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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.  |
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Your backfocus should be 55mm you have to much in the 32mm section B
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Hi .. the correct back focus from the back of the flattener to the sensor is 63mm plus 1/3 of the thickness of whatever filter in the filter draw.. this may need fine tuning with thin spacers.. Currently you have it about 20mm too long. HTH Dave |
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Anne-Maree McComb: The 55mm that Skywatcher quote is from the rear of the 8 mm adapter.. not the rear of the flattener ie 63 minus 8 = 55 .. |
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Well, the backfocus should be 63mm from the back of the flattener to the sensor. Part A is pretty much obligatory, so from there on it’s pretty much the standard 55mm. Assuming you got the measurement of D correct and C is directly attached to it there’s 11,5mm left for B. Now there’s seems to be a ring, which convert whatever size the filterdrawer is to M48, this add 2mm. So we’re down to 9,5mm for B. 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. What I do for myself is make a drawing of what I think it should be and a caliper for measurements. hope this make sense. |
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Hello! I have the same camera and telescope as you. You need to replace the 32 mm adapter with an 11.5 mm long adapter. To the value of 11.5 mm, you need to add 1/3 of the filter's thickness. Since I use Astronomik filters, which are 1 mm thick, I must add 0.3 mm to the 11.5 mm value (i.e. the total length is 11.8 mm). Best, M.
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Remove the 32mm spacer and put in the correct one to make 55mm from the back of the 8mm adapter to the sensor… so looks like you need a 12mm spacer, plus approx 0.5mm to allow for the filter, so 12.5mm in the place of the 32mm one you have there… |
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Hello! As they say, it is necessary to take into account the thickness of the filters. I use two Artesky spacer rings of 0.5 and 0.15 mm to compensate the 1/3 of 2 mm of the filters with very good results. Total BF = 55.65mm Clear Skies! ✨🔭 |
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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. hi just reduce the section B and don't forget to add back focus of the filter CLEAR SKIES Brian |
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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:
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). |
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Bronco Oostermeyer: Where u will put the spacer, any place in the image's train or just after the filter ? |
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Bronco Oostermeyer: 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. |
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Bronco Oostermeyer:Bronco Oostermeyer: 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 ? |
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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. |
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If you look at the Skywatcher 100mm manual, you will see it does NOT include the 8mm adapter so the distance is 63mm.
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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? Brian, the Baader fine adjustment spacers only come in 0.3mm, 0.5mm and 1mm. However, without it you’ll be at 63.5mm which may be close enough. Personally I like having a bit more wiggle room +/- 1 or 2mm in case the stated optimal back focus is not exact in practice. |
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The sensitivity of the BS depends a bit on aperture (f/5.5 is not that demanding) and the size of the sensor. OP, which is a bit quiet btw, doesn’t state what camera he’s using. A 533 would a lot more forgiving than a 6200 (to name something).
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Its an 268c ie APSc format
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Bronco Oostermeyer:Bronco Oostermeyer: It doesn’t matter where you put the spacer as long as it’s between the sensor and the flattener, as it’s the distance from sensor to flattener that changes with filters added, so even though the backspace is not increased until after the filter it’s the overall distance from flattener to the sensor that matters and needs adjusting… |
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I just returned from travel for work and had a few packages waiting for me. I've adjusted the backfocus for my setup and now just need some clear skies to be able to verify backfocus. Thanks all for your help so far, I really appreciate the advice and guidance.  |
