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First light to me AP130 EDF 6. One optical question about optical vignetting / field illumination


alan.dang@...
 
Edited

I finally was able to get first-light on my AP130 EDF f/6 tonight.  Accuweather had astronomy forecast at a poor 4/10, but the clouds were absent in the area of Orion. The superstition of a new telescope still held though. I was only able to get 30 minutes of data before a pair of racoons decided to visit which led me to pack up and quickly call it a night.  I guess they were curious too.

EOS Ra with Canon R-to-EF adapter
Vixen SXD2, guided with a MGEN-3 and Canon 180/3.5 mm lens
I had my dithering set for every 15 shots, so I only had one shift.

Even from Bortle 7 skies, I was able to pull out (with noise) a lot of the dust around M42.  I used to think that aperture and f-ratio were all that mattered when capturing data, but I think I have now recognized that sharper refractors help to pull out more nebulosity the same way that being in focus helps rather than being out of focus.  (The other benefit being thermal stability and less focus drift.)

Optical Question: Optical Vignetting vs. Field Illumination vs. F-Ratio
My Vixen VSD100 f/3.8 generates spikes in bright stars.   One of the Japanese reviews of the VSD100 pointed out that the rear element is too small, so these artifacts are un-avoidable.  These are spikes that "look" like pinched optics.   I have also seen (but cannot find) a post on a message board explaining how the "cat-eye bokeh" effect for daylight photos is optical vignetting and that is why we see those artifacts.  That said, both a 33x44 sensor and 24x36mm sensor perform well enough on the VSD100 that you can get away without taking flats.

The AP130EDF had a lot more light fall-off at the corners than I expected.  One confounding factor is that I was using the official Canon RF to EF adapter for the first time.  This adds rectangular baffles useful for daylight photography but may decrease clear aperture and potentially cause the diffraction artifacts seen around Iota Orionis in my photo.  With my VSD100, I was using a Hutech adapter which is slightly off in terms of backfocus but has no baffles.  However, my initial thought was that illumination at the corners were poorer (more light fall-off) but the actual stars are almost perfect and show none of the optical vignetting artifacts seen with the VSD100.

Both the VSD100 and AP130EDF were designs that worked with 6x7cm film.  Am I seeing the difference between optical vignetting from a hard stop versus optical vignetting from curvature of light?  Or are the presence/absence of diffraction spikes the difference between f/3.8 and f/6+flattener?

Here's my AP130 EDF shot from tonight.  The sharpness is incredible, matching the visual clarity of the scope.



and the VSD100 photo.  Notice the spikes on the bright stars.  I took more artistic license with this one


Roland Christen
 

The 130F6 lens will cover a large format and has been used in the past with 6x7 film negs. The limitations will come at the rear if the camera adapters have small openings. The lens itself is not limited, and the focuser can also handle a wide field without vignetting.

Rolando


-----Original Message-----
From: alan.dang@...
To: main@ap-ug.groups.io
Sent: Sun, Apr 4, 2021 4:15 am
Subject: Re: [ap-ug] First light to me AP130 EDF 6. One optical question about optical vignetting / field illumination

[Edited Message Follows]
I finally was able to get first-light on my AP130 EDF f/6 tonight.  Accuweather had astronomy forecast at a poor 4/10, but the clouds were absent in the area of Orion. The superstition of a new telescope still held though. I was only able to get 30 minutes of data before a pair of racoons decided to visit which led me to pack up and quickly call it a night.  I guess they were curious too.

EOS Ra with Canon R-to-EF adapter
Vixen SXD2, guided with a MGEN-3 and Canon 180/3.5 mm lens
I had my dithering set for every 15 shots, so I only had one shift.

Even from Bortle 7 skies, I was able to pull out (with noise) a lot of the dust around M42.  I used to think that aperture and f-ratio were all that mattered when capturing data, but I think I have now recognized that sharper refractors help to pull out more nebulosity the same way that being in focus helps rather than being out of focus.  (The other benefit being thermal stability and less focus drift.)

Optical Question: Optical Vignetting vs. Field Illumination vs. F-Ratio
My Vixen VSD100 f/3.8 generates spikes in bright stars.   One of the Japanese reviews of the VSD100 pointed out that the rear element is too small, so these artifacts are un-avoidable.  These are spikes that "look" like pinched optics.   I have also seen (but cannot find) a post on a message board explaining how the "cat-eye bokeh" effect for daylight photos is optical vignetting and that is why we see those artifacts.  That said, both a 33x44 sensor and 24x36mm sensor perform well enough on the VSD100 that you can get away without taking flats.

The AP130EDF had a lot more light fall-off at the corners than I expected.  One confounding factor is that I was using the official Canon RF to EF adapter for the first time.  This adds rectangular baffles useful for daylight photography but may decrease clear aperture and potentially cause the diffraction artifacts seen around Iota Orionis in my photo.  With my VSD100, I was using a Hutech adapter which is slightly off in terms of backfocus but has no baffles.  However, my initial thought was that illumination at the corners were poorer (more light fall-off) but the actual stars are almost perfect and show none of the optical vignetting artifacts seen with the VSD100.

Both the VSD100 and AP130EDF were designs that worked with 6x7cm film.  Am I seeing the difference between optical vignetting from a hard stop versus optical vignetting from curvature of light?  Or are the presence/absence of diffraction spikes the difference between f/3.8 and f/6+flattener?

Here's my AP130 EDF shot from tonight.  The sharpness is incredible, matching the visual clarity of the scope.



and the VSD100 photo.  Notice the spikes on the bright stars.  I took more artistic license with this one


alan.dang@...
 

Thanks for the insight -- that clarifies that it must be the effect of the camera adapter then.  Could the corners also be causing the small diffraction spikes I saw around Iota Orionis?


Roland Christen
 


Could the corners also be causing the small diffraction spikes I saw around Iota Orionis?
Yes, for sure. Anything that disturbs the light path (cuts into the light path) will cause a diffraction spike.

Rolando


-----Original Message-----
From: alan.dang@...
To: main@ap-ug.groups.io
Sent: Wed, Apr 7, 2021 3:13 am
Subject: Re: [ap-ug] First light to me AP130 EDF 6. One optical question about optical vignetting / field illumination

Thanks for the insight -- that clarifies that it must be the effect of the camera adapter then.  Could the corners also be causing the small diffraction spikes I saw around Iota Orionis?