How do you calculate your back focus?

[Corpze]

I have always wondered how you guys calculate or work out the right back focus distance in your setups.

In the beginning of my astrophotography journey, i just wen't with the standard 55mm which seems to be a standard measurement for more or less every OEM corrector/reducer.

In the later years, I have been more and more keen to use a excel spread sheet to roughly calculate the back focus necessary, and to see what kind of adapters I need to order.
But the two last setups, the calculated distance and "real", measured distance can vary by almost 1mm.

What is your experience  and practice when setting up a new camera train?
I recorded and put a video together of how I did last time.

/Daniel
 

 

[teoria_del_big_bang]

I just use the recommended BF from the flattener manufacturer, which as you say 55mm seems to be a common measurement, then add 1/3 thickness of my filters that are after the flattener (so if I have a LP filter in the train but before the flattener that is not included.

So for my setup that is 55.7 mm.
Then I put my spacers together and connect them to the filter wheel, and rotator if using one, and actually measure the total length with digital verniers as accurately as I can and use that distance as the actual distance not what the filter wheel and spacer manufacturers say they are. Then tune the distance in with different spacers until it is as close to the required distance as I can get with my spacers I have.

I then do some imaging and study the stars in the very corners to see if the distance is correct.

I try to have some of THESE thin spacers, or shims, in the set-up and start off with a 0.5mm in there so that if I need to shorten the BF to make stars correct I can take it out, or replace with a 0.3, or I can add spacers or go up to 1 mm until I get the stars perfect, usually being less than 0.3 mm out doesn't really make any noticeable distortion, to my eyes anyway.

So trial and error really I guess.

Very interesting video though and very interested in that optical rig 🙂 

Steve 

Edited August 27, 2021 by teoria_del_big_bang

[ollypenrice]

I was interested in your bench test for camera tilt. Do you have more information on how that works?

Olly

[davew]

1 hour ago, ollypenrice said:

I was interested in your bench test for camera tilt. Do you have more information on how that works?

Olly

Just in case he doesn't see the post,

Daniel has some other videos here that may well help - https://www.youtube.com/channel/UCm50WzpT1YKCwghW13b6k-g

Dave.

[Corpze]

2 hours ago, ollypenrice said:

I was interested in your bench test for camera tilt. Do you have more information on how that works?

Olly

That works really well, you can find the original article on Starlight xpress QA page.

5 hours ago, teoria_del_big_bang said:

I just use the recommended BF from the flattener manufacturer, which as you say 55mm seems to be a common measurement, then add 1/3 thickness of my filters that are after the flattener (so if I have a LP filter in the train but before the flattener that is not included.

So for my setup that is 55.7 mm.
Then I put my spacers together and connect them to the filter wheel, and rotator if using one, and actually measure the total length with digital verniers as accurately as I can and use that distance as the actual distance not what the filter wheel and spacer manufacturers say they are. Then tune the distance in with different spacers until it is as close to the required distance as I can get with my spacers I have.

I then do some imaging and study the stars in the very corners to see if the distance is correct.

I try to have some of THESE thin spacers, or shims, in the set-up and start off with a 0.5mm in there so that if I need to shorten the BF to make stars correct I can take it out, or replace with a 0.3, or I can add spacers or go up to 1 mm until I get the stars perfect, usually being less than 0.3 mm out doesn't really make any noticeable distortion, to my eyes anyway.

So trial and error really I guess.

Very interesting video though and very interested in that optical rig 🙂 

Steve 

I agree with everything - a lot of shims makes it easier to adjust, and the caliper is probably your best friend 😃

 

[ollypenrice]

1 hour ago, davew said:

Just in case he doesn't see the post,

Daniel has some other videos here that may well help - https://www.youtube.com/channel/UCm50WzpT1YKCwghW13b6k-g

Dave.

Thanks, that's perfect.

Olly

[Padraic M]

Not at the same level of astro quality I know, but I'm struggling with a C8 (non-EdgeHD) with the Celestron 0.63x FR. There's no/little/confusing information about what back focus is required, and from which point on the reducer is it measured.

Procedurally, if the back focus distance isn't known but the reduction factor is, am I safe to test at two or more different distances, platesolve to establish the achieved focal length at each distance, and calculate the back focus distance that gives the required reduced focal length? (in my case, 2032mm x 0.63 = 1280mm)

 

[newbie alert]

3 hours ago, Padraic M said:

Not at the same level of astro quality I know, but I'm struggling with a C8 (non-EdgeHD) with the Celestron 0.63x FR. There's no/little/confusing information about what back focus is required, and from which point on the reducer is it measured.

Procedurally, if the back focus distance isn't known but the reduction factor is, am I safe to test at two or more different distances, platesolve to establish the achieved focal length at each distance, and calculate the back focus distance that gives the required reduced focal length? (in my case, 2032mm x 0.63 = 1280mm)

 

With my xlt 8 inch I've altered the backfocus as my stars looked wrong, celestron say to measure to the threads of the reducer but the adapter doesn't thread fully on.. So I'm using this measurement to the flat of the reducer @106.67

I'm using astronomik filters that are 1mm thick, and plate solving shows my fl as 1317, not as 1280 as it's calculated fl