6 replies to this topic

[DTO]

My current setup is

- Stellavue SV80-A with the SFFR-80 reducer .80 - Back focus flatterer to (edit cause I'm a tired ****) 55mm - https://www.stellarv...f-7-refractors/

- ZWO ASI678MC - back focus of 12.5mm gap from sensor to ring threads - https://www.zwoastro...t/zwo-asi678mc/

 

 

From the above I have a reducer with a stated backfocus of 55mm and a camera with a focus gap of 12.5mm.   So from this I assume I need a spacer of 42.5mm.  I have a Baader variable spacer that I have set up to 42.5. 

 

The seeing here lately has been crap but I've been trying to dial in the focus, tracking etc. for my rig with the moon.   Caught it a couple nights ago and partially last night in not ideal conditions so I'm not sure if my lack of sharpness is because of my backfocus attempts or just the conditions I've been having. 

 

I put the question of this setup out over a year ago but want to confirm if this is correct.   I find it terribly odd that there are so many cameras with a backfocus of 12.5 mm but no specifically built 42.5 spacers.   Finding out about the variable I have took someone here knowing this piece of kit existed.

 

so,

- is my determination of 42.5 backfocus spacer correct or am I missing something (No filters to adjust for in this case...will add filters in time)

- how do I test for and nail the backfocus (maybe my spacer is off by fractions of a mm)

Thanks

Edited by DTO, 14 March 2025 - 11:30 AM.

[Borodog]

Uh . . . 50 - 12.5 = 37.5.

[DTO]

Uh . . . 50 - 12.5 = 37.5.

yep....been one of those.  up late with the eclipse.  thank you

 

50 should have been 55.    or the distance a DSLR has with a T-Ring adapter.   so the  42.5 was right.   right answer, failed at showing my work

Edited by DTO, 14 March 2025 - 11:33 AM.

[Borodog]

In that case, yes, all you need is 42.5mm of spacer. Unless you use a filter drawer  or wheel, the filter usually screws to the front side of the reducer/flattener, so it has no effect on the back focus.

[DTO]

In that case, yes, all you need is 42.5mm of spacer. Unless you use a filter drawer  or wheel, the filter usually screws to the front side of the reducer/flattener, so it has no effect on the back focus.

so strange that no one makes one.   can get spacers in 1, 2, 3, 5, 10, 15, 20 etc.  but I've got it solved for now.  

 

do you know any tips/tricks to test and know that the back focus is as tight as it can be?  with the variable spacer I can go anywhere from 30 to 46.  I went down to 40 and that was obviously wrong, could not get focus.   would using a bhatinov be of any use?

[Borodog]

so strange that no one makes one.   can get spacers in 1, 2, 3, 5, 10, 15, 20 etc.  but I've got it solved for now.  

 

do you know any tips/tricks to test and know that the back focus is as tight as it can be?  with the variable spacer I can go anywhere from 30 to 46.  I went down to 40 and that was obviously wrong, could not get focus.   would using a bhatinov be of any use?

Image a star field and then look at the corners.

 

 

 

The 678 is a relatively small sensor. The smaller the sensor, the less sensitive to the back focus. I don't think you'll have much trouble.

[DTO]

Image a star field and then look at the corners.

 

 

 

The 678 is a relatively small sensor. The smaller the sensor, the less sensitive to the back focus. I don't think you'll have much trouble.

brilliant.  thank you