Back Focus Help for new Gear

[Leti Theobald]

Hello all,

I just got some new gear and hoping someone can just double check my measurements to get the correct back focus.

• Scope: WO Zenithstar 73II
• WO Optics Adjustable Flattener for Z73
• ZWO Electronic Filter wheel  (20mm) - and it has a 5mm T2(M42)-M48 Adapter stuck to it which I can't remove
• ZWO Filters
• ZWO 2600MM

So, this is how I calculated which means I need to have the adjustment on the flattener at 5.2mm

 

[symmetal]

Yes, your calculation is correct. 🙂

However, you haven't allowed for the glass in the way between the FF and sensor which extends the required back-focus distance by around 1/3 the total thickness of the glass, due to the different refractive indices of air and glass. So if you have say 2mm thick filters and 1mm sensor protect glass, for a total of 3mm. you need to extend the back focus distance by 1mm which makes your FF setting 6.2mm rather than 5.2mm.

Manufacturers back focus distances are quoted for only air between the FF and the sensor, and it's up to the user to change this distance as necessary, if anything other than air is introduced between them.

Alan

 

[Leti Theobald]

That’s great, so my ZWO filters are 7mm, so I’ll add another 2.3mm increasing it to 7.5mm 

correct?

[symmetal]

Not quite. It's the thickness of the filters that matters which is 2mm for the Zwo narrowband filters. You've confused the 2mm optical thickness with the  optical bandpass width which is 7nm (nanometres). Note it's nm and not mm for the bandpass width. 1 nm is 1 millionth of a mm. 😉

The Zwo ASI2600MM has a 2mm thick protect window so your total glass in the way is 4mm. 1/3 of 4mm is 1.33mm. So your FF adjustment should be 5.2 + 1.33 which is about 6.5mm. You don't need to be supercritical in this setting. For small sensors it won't make any difference whether you use 5.2 or 6.5mm but the larger the sensor diameter the more necessary it becomes to get the spacing right. 🙂 

Alan

Edited July 3, 2023 by symmetal

[Leti Theobald]

6 hours ago, symmetal said:

Not quite. It's the thickness of the filters that matters which is 2mm for the Zwo narrowband filters. You've confused the 2mm optical thickness with the  optical bandpass width which is 7nm (nanometres). Note it's nm and not mm for the bandpass width. 1 nm is 1 millionth of a mm. 😉

The Zwo ASI2600MM has a 2mm thick protect window so your total glass in the way is 4mm. 1/3 of 4mm is 1.33mm. So your FF adjustment should be 5.2 + 1.33 which is about 6.5mm. You don't need to be supercritical in this setting. For small sensors it won't make any difference whether you use 5.2 or 6.5mm but the larger the sensor diameter the more necessary it becomes to get the spacing right. 🙂 

Alan

Thank you so much Alan, super helpful!

hopefully I can try tonight ☺️