10 replies to this topic

[aabusara]

Hello everyone. I wanted to ask the cerebral / technical folks over here on how to best approach this issue. 

 

I recently aquired an AG Optical Convergent FA12 and I am trying to figure out the best way to build this imaging train to:

 

1. Be able to use with a ZWO EAF. 

2. Utilize the current system.

 

If you are not familiar with this OTA, it is now discontinued. It is harmer wynne cassegrain with an aperature of 12.5", f/5 (FL 1570mm). The backside of the telescope has a Male Optec 3600 dovetail. My OTA came with a perinstalled Optec Leo Focuser , Optec 3000 50mm extension and Optec 3000 variable Drawtube Adapter  / Extender.

 

I have the 6200mm pro, bolted to a 7 position 2" ZWO EFW, which in turn is bolted to an OAG-L with an M68 tilt adapter (female threads) (consumes all together 55mm of backfocus). The backfocus for the OTA is 213mm from the backplate. 

 

I attempted to image with a Baader Diamond Steeltrack SC 2" focuser that I already have and it suffered from severe circular vignetting. The focuser telescope side attached 65mm from the backplate. So at this point the cone of light was constrained to a clear diameter of 55.4mm. Between the backplate and the Focuser was the Optec 3000 Drawtube adapter that had a clear diameter of 67.3mm. 

 

Is there a way to exactly calculate the cone of light diamter needed to illuminate 50mm image circle on the FF sensor at varios distances from the back plate? 

 

Do you think a 3" Rack and pinion focuser like the one offered by Telescope express with a clear diameter of 84mm would be sufficient to get rid of any light cone constraintes? 

 

I am also going to attempt using the Optec leo but I am ashamed to say I am still not proficient with using a mini PC and I am working on it. I am highly addicted to the ASIair. I do not like how bulky this focuser is as it needs another control box, and the travel is really limited. 

 

Thanks!

Edited by aabusara, 29 December 2023 - 05:03 PM.

[aabusara]

Here is a sample flat from my current setup with the 2" focuser.  I am still not 100% it is the focuser, not just the constraints of a 50mm filter. Is this an expected flat with an f/5 system, with 6200mm pro and an EFW that is bolted with 50mm unmounted circular filters?

 

This is an autostretched flat from the Ha filter in PI. 

 

Edited by aabusara, 30 December 2023 - 03:49 PM.

[Broz22]

I  have a CDK 14 with an Optec Gemini focuser. When I use the Planewave .66 reducer I get similar vignetting as what you show above. It pretty much calibrates out, except for a small amount in the corners of the final light stack that I usually have to crop out. Have you tried just processing all the way through calibration and stacking to see how it looks? And running the focuser through NINA works really well. Bit of a learning curve, but very capable and flexible control and acquisition program. Patriot Astro's vids are great to get things up and running.

John

[aabusara]

I  have a CDK 14 with an Optec Gemini focuser. When I use the Planewave .66 reducer I get similar vignetting as what you show above. It pretty much calibrates out, except for a small amount in the corners of the final light stack that I usually have to crop out. Have you tried just processing all the way through calibration and stacking to see how it looks? And running the focuser through NINA works really well. Bit of a learning curve, but very capable and flexible control and acquisition program. Patriot Astro's vids are great to get things up and running.

John

 

I think you are right. I was surprised by how well it calibrated out with flats, darks and bias frames.Here is my first fully processed result from 4 hours of integration. Do you think the light dropoff is just what one should expect from a 50mm unmounted filter on a FF? 

 

[Broz22]

It looks that similar to my photos (with the reducer), you might want to crop a bit to fix the corners.

John

[AstroPaolo]

in FF for a real no vignetting you should use a 50mm square filters and a 3" focuser

 

here you can calc the filter size for your configuration

https://astronomy.to...ccd_filter_size

 

consider that even if you use a nomount filter you don't use the hole space, atleast 2mm are used by the wheel borders so let say you have a 48mm of aperture

Edited by AstroPaolo, 07 January 2024 - 06:43 PM.

[Dean J.]

I think you are right. I was surprised by how well it calibrated out with flats, darks and bias frames.Here is my first fully processed result from 4 hours of integration. Do you think the light dropoff is just what one should expect from a 50mm unmounted filter on a FF? 

 

If you can correct the illumination with the flat frame, which appears to be the case with your setup, then I don't see any problem with going forward as is.  I used to have a full frame camera with a built-in filter wheel - the old SBIG STL-11000 - and it worked fine with the 50mm unmounted filters.  If you are happy with your calibrated result, then why throw money at it?
 

[aabusara]

in FF for a real no vignetting you should use a 50mm square filters and a 3" focuser

 

here you can calc the filter size for your configuration

https://astronomy.to...ccd_filter_size

 

consider that even if you use a nomount filter you don't use the hole space, atleast 2mm are used by the wheel borders so let say you have a 48mm of aperture

50mm square filters are probably not an option right now. But I can work on getting a 3" focuser.

 

 

If you can correct the illumination with the flat frame, which appears to be the case with your setup, then I don't see any problem with going forward as is.  I used to have a full frame camera with a built-in filter wheel - the old SBIG STL-11000 - and it worked fine with the 50mm unmounted filters.  If you are happy with your calibrated result, then why throw money at it?
 

 

I suppose I am loosing significant signal in these corners? I like to stitch Mosaics a lot, and if the corners have drastically less SNR than the center of the image, I'd imaging it will affect the outcome. At least that's my thinking. 

[aabusara]

Just an update to this. I changed my 2" focuser to a 3" focuser from Telescope express, and the Vignetting is significantly improved.

 

3 inch focuser (when stretched to the same level as the 2" focuser autostretch):

 

 

2 inch focuser:

Edited by aabusara, 27 January 2024 - 09:26 AM.

[Spaceman 56]

thats quite a difference Aabusara.

 

very noticeable. well done.

[aabusara]

thats quite a difference Aabusara.

 

very noticeable. well done.

 

Thanks! Yea the process of getting flats is much more pleasent now.