The Ultimate Back Focus Guide to Reach Zenith with a Focal Reducer on a SE, Evolution or CPC Mount

[curtismacc]

Focal reducers are an essential tool for EAA used to speed up the optics and get wider fields of view but reaching zenith with one on some Alt-Az mounts can be a challenge. The results from the limited clearance between the back of the OTA and the base of the mount on fork mounts like the Celestron Nexstar SE/Evolution and CPC mounts. Celestron's 6.3X reducer requires a back spacing of 105mm which makes it difficult for a camera to the clear the mount at this distance, especially a cooled camera. It can be done and there are posts on multiple forums showing some of the ways to do it but not always with a complete list of the "plumbing" needed. Recently I decided to take a comprehensive look at all of the possible options. With the help of folks on CN, many days of searching multiple vendor's inventory of adapters/connectors/extensions, copious purchases of the same and a lot of experimentation I finally came up with the parts for 4 different solutions to the problem which I want to share so that no one has to go through the discovery process again. Of course, I am certain that there can be variations on the solutions I came up with. I had 4 basic objectives with my approach:

1. Use the least number of parts
2. Minimize the cost
3. Provide additional parts options which may be more costly but are more versatile
4. Allow for the ability to vary the spacing easily to achieve more or less focal reduction

There are 2 basic approaches which are well know:

1. A straight through design with no extra optics in the light path. This typically requires a longer rail or rail extension and may not allow for use of a cooled camera in all cases.

2. A right angle design using a diagonal. This obviously adds glass to the light path and can reduce the clear aperture but will work with a cooled camera. Unfortunately most diagonals do not specify their optical path length and, are generally much too long to be useful.

If one wants to add a filter drawer the 2 approaches become 4. While you can find each of these approaches mentioned somewhere on line the exact components needed are not always specified and may not be the cheapest or simplest approach. I wanted to put everything together in a single place with all the detail necessary to replicate the solutions with links to all of the components I used for each approach along with some options in case one wants to try something a little different.

I also had to address the question of the correct back spacing for the Celestron 6.3X reducer and how to measure it. If you check on line you will find that the most common answer is 105mm but you will also find many quoting anywhere from 100 to 110mm. I make the case that the design back spacing is 105mm which I demonstrate how to achieve in each of the 4 configurations. The question of exactly where on the focal reducer to measure it from is the one where I see the most disagreement. Here too, I make the case that the measurement point is simple and, in retrospect, should be obvious.

After weeks of experimenting with various components which I bought expressly for this I put together a video with step by step instructions along with demonstrations that each approach does indeed work. You can find the video here https://www.youtube.com/watch?v=_oF5gBMiY0g If you watch the video you can tell from the change in clothes throughout the duration of the video that it took me a long time to get this right. As always, I welcome feedback both positive and not so positive. And would like to hear any difference in opinion as to the correct back spacing and point of origin of the measurement.

My hope is that this will save others a lot of time, frustration and money.

Regards,
Curtis

[admin]

Hi Curtis,

thanks for sharing this detailed write up and video. I remember hitting the very same issue back before I switched to the dark(er) side of having an EQ mount, so I have felt the pain!

cheers,

Robin

[turfpit]

Curtis

Thanks for the video detailing how to deal with this issue and also the comprehensive list of links in the text below the video. My first scope was a Celestron 6SE so I have faced the issues you discuss.

I hope acquiring all the parts for the solutions didn't bankrupt you!

Dave

[curtismacc]

Robin & Dave

Thanks for the positive feedback. Hopefully folks who are addressing the same challenge will find my video and not have to do all the legwork we went through.

Not bankrupt, but I think I spent close to $500 on the parts for this, only some of which I actually need for myself. The rest was just to show all the options. Fortunately I was able to return two items on Amazon that weren't actually used and I should get close to $200 back, hopefully.

On to the next one.

Regards,
Curtis

Edit: I should make it clear to anyone looking to do this that it won't cost $500. The simplest solution is the straight through design with a rail extension. In the video I show that this can be done for $107 assuming that you already have the focal reducer. You could spend more if you want one of the adapters that can be used to fine tune the spacing. The solution using a diagonal gives the most clearance and allows for a cooled camera but is more expensive, $267, driven mostly bu the cost of the diagonal.