Hello everyone,
Now that I have the AG Optical FA12 (12.5", f/5, fl 1570mm), with a stated corrected field of 50mm, I am trying to make sure I am utilizing as much of that cone of light as possible. My imaging train currently is a little bit weird and not typical, simply because it is what I had right now to get it to image at all. Before I go on buying spree of a bunch of expensive adapters and focusers, I want to make sure they will materially improve the image, and that they are necessary.
Here is my imaging train:
Back of the OTA baffle is an Optec 3600 male dovetail on a tip/tilt plate > Optec Leo focuser (31mm thickness >Optec 3000 to 3.29" SCT threads adapter> Baader Diamond Steeltrack SCT focuser> M68 30mm extension> M68 ZWO tilt plate > OAG-L> ZWO 2" 7 position EFW>6200mm pro (The OAG/EFW/Camera are bolted together in one unit).
You might ask why I have two focusers. It is because the Optec Leo came with the OTA, and I don't have any Optec 3600 > 3.29" adapters yet, or any SCT>M68mm adapters yet. I curretly use the ZWO EAF to focuse. The Optec Leo focuser is just being used as an adapter here.
So after this introduction to the imaging train, here is my question:
I do notice a steep drop off in the light in the corners of my full frame 6200mm pro with the 50mm round filters. That drop off does calibrate out with flat frames however to produce an acceptable result.
The question is, is this what everyone sees when using an f/5 scope on the 6200mm or other full frame sensors and a 50mm round filter? Or am I being constrained by my 2" Baader Steeltrack focuser?
Here is an example of what my flat frame looks like with an autostretch. With my light frames I do see stars all the way to the corner so it is not zero illumination, just a significant drop off that calibrates well with flat frames.
Flat frame example with autostretch:
These are the only two examples of the two targets that I managed to image so far since I got this OTA last week (just about 4 hours for each target, so not much time) after full calibration and post processing. Note that I only cropped a minor amount for stacking artifacts (about 50-100 pixels in each axis). So this is pretty much near the full frame sensor size:
These are narrowband images. I did not test it with a broadband target just yet to see if the flat frames work well for those filters. I suspect it will be harder.
Do you think using a 3" focuser for the entire imaging train would improve the vignetting? Or what I am seeing is normal and expected with an f/5 system and this is the limit of a 50mm round filter on a full frame sensor?
Thanks.
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What makes you think that the vignetting is due to either the filter or focuser?
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andrea tasselli:
What makes you think that the vignetting is due to either the filter or focuser?
What else could it be?
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The secondary, for example. Nearly never the filter. Sometimes the focuser. As I don't have the mechanical layout of the system I can't tell you whether either the focuser or the secondary are properly sized for a full frame imager at a predefined % illumination at the corners (i.e., 22mm from the optical axis). Off the cuff I'd say the your vignetting looks pretty normal for such a system but this is no substitute from actual calculations. For one I'd find out what is your % illumination drop at the corner to see where you are, which is easily achieved from the flat.
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andrea tasselli:
The secondary, for example. Nearly never the filter. Sometimes the focuser. As I don't have the mechanical layout of the system I can't tell you whether either the focuser or the secondary are properly sized for a full frame imager at a predefined % illumination at the corners (i.e., 22mm from the optical axis). Off the cuff I'd say the your vignetting looks pretty normal for such a system but this is no substitute from actual calculations. For one I'd find out what is your % illumination drop at the corner to see where you are, which is easily achieved from the flat.
The secondary is quite massive. It is 7.1 inches in diameter (57% linear obstruction). Here are the specs from the manufacturer:
http://agoptical.com/convergent-fa12/
Optical Tube Assembly
- 43 lbs. carbon fiber tube assembly
- Tube length: 27.25 inches
- 12.5″ (.3m) low expansion substrate F5 optical system
- 50mm corrected imaged circle, 1.8 degrees
- Cooling fans and optional Thermal Control System
- Dust covers
- All CNC machined components, all stainless steel fasteners
Optical System
- 12.5″ F5 corrected optical system with integral field corrector
- 1570mm focal length
- 7.1″ secondary obstruction (57% linear obstruction)
- Performance: 5 micron RMS 15mm off-axis, 7 micron RMS 25mm off-axis
- 5.75″ of back focus from Optec Gemini focuser
- 96% enhanced aluminum coatings
- Two-element permanently-aligned field corrector with broadband anti-reflection coating on all surfaces (<.5% reflectivity from 400 to 700 nm)
- 3.95″ of back focus with Optec TCF-S3i focuser installed
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Had a quick look on the back of a similar system (RC) and you aren't going to get much of an improvement beyond a 2.5" focuser. There will be some vignetting with a 2" focuser. Since is an iDK is difficult to come at simple geometrical relationships but the ones I've used had similar light drop-offs that I see here.
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andrea tasselli:
Had a quick look on the back of a similar system (RC) and you aren't going to get much of an improvement beyond a 2.5" focuser. There will be some vignetting with a 2" focuser. Since is an iDK is difficult to come at simple geometrical relationships but the ones I've used had similar light drop-offs that I see here.
That's perfect and extremely useful. I will stop obsessing about it now. I am waiting on an adapter to try it with the 3" focuser and see if there is any improvement. Will report back.
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Take this with a very tiny grain of salt as I don't have any experience with the equipment you have, but a 2" focuser definitely seems too small for a full frame chip. I suspect your filter size is fine, and I bet the larger focuser will open it up significantly.
That being said, the vignetting calibrates out just fine - so who cares?  |
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Take this with a very tiny grain of salt as I don't have any experience with the equipment you have, but a 2" focuser definitely seems too small for a full frame chip. I suspect your filter size is fine, and I bet the larger focuser will open it up significantly.
That being said, the vignetting calibrates out just fine - so who cares?
Thanks Jay! I will try to test it with the 3inch focuser. I am concerned about the vignetting for two reasons. First is because I will be losing SNR in those corners, and I like to do mosaics a lot and that can be problematic. The second is it is significantly more prominent in broadband images. I have not had a chance to test either of those things yet.
Will see!
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FWIW: I image with a 6200MM on a 10" f/4 Newt and get negligible vignetting. 3" FeatherTouch focuser. ZWO OAG-L. 50mm unmounted filters.
Example: https://www.astrobin.com/6gbmul/ |
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Jon Main:
FWIW: I image with a 6200MM on a 10" f/4 Newt and get negligible vignetting. 3" FeatherTouch focuser. ZWO OAG-L. 50mm unmounted filters.
Example: https://www.astrobin.com/6gbmul/
Thanks for the feedback. Do you mind sharing a single autostretched flat frame ( or one raw luminance flat frame fit file). Would be really appreciated!
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Ashraf AbuSara:
Jon Main:
FWIW: I image with a 6200MM on a 10" f/4 Newt and get negligible vignetting. 3" FeatherTouch focuser. ZWO OAG-L. 50mm unmounted filters.
Example: https://www.astrobin.com/6gbmul/
Thanks for the feedback. Do you mind sharing a single autostretched flat frame ( or one raw luminance flat frame fit file). Would be really appreciated!
No problem! Single L flat FIT: https://1drv.ms/u/s!AgdoWVwj0fVaixaftsGribEABAVP?e=4lTvnd |
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Jon Main:
Ashraf AbuSara:
Jon Main:
FWIW: I image with a 6200MM on a 10" f/4 Newt and get negligible vignetting. 3" FeatherTouch focuser. ZWO OAG-L. 50mm unmounted filters.
Example: https://www.astrobin.com/6gbmul/
Thanks for the feedback. Do you mind sharing a single autostretched flat frame ( or one raw luminance flat frame fit file). Would be really appreciated!
No problem! Single L flat FIT: https://1drv.ms/u/s!AgdoWVwj0fVaixaftsGribEABAVP?e=4lTvnd
I will check it out today, thanks!
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Jon Main:
Ashraf AbuSara:
Jon Main:
FWIW: I image with a 6200MM on a 10" f/4 Newt and get negligible vignetting. 3" FeatherTouch focuser. ZWO OAG-L. 50mm unmounted filters.
Example: https://www.astrobin.com/6gbmul/
Thanks for the feedback. Do you mind sharing a single autostretched flat frame ( or one raw luminance flat frame fit file). Would be really appreciated!
No problem! Single L flat FIT: https://1drv.ms/u/s!AgdoWVwj0fVaixaftsGribEABAVP?e=4lTvnd
I've never seen an L flat with such little vignetting! LOL
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Jon Main:
Ashraf AbuSara:
Jon Main:
FWIW: I image with a 6200MM on a 10" f/4 Newt and get negligible vignetting. 3" FeatherTouch focuser. ZWO OAG-L. 50mm unmounted filters.
Example: https://www.astrobin.com/6gbmul/
Thanks for the feedback. Do you mind sharing a single autostretched flat frame ( or one raw luminance flat frame fit file). Would be really appreciated!
No problem! Single L flat FIT: https://1drv.ms/u/s!AgdoWVwj0fVaixaftsGribEABAVP?e=4lTvnd
I've never seen an L flat with such little vignetting! LOL
Right? I was pleasantly surprised. Maybe I'll do a build write up someday!
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Has to be vignetting due to the Baader Diamond Steeltrack SCT focuser. You essentially have restricted the light to a 2-inch opening over 80mm from the sensor. You can use the CCD Filter Size calculator at astronomy.tools to work the solution backwards. If you enter your equipment (assuming the IDK 12.5 is the same dimensions as your FA12) and then enter the distance from the Baader focuser to the sensor (estimating 85 mm), it says you would need a filter 58mm big at that distance...which is greater than 2 inches.
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George Chappel:
Has to be vignetting due to the Baader Diamond Steeltrack SCT focuser. You essentially have restricted the light to a 2-inch opening over 80mm from the sensor. You can use the CCD Filter Size calculator at astronomy.tools to work the solution backwards. If you enter your equipment (assuming the IDK 12.5 is the same dimensions as your FA12) and then enter the distance from the Baader focuser to the sensor (estimating 85 mm), it says you would need a filter 58mm big at that distance...which is greater than 2 inches.
Excellent idea! Never thought about using the filter tool to see how the light would be constrained a certain distance from the sensor. The distance in my imaging train is actually much farther than that too. I had to add two M68 adapter, in addition to the OAG/EFW between the focuser and the sensor.
I'll update when I get the adapters for the 3" focuser.
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An update to this topic. I changed the 2" focuser to a 3" focuser from telescope express. It made a huge difference. So the focuser is the culprit, confirmed.
3inch focuser.
2" inch focuser:
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With 50mm filters I put my hand on fire that you’ll not have problems with a full-frame sensor.
The vignetting can come from or the secondary mirror size, or the size of the chamber of your spacer rings our the focuser chamber size.
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Ashraf AbuSara:
An update to this topic. I changed the 2" focuser to a 3" focuser from telescope express. It made a huge difference. So the focuser is the culprit, confirmed.
3inch focuser.
2" inch focuser:
Looks great now!
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Observatório Astrográfico do Boqueirão:
With 50mm filters I put my hand on fire that you’ll not have problems with a full-frame sensor.
The vignetting can come from or the secondary mirror size, or the size of the chamber of your spacer rings our the focuser chamber size.
Yes it turned out to be the size of the focuser. The secondary mirror is a massive 7.1 inch occupying 57% of the aperture linearly. So don't think it is contributing to it at all. The smallest spacer / adapter is M68, and that is just between the OAG-L and the focuser.
Jon Main:
Ashraf AbuSara:
An update to this topic. I changed the 2" focuser to a 3" focuser from telescope express. It made a huge difference. So the focuser is the culprit, confirmed.
3inch focuser.
2" inch focuser:
Looks great now!
Thanks for the help!
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