The Celestron Reducer backfocus debate

[pmrid (Peter)]

I’m in the process of re-purposing an older Celestron C11 SCT. One part of that is the addition of a Celestron F6.3 reducer. But I find Celestron have not been forthcoming with any info on the optimal backfocus requirement of this item.

The Description on Bintel’s site claims a 24mm image circle and 85mm backfocus. And yet, a bit of scouring Cloudy Nights and the like more often than not comes up with a figure of 105mm.

Some of the discussions claim that at 105mm, there is serious distortion of stars on the outer making OAGs unworkable.

I gather this has nothing to do with the ccd/camera being used but is entirely a function of the reducer.

Has anyone come up with anything definitive on this question. Celestron themselves offer no assistance.

[bojan]

Peter,

I (with the help from JA and Damien) did quite a lot of investigation on both Celestron and Meade FR, see below especially post #32):
https://www.iceinspace.com.au/forum/...d.php?t=173640

[Dave882 (David)]

I've used the celestron 0.63x on a number of larger scts, c8, m10, c14 and have generally aimed at 105mm (but +/- a mm or2 depending on the scope). Stars are decent across a 4/3" sensor (asi294), and just need to do a bit of a crop on an APS-C sensor (asi2600). Its going back a while but I think I remember trying 145ish mm as was recommended by a few people and got quite bad results. 85mm is not possible for me to fit an OAG and filter drawer so never tried it. The corrected image circle produced by the reducer is indeed about 24mm, but there is still a bit of useable image circle outside of this IMO.

Regarding using the OAG - this is when things get interesting. The illumination of an OAG is right on the outer edge of this circle. What I found was this means its almost impossible to get full illumination of the OAG prism, and for some reason I found that this gets worse with the larger aperture scts. I had to push the prism just inside the cameras imaging circle (just enough so that flats would fix it ok), and even then I still had crazy comet shaped stars to guide with, and only on approx 1/2 of the guide sensor used. Now, you can still guide on a comet-shaped star and this is pretty much how I did all my guided shots, but the real problem was that with such a small percentage of the guide sensor illuminated, there were so many targets I just gave up on due to inability to find a guide star.

I recently got the Starizona SCT corrector -L for the c14, and this has all changed. I'm not convinced that the optics are any better than Celestron's reducer, but I get full illumination of the guide sensor even when using the asi2600 and much better stars to guide with. Plus lots more backspacing to work with.

[pmrid (Peter)]

Thanks a lot guys. The OAG issue is one I was concerned about in particular. I have found an old Celestron OAG that I can use and achieve 85mm BF including a ZWO filter wheel and ASI1600MM. And i have an adapter I can throw in that will give me 104mm so I’ll give both a run as soon as the weather clears.

[pmrid (Peter)]

Well, interesting development. Following the”85mm backfocus” school of thought, I managed to get ASiair to do a polar alignment routine at a focal length of 1784 meaning the effectiveness of the Celestron F6.3 reducer was pretty bang on at that FL. The scope is a C11.

On the down side, I found the OAG was pretty useless. Even binned x2 the ASI120MM could barely detect a star at 5 second exposures. So I’ll dump the OAG and go back to a piggyback guide scope. I have an 80mm Lumicon guidescope with a FL of 300mm. I’m hoping that’s going to be enough for guiding at the FL of 1784. Fingers crossed and tomorrow night will be the moment of truth.

[evltoy (Wayne)]

Hey Peter

My below comments are only from my personal experience with an 8" SCT, but my help.

I too had almost the same problem with stars until I upgraded from the asi120mm to the asi290mm. I have the older celestron radial OAG and after going to the 290mm its been awesome at 1280mm FL, yet to try 2032mm. I do also hear the zwo OAG is not well suited for the SCT's.

Backfocus! Gees what a drama I went through. All I can say is if I had known about plate-solving and had my ASIair back then, I wouldn't have spent a painful 12 months wasting good nights and buying all these rings etc. All the numbers you hear out there is "ball-park" start with 105mm, plate-solve (focus first) and that will tell you your FL. The closer you are to your spec'd FL you know your back focus is close to correct. I only just work this out when my ASIair told me my FL was 1276mm after a plate-solve and my physical setup was 1280mm. This is with a 105mm +/- 1mm back focus.

Also, a 300mm FL guide scope will not cut it. You are going to get flexture when you main scope is 1764mm and beyond. I was using a 280mm 60mm Zwo guider that could never give me round stars due to flexture. I moved to a ST80 (400mm) that helped heaps at a main FL of 1280mm, but still has flexture until going OAG. Also note my guiding was well under .5 at FL of 280mm & 400mm.

[pigeond (Howell)]

I got a used C6 recently so I thought I'll share my (limited) experience too with the 0.63 reducer/corrector.

I was trying with the common 105mm backfocus at first and it is "ok" for the center but definitely far from perfect. CCDInsepector suggested 60% curvature.

I then stumbled across the 85mm as listed on Bintel's so I tried but it was definitely not it and looked pretty bad.

I'm using OAG too and it can very tricky with this setup due to how narrow the light path is on the C6, as opposed to C8 and up, as noted by many on the CN forums.

Another thing I noticed is when I was doing 105mm with filter drawer and the Baader Neodymium Moon & Skyglow Filter, I get much worse image, with or without adding the 0.3mm according to the +filter thickness / 3 rule. This is something that I still haven't figured out yet.

BTW these are all done with an ASI294MC Pro.

[evltoy (Wayne)]

Originally Posted by pigeond

I then stumbled across the 85mm as listed on Bintel's so I tried but it was definitely not it and looked pretty bad.

Bintel should remove this as it is very wrong. The Celestron 93633-A was designed to be used with or without the reducer for a DSLR (55mm back focus) setup. The Celestron 93633-A is about 50mm. There is a reason for this length especially when using the reducer with a DSLR.... 105mm

Once I used the 105mm rule with my 294MC everything worked perfectly.

Originally Posted by pigeond

Another thing I noticed is when I was doing 105mm with filter drawer and the Baader Neodymium Moon & Skyglow Filter, I get much worse image, with or without adding the 0.3mm according to the +filter thickness / 3 rule. This is something that I still haven't figured out yet.

BTW these are all done with an ASI294MC Pro.

You are correct to factor the thickness of the filter. I never have and I haven't seen much negative impact by using my M&S Glow or my ZWO duo.

Just to note. you will never get perfect round stars to the edges of a non-hd SCT. Even a EdgeHD will only give you "better" but not perfect round stars on the edges. and the Celestron .63 reducer is not that good quality either.