Back focus

[Rustang]

In my research into a new scope and what I may need, I've come across something else I need confirmation on my understanding of. You know me and this hobby, this seems to be the case with most of it! 😄

So if a scope has 170mm of back focus, the cameras sensor needs to be 170mm from the back of the scope to obtain focus, correct? So where I'm getting confused is if then the field flattener has a back focus of 123mm where does that then sit in regards to the back of the scope/ camera sensor?

The sensor in my camera (QHY-9) I believe is 15.5mm from the front of its casing, then there's 22mm of filterwheel so I'm already 37.5 at the casing of the filter wheel, would I then need spacers to make sure the field flattener's glass is 85.5mm away from the camera sensor then ? 

Also I have read you need to take into account filters when calculating back focus? Can you confirm what the calculation is? I believe my Baader 1.25 NB filters are 2mm thick.

 

This will also help with my current setup as it appears with the stars direction of travel going 'around' the edges that my sensor is to far from the reducer.

[tooth_dr]

@Rustang what scope are you using? I haven’t come across a scope with a specified back focus distance unless it has an integrated flattener.  In which case you don’t need the other flattener.

You are correct that the camera is 15.5mm plus 22mm for the filterwheel.  The filters thickness is multiplied by 1/3 to get the additional back focus ie .7mm in your case.

123 - 15.5 - 22 - 0.7mm = spacers needed.

Does that help?

[Rustang]

1 hour ago, tooth_dr said:

@Rustang what scope are you using? I haven’t come across a scope with a specified back focus distance unless it has an integrated flattener.  In which case you don’t need the other flattener.

You are correct that the camera is 15.5mm plus 22mm for the filterwheel.  The filters thickness is multiplied by 1/3 to get the additional back focus ie .7mm in your case.

123 - 15.5 - 22 - 0.7mm = spacers needed.

Does that help?

Thanks Adam, yeah that helps, just need to see what I'm missing/not understand in regards to the scope back focus then. I'm still using the SW 80 ED which I had posted a thread about in regards to the stars in the corners, they seem to be traveling around the outside of the image and this could well be as I've seen on the web, the sensor being to far from the reducer.

How to Set the Back Focus for Your Telescope (Guide) (optcorp.com)

In regards to the scope I'm considering getting is the TS optics photo line 80, its 2ich flattener (TS Flat 2) is stated to have a back focus of 123mm for the 480 focal length and the scope has a stated back focus of 170mm from the thread on the back of the scope. 

Teleskop-Express: TS-Optics PHOTOLINE 80mm f/6 FPL53 Triplet APO - 2.5" RAP Focuser

Edited August 10, 2021 by Rustang

[tooth_dr]

1 hour ago, Rustang said:

the scope has a stated back focus of 170mm from the thread on the back of the scope

I've not really noticed this statistic on a scope before, not sure what it means.

[symmetal]

1 minute ago, tooth_dr said:

I've not really noticed this statistic on a scope before, not sure what it means.

It could just be the distance an object at infinity would be brought to focus without any correctors etc. in the way. The corrector back focus spacing is the critical distance to maintain, the scope back focus would then be modified by the corrector anyway, with the focuser itself correcting for the modified scope back focus. As long as you can focus the object then the scope back focus distance is met by default.

Alan

[CraigT82]

I think it just means that the objective forms it's image (the focal point) at a spot which is 170mm away from the end of the thread on the fully racked in focuser drawtube, or 135mm away from the 2" adaptor.

This seems about right as if you stick a 2" diagonal in there for visual use that would have an optical path about 100mm maybe so you would need to rack the focuser out by 35mm to reach focus.

Obviously when imaging the camera sensor would need to be placed at this point, which is easily done by using the focuser! 

The TSAflat2 would go in the focusers 2" adaptor and then the camera would need to be placed 123mm from the back of the flattener (with filter wheel etc in this 123mm space).... and then the whole lot racked out by about 10mm or so to get the sensor to the focus point 

 

 

[Rustang]

Thanks guys, I think your right, it just got confusing reading that the scope had back focus but I think you have explained what it probably means. Vlad had pretty much explained and calculated the above to me but I've only got myself to blame for re confusing it so thanks for all the confirmation 👍

Edited August 10, 2021 by Rustang