Anne-Maree McComb:
Your backfocus should be 55mm you have to much in the 32mm section B
Hello, I need help correcting the backfocus on my setup which includes a Sky-Watcher Esprit 100ED with a QHY268C. The picture below shows my current setup and my calculations and potential solution to get to the right backfocus. I would appreciate any help the community can provide as I am new to astrophotography and a lot of this is a little bit confusing to me.
Bronco Oostermeyer:
However, filters add length to your required backfocus, usually 1/3 of the thickness of the filter. So, in my conclusion would be that you would need a spacer ring of 10mm. Maybe buy something like https://www.ganymedes.nl/product/baader-m48-afstelringen-set/ to smooth out the last few 1/10’s of mm.
Bronco Oostermeyer:
However, filters add length to your required backfocus, usually 1/3 of the thickness of the filter. So, in my conclusion would be that you would need a spacer ring of 10mm. Maybe buy something like https://www.ganymedes.nl/product/baader-m48-afstelringen-set/ to smooth out the last few 1/10’s of mm.
Where u will put the spacer, any place in the image's train or just after the filter ?
Bronco Oostermeyer:Bronco Oostermeyer:
However, filters add length to your required backfocus, usually 1/3 of the thickness of the filter. So, in my conclusion would be that you would need a spacer ring of 10mm. Maybe buy something like https://www.ganymedes.nl/product/baader-m48-afstelringen-set/ to smooth out the last few 1/10’s of mm.
Where u will put the spacer, any place in the image's train or just after the filter ?
Standard practice is to keep your filters as close to the camera as possible, so that leaves section B in the photo of the OP.
Hello, I need help correcting the backfocus on my setup which includes a Sky-Watcher Esprit 100ED with a QHY268C. The picture below shows my current setup and my calculations and potential solution to get to the right backfocus. I would appreciate any help the community can provide as I am new to astrophotography and a lot of this is a little bit confusing to me.
=14pxply here ***
Thank you all for the help. I had not taken into account the filter so I really appreciate that point. That means the required backfocus is actually 63mm + 1/3 of the filter thickness (2mm), so 63.6mm, correct?
I'm going to order a M54 to M48 adapter (2mm) and a 10mm Spacer Ring plus some Baader fine adjustment spacer rings.
My new calculation is:
- Field Flattener attach ring: 8mm
- Blue Fireball M48x0.75 Thread Spacer Ring with 10mm Extension: 10mm
- ZWO M54 Male Thread to M48 Female Thread Low Profile Adapter: 2mm
- Baader Fine-Adjustment Aluminum Spacer Ring Set for M48 Threads: 0.1 mm
- ZWO Filter Drawer M54: 20 mm
- QHY268C backfocus (from edge of rotation ring to sensor plane): 23.5mm
- 8mm + 10mm + 2mm + 0.1 mm + 20mm + 23.5mm = 63.6 mm (should be 63mm + 1/3 of filter thickness = 63.6)
PS: I also realized I had an extra 2mm in my calculation. The picture is correct and equals 83.5 mm (not 85.5mm as in my original post).
Bronco Oostermeyer:Bronco Oostermeyer:
However, filters add length to your required backfocus, usually 1/3 of the thickness of the filter. So, in my conclusion would be that you would need a spacer ring of 10mm. Maybe buy something like https://www.ganymedes.nl/product/baader-m48-afstelringen-set/ to smooth out the last few 1/10’s of mm.
Where u will put the spacer, any place in the image's train or just after the filter ?
Standard practice is to keep your filters as close to the camera as possible, so that leaves section B in the photo of the OP.
Thx Bronco, but there is something I never understood, despite I'm not that picky with the focal length.
For example with a 2mm thick filter, you will need to add 1/3 = 0.6mm
But the focal plane is changed after the filter not before, so optically the 0.6 spacer should be just after the filter, in between the filter and the camera.
I'm right or there is something else I missed ?
