most likely the custom adaptor which is made locally by a machinist and I am not sure what his precision is.

I think I will invest in both a tilt adjuster and also make a new custom adaptor. Can someone please verify what distance to specify with Precisionparts, if the camera + EFW + OAG take up 56 mm? Is it 8.1 mm, which is what I used with the current adaptor?


M Hambrick
 

Henry

The 64.1 mm backfocus is dimension X in the sketch below.

Mike


Roland Christen
 

Tilt might be introduced by the three thumb screws. If they have sharp points they may not allow the TCC to seat squarely onto the adapter. We have changed these screws to have flat bottoms which allow for proper seating. We can either send you a new set if yours have sharp tips, or you can grind or file off the tip yourself on the ones you have.

Roland Christen
Astro-Physics Inc.



-----Original Message-----
From: Henry Kwok <henry.ck.kwok@...>
To: main@ap-ug.groups.io
Sent: Sat, Jul 30, 2022 12:32 am
Subject: Re: [ap-ug] Stowaway, 92TCC and corner stars

Thanks for the link to the article, Bill. Very useful. I have used ASTAP on the images with my AP130 and my AP92. the AP130 has a little aberration and the HFD at the corners are slightly different such that the "box" is not perfectly square. However, with the AP92 the stars on one side have vastly different HFD than the other side, such that the box looks like a trapezoid. I guess this means I have tilt with my AP92, as spacing should not have a gradient like that. Question though is, where is the tilt coming from? Before investing in a tilt adjuster, wouldn't it be better to find out where the tilt is introduced? The imaging chain (CMOS + FW + OAG) is solidly bolted together, and there is little tilt when using it with the AP130, so presumably the tilt lies with the custom adaptor or the AP92/TCC92 itself? Is there a easy way to isolate? Some kind of rotation test??


Roland Christen
 

If the machinist made your adapter in two setups it will definitely have tilt. You can measure that by using a micrometer and measure 3 points around the length of the adapter. If they vary by .001" it is enough tilt to affect the image.

Roland



-----Original Message-----
From: Henry Kwok <henry.ck.kwok@...>
To: main@ap-ug.groups.io
Sent: Sat, Jul 30, 2022 1:22 am
Subject: Re: [ap-ug] Stowaway, 92TCC and corner stars

Yeah... I am sure the scope and the 92TCC are not to blame. So it is most likely the custom adaptor which is made locally by a machinist and I am not sure what his precision is.

I think I will invest in both a tilt adjuster and also make a new custom adaptor. Can someone please verify what distance to specify with Precisionparts, if the camera + EFW + OAG take up 56 mm? Is it 8.1 mm, which is what I used with the current adaptor?


Henry Kwok
 

Hello Roland,

Yes, the thumb screws holding the TCC in the DOVELOC are indeed pointed. In fact, they are so pointed that they make small marks on the TCC itself. See photos attached. I guess this could potentially be the problem? I am actually hoping that it is, as it may be an easy fix...

It would be very nice if you can send me a set of new ones, but I am now back in New Zealand (I was living in NYC up until recently) and the only way you can send me a set is by UPS/Fedex as USPS is currently not sending to NZ due to the pandemic.

In the mean time, I may try to grind the screws a bit to see if this will help. How much shall I grind them by? Can you please post a photo of what the screws should look like? Should the tips be completely flat? Shall I invest in some teflon tip screws that AgenaAstro sells?

 


Henry Kwok
 

I have been plowing through the forum here of topics on 92TCC/Stowaway, and I have come across a few more posts relating to tilt:

(1) On 10/26/2021, Roland replied in a thread: "It is possible for the camera adapter dovetail attachment to be tilted – you can check that visually. Make sure that it seats flat against the focuser end when you tighten the three thumb screws."

Can you please tell me how to ensure the TCC is sitting flat? To the naked eye it seems to be well seated. Do you need to install the TCC with the scope vertical?

(2) Mike Hambrick replied in the same thread with illustration: "I believe that the root cause of this issue is that the lock screw holes in the Doveloc adapter are drilled in the wrong location (too close to the back face by about 0.030"). However, you can correct this issue by filing the sharp tip off of the lock screws so that only the tapered part makes contact with the angled shoulder of the 92TCC." 

I don't understand the last sentence: "iling the sharp tip off of the lock screws so that only the tapered part makes contact with the angled shoulder of the 92TCC."

(3) In another tread, Mark posted again: "As for other comments, pay attention to the locking screws that hold the 92FF or 92TCC into the Doveloc adapter. If you see any indentions in the bottom of the groove of your 92FF or 92TCC it means that the lock screws are not properly holding the corrector in position. You will need to slightly blunt the tips of the locking screws with a file so that the tapered section of the locking screws is making contact with the tapered section of the groove in the 92FF or 92TCC."

If I understand it correctly, I should grind the tip so that the point of contact between the screw and the TCC is on the sloped edge of the groove, not the bottom of the groove. Is this correct?

Sorry for the questions. I am not very mechanical minded and found it difficult to picture why this matters and how the solution work.


Kurt Mihalco
 

Yes to this –

If I understand it correctly, I should grind the tip so that the point of contact between the screw and the TCC is on the sloped edge of the groove, not the bottom of the groove. Is this correct?
Kurt

 

From: main@ap-ug.groups.io [mailto:main@ap-ug.groups.io] On Behalf Of Henry Kwok
Sent: Saturday, July 30, 2022 12:45 PM
To: main@ap-ug.groups.io
Subject: Re: [ap-ug] Stowaway, 92TCC and corner stars

 

I have been plowing through the forum here of topics on 92TCC/Stowaway, and I have come across a few more posts relating to tilt:

(1) On 10/26/2021, Roland replied in a thread: "It is possible for the camera adapter dovetail attachment to be tilted – you can check that visually. Make sure that it seats flat against the focuser end when you tighten the three thumb screws."

Can you please tell me how to ensure the TCC is sitting flat? To the naked eye it seems to be well seated. Do you need to install the TCC with the scope vertical?

(2) Mike Hambrick replied in the same thread with illustration: "I believe that the root cause of this issue is that the lock screw holes in the Doveloc adapter are drilled in the wrong location (too close to the back face by about 0.030"). However, you can correct this issue by filing the sharp tip off of the lock screws so that only the tapered part makes contact with the angled shoulder of the 92TCC." 

I don't understand the last sentence: "iling the sharp tip off of the lock screws so that only the tapered part makes contact with the angled shoulder of the 92TCC."

(3) In another tread, Mark posted again: "As for other comments, pay attention to the locking screws that hold the 92FF or 92TCC into the Doveloc adapter. If you see any indentions in the bottom of the groove of your 92FF or 92TCC it means that the lock screws are not properly holding the corrector in position. You will need to slightly blunt the tips of the locking screws with a file so that the tapered section of the locking screws is making contact with the tapered section of the groove in the 92FF or 92TCC."

If I understand it correctly, I should grind the tip so that the point of contact between the screw and the TCC is on the sloped edge of the groove, not the bottom of the groove. Is this correct?

Sorry for the questions. I am not very mechanical minded and found it difficult to picture why this matters and how the solution work.


Roland Christen
 


I guess this could potentially be the problem?
Yes, this is the problem. When you flatten the tips, install the TCC and tighten the 3 screws evenly. Then check that the TCC body is tight against the mating adapter all the way around the circumference. There should be no airgap between them.

Roland

-----Original Message-----
From: Henry Kwok <henry.ck.kwok@...>
To: main@ap-ug.groups.io
Sent: Sat, Jul 30, 2022 2:13 pm
Subject: Re: [ap-ug] Stowaway, 92TCC and corner stars

Hello Roland,

Yes, the thumb screws holding the TCC in the DOVELOC are indeed pointed. In fact, they are so pointed that they make small marks on the TCC itself. See photos attached. I guess this could potentially be the problem? I am actually hoping that it is, as it may be an easy fix...

It would be very nice if you can send me a set of new ones, but I am now back in New Zealand (I was living in NYC up until recently) and the only way you can send me a set is by UPS/Fedex as USPS is currently not sending to NZ due to the pandemic.

In the mean time, I may try to grind the screws a bit to see if this will help. How much shall I grind them by? Can you please post a photo of what the screws should look like? Should the tips be completely flat? Shall I invest in some teflon tip screws that AgenaAstro sells?

 


Roland Christen
 


I should grind the tip so that the point of contact between the screw and the TCC is on the sloped edge of the groove, not the bottom of the groove. Is this correct?
Yes, this is what you need to do so that the mating parts sit tight against each other.

Roland


-----Original Message-----
From: Henry Kwok <henry.ck.kwok@...>
To: main@ap-ug.groups.io
Sent: Sat, Jul 30, 2022 2:45 pm
Subject: Re: [ap-ug] Stowaway, 92TCC and corner stars

I have been plowing through the forum here of topics on 92TCC/Stowaway, and I have come across a few more posts relating to tilt:

(1) On 10/26/2021, Roland replied in a thread: "It is possible for the camera adapter dovetail attachment to be tilted – you can check that visually. Make sure that it seats flat against the focuser end when you tighten the three thumb screws."

Can you please tell me how to ensure the TCC is sitting flat? To the naked eye it seems to be well seated. Do you need to install the TCC with the scope vertical?

(2) Mike Hambrick replied in the same thread with illustration: "I believe that the root cause of this issue is that the lock screw holes in the Doveloc adapter are drilled in the wrong location (too close to the back face by about 0.030"). However, you can correct this issue by filing the sharp tip off of the lock screws so that only the tapered part makes contact with the angled shoulder of the 92TCC." 

I don't understand the last sentence: "iling the sharp tip off of the lock screws so that only the tapered part makes contact with the angled shoulder of the 92TCC."

(3) In another tread, Mark posted again: "As for other comments, pay attention to the locking screws that hold the 92FF or 92TCC into the Doveloc adapter. If you see any indentions in the bottom of the groove of your 92FF or 92TCC it means that the lock screws are not properly holding the corrector in position. You will need to slightly blunt the tips of the locking screws with a file so that the tapered section of the locking screws is making contact with the tapered section of the groove in the 92FF or 92TCC."

If I understand it correctly, I should grind the tip so that the point of contact between the screw and the TCC is on the sloped edge of the groove, not the bottom of the groove. Is this correct?

Sorry for the questions. I am not very mechanical minded and found it difficult to picture why this matters and how the solution work.


Wenhan Chang
 

Hi Henry,

Here is a photo comparing the locking screws for diagonal/eyeypiece (left) and for 92FF/TCC (right), both from AP. Unlike screws for eyepiece, the 92TCC screw doesn't need to be too flat.



Hope it helps.

Best,
Wenhan


M Hambrick
 

Hi Henry

Your photo showing the dimples in the groove of the Doveloc adapter look exactly like what I experienced. Take a look at the picture below. This is a photo of the original Doveloc screw after I blunted the tip of it. What you are trying to achieve is to have the tapered end of the Doveloc screw making contact with the tapered shoulder on the Doveloc adapter. This angular contact will "pull" the adapter up tight against the back face of the focuser assembly. There is another sketch below that shows the concept.

If you have a file, you can blunt the tips of the screws yourself. Work slowly so as not to remove too much material, but it will seem like you are having to file the tip down quite a lot.

Mike

Mike




Gary Saunders
 

The one on the right in this photo is grounded down. The left and center are not. I ‘m not sure if you can make it out.

I used sandpaper on a flat surface to remove the tip. Its not much that needs to be remove. I did do all three.

 

Gary Saunders

 

Sent from Mail for Windows 10

 

From: Henry Kwok
Sent: Friday, July 29, 2022 11:48 PM
To: main@ap-ug.groups.io
Subject: [ap-ug] Stowaway, 92TCC and corner stars

 

Hello,

Questions about trying to get good corner stars with my Stowaway + 92TCC.

I have been trying to get good corner stars with my Stowaway + 92TCC + APS-C size QHY268m camera. No matter what I tried the corner stars are ever slightly malformed. The same camera / FW / OAG are used interchangably with my AP130GT + 67PF flattener with no problem, via two different custom adaptors to get the required back focus distances for the two set ups. I can tweak the corner star shape a bit by manually fine tuning the focus while watching the corners, but it is not going to be good enough for auto focus. I am not sure if I am expecting too much of the 92TCC, as it is supposed to be able to cover an APS-C size sensor. I don't know if there are other things to try, or whether I should just buy a the 92FF and accept a slower speed.

Just a bit of sanity check:
- The back focus of 64.1 mm for 92TCC, where is it measured from?  With the custom adaptor I designed and had made, I asked for a effective length of 8.1 mm, measured from the very end of the 92TCC (i.e. the female 2.7" thread portion of the adaptor which goes onto the 92TCC does not contribute to the effective length). With that this gives me 64.1-8.1=56 mm, which is the amount taken up by my camera+OAG+FW.

- Is the 92TCC designed to give pinpoint corner stars using a APS-size sensor with pixel size of only 3.75 micron? Or am I asking too much of it? Would a 92FF mitigate the corner problem?

- The fact that the star shape can be tweaked with focus, does it suggest that the problem may be to do with spacing? or is it tilt (given the AP130GT with 67PF is slower it may not manifest).  How do I isolate the problem?

Corner star shape bothers me, even though it is probably not important for most compositions, and I am keen to get to the bottom of it.

Thank you