Continued: (Field curvature with Flatteners and compressors)


Roland Christen
 

Field correction of the 140EDL refractor using the QuadTCC telecompressor:

In order to cover a wide field and compress the image to get faster focal ratios, we have the QTCC. This compressor was originally designed for our 130 GTX refractor for use with the 3.5" focuser. However, it has been requested for other scopes such as the TEC 140 to 180 triplets, and our own triplets of various sizes. The amount of back focus required depends on the scope's field curvature and some other factors. The faster the lens, the more back focus is required to get a flat field. So the question is what is the best back focus for my camera and my scope. The correct answer is - it depends....

For an example, below I have an analysis of the AP140 F7.5 refractor (which we made in limited quantities a long time ago). As the images below show, the optimum back focus changes as the size of the field changes. The reason for that is that the shape of the field curvature doubles back on itself, at first ever so slightly undercorrected and then as the limit is reached it goes quickly into overcorrection. Even a small change in the field size will produce significant deterioration of the corner stars. So the spacing becomes more and more critical as the chip size gets larger. Any tilt in the camera will cause one corner's stars to be perfect and the opposite corner to be quite out of focus with wonky star shapes.

Frame 1 shows the optimum spacing for a 42mm field. Frame 2 shows that with the same spacing the stars at the edge of a 51mm field go quickly out of focus. Frame 3 shows that decreasing the spacing by 1.25mm results in much better edge sharpness. For reference, the Airy Disc diameter at 140mmF5.39 is 7.2 microns.




--
Roland Christen
Astro-Physics


lmbrabec@...
 

Really cool information Roland!  Would you by chance have any data to share with TEC140FL with the QTCC?  I'm early on trying to fine tune my new TEC140FL with a QTCC and ZWO ASI294MC Pro camera to minimize star elongations in the corners of the image.  I'm curious how sensitive it might be to minor changes in the back focus versus the spec. of 80.8 mm +/- 1.0 mm (I also have the 18.3 mm spacer for use with the TEC140).  All the best, Scott


Roland Christen
 

The TEC 140 FL is the same as the ED model and would use the same spacer. For each 3mm filter and 3mm cover glass you need to add 1mm of back focus. If the camera company specifies the chip distance as the optical path, then the cover glass 1mm is not added. I am not familiar with your camera, but at least for the filters you need to add 1/3 of the filter thickness to the back focus calculations. For smaller chips the ideal back focus distance will be between 1/2 and 1mm further than for the largest full frame chips. So plan on adding 1 to 2mm and see what you get.

Rolando



-----Original Message-----
From: lmbrabec@...
To: main@ap-gto.groups.io
Sent: Tue, Dec 29, 2020 8:12 pm
Subject: Re: [ap-gto] Continued: (Field curvature with Flatteners and compressors)

Really cool information Roland!  Would you by chance have any data to share with TEC140FL with the QTCC?  I'm early on trying to fine tune my new TEC140FL with a QTCC and ZWO ASI294MC Pro camera to minimize star elongations in the corners of the image.  I'm curious how sensitive it might be to minor changes in the back focus versus the spec. of 80.8 mm +/- 1.0 mm (I also have the 18.3 mm spacer for use with the TEC140).  All the best, Scott

--
Roland Christen
Astro-Physics


lmbrabec@...
 

Thank you so much Rolando!  Your advise is much appreciated!  All the best, Scott
Oh by the way..... I love my Mach2 mount!


Terri Zittritsch
 

Hi Roland, I have the standard TEC140ED,  just like your own.   Can I assume the same as above?  I have both of your grenades, the reducer and flattener.   Can I assume same spacing for both?   In the past I think your spec said 99.1 for the tec140 so I think you’re now saying that it’s 98.55 for a full frame (42mm given 43.55 is a full frame).    Since I use the dedicated TEC 18.3 spacer along with an ordered precise parts spacer, I think I can order your 16.x mm spacer (for another scope) and then some 1mm rings to adjust.   Before I do this I want to try it out first to see where I sit.  But as you can understand, I don’t have a lot of 68mm tubes and spacers lying around like I do for T-ring size.   It could make this whole adjustment process a bit expensive.    
Also, you mention a 51mm image circle.   I have a full frame sensor which is around 43mm (widest point), so do I use the 42mm or 51mm set point?   My simplistic view is that I’d use the 42 since it’s only a 1/2mm per corner vignette.   But maybe I’m looking at this too simply and you’re telling us about the 51mm image circle because we need 51mm to get a fully illuminated 36 film frame.  I’m a bit unclear on the message here.   Sorry, a bit dense.

On the 92TCC, for the Stowaway, I see it talks about a 40mm image circle, which is a little smaller than full frame (43 and change).. which I’m not overly worried about unless I’m looking at this wrong.   Originally I wasn’t concerned but now wondering if I’m looking at this correctly and need to hear from you.

Sincere thanks for your and Marj’s support this year.  

Best wishes for you and your family in the new year..   Let’s hope for a better one!

Terri


Roland Christen
 

Try the spacer that comes with the reducer. It should work fine for a 42mm / 43.55mm diagonal dimension.
Spacing for the field flattener is less critical.

Roland



-----Original Message-----
From: Terri Zittritsch <theresamarie11@...>
To: main@ap-gto.groups.io
Sent: Thu, Dec 31, 2020 4:27 pm
Subject: Re: [ap-gto] Continued: (Field curvature with Flatteners and compressors)

Hi Roland, I have the standard TEC140ED,  just like your own.   Can I assume the same as above?  I have both of your grenades, the reducer and flattener.   Can I assume same spacing for both?   In the past I think your spec said 99.1 for the tec140 so I think you’re now saying that it’s 98.55 for a full frame (42mm given 43.55 is a full frame).    Since I use the dedicated TEC 18.3 spacer along with an ordered precise parts spacer, I think I can order your 16.x mm spacer (for another scope) and then some 1mm rings to adjust.   Before I do this I want to try it out first to see where I sit.  But as you can understand, I don’t have a lot of 68mm tubes and spacers lying around like I do for T-ring size.   It could make this whole adjustment process a bit expensive.    
Also, you mention a 51mm image circle.   I have a full frame sensor which is around 43mm (widest point), so do I use the 42mm or 51mm set point?   My simplistic view is that I’d use the 42 since it’s only a 1/2mm per corner vignette.   But maybe I’m looking at this too simply and you’re telling us about the 51mm image circle because we need 51mm to get a fully illuminated 36 film frame.  I’m a bit unclear on the message here.   Sorry, a bit dense.

On the 92TCC, for the Stowaway, I see it talks about a 40mm image circle, which is a little smaller than full frame (43 and change).. which I’m not overly worried about unless I’m looking at this wrong.   Originally I wasn’t concerned but now wondering if I’m looking at this correctly and need to hear from you.

Sincere thanks for your and Marj’s support this year.  

Best wishes for you and your family in the new year..   Let’s hope for a better one!

Terri

--
Roland Christen
Astro-Physics


Max Mirot
 

Roland,

Have you ever done a ray trace of the QuadTCC for the older AP 130 EDF F6 ( oil spaced) ?
Some of us would like to know if it is likely to give acceptable results at full frame.

Thanks 

Max


Dean Jacobsen
 

Excellent info.  I have a TEC140FL on order and have been planing on using the QTCC and either an APS-C or a 4/3' sized CMOS chip with it.
--
Dean Jacobsen
http://astrophoto.net/wp/
Image Gallery - http://astrophoto.net/wp/image-gallery/
Astrobin Image Gallery - https://www.astrobin.com/users/deanjacobsen/ 
Amateur Radio Call Sign - W6DBJ