C9.25HD Back Focus - the quest to 146.05mm

[Maideneer]

Just received my brand new scope and can't even use it yet!  The forgetful folks at Celestron forgot to pack my counterweight bar in the mount box so I'm waiting for them to send me the piece as I wait in (im)patience.  However, this does allow me the time to make sure I have everything I need for imaging.  Correct back focus distance for this model is 146.05mm and I need some assistance getting to that point, as I will be using my ZWO ASI224MC (non-cooled).

Assuming I pick up the T-Adapter which I don't have yet, that's 91.05mm.  Now then, the focus distance to sensor here is 12.5mm which means I need 42.5mm to reach the goal.

Here is what I can't figure out:
If you look at the attached diagram, is it telling me the 12.5mm distance is including the ring?  If it is, then I am then trying to figure out how best to get the needed 42.5mm through various extenders.  ZWO makes 21 and 16.5mm extenders but I don't believe make a 5mm one.

So assistance from the more experienced folks here is needed from my end. As always, thanks for your help!

[CraigT82]

Yes 12.5mm including the flange. You can buy any T2 extension tubes, you don't need to stick to ZWO (unless you really want to)

https://www.firstlightoptics.com/adapters/astro-essentials-t2-m42-extension-tube-set-4mm-5mm-6mm-7mm-8mm-9mm.html

[Maideneer]

34 minutes ago, CraigT82 said:

Yes 12.5mm including the flange. You can buy any T2 extension tubes, you don't need to stick to ZWO (unless you really want to)

https://www.firstlightoptics.com/adapters/astro-essentials-t2-m42-extension-tube-set-4mm-5mm-6mm-7mm-8mm-9mm.html

Ah, that's sort of what I was getting at.  It's just a piece of metal so I'm not a ZWO adherent in that sense lol...I just didn't know how to search for these things. Cheers sir!

[Maideneer]

I have a huge longshot favor to ask someone.  I found a 5mm spacer tube but there is a little twist with it where the female thread is 3.5mm and the corresponding male thread MUST be 3.5mm to match it otherwise it will not screw in fully and the total extension provided will be greater than 5mm.

If there is ANYONE out there with these ZWO 21mm and/or 16.5mm spacer tubes out there, could you possibly measure the male thread portion and tell me if either of those equal 3.5mm in length?  Cannot find any info here on the interwebs.

Edited July 30, 2021 by Maideneer

[jacko61]

I can measure mine in the morning but be aware that the zwo 16.5mm extension is M42 male - M48 female.  I don't think they do an M42 - M42.  This is causing me a headache this weekend trying to figure the 105mm backspacing on my C8 / meade reducer.

Graeme

 

[Maideneer]

37 minutes ago, jacko61 said:

I can measure mine in the morning but be aware that the zwo 16.5mm extension is M42 male - M48 female.  I don't think they do an M42 - M42.  This is causing me a headache this weekend trying to figure the 105mm backspacing on my C8 / meade reducer.

Graeme

 

The 16.5mm will be the last piece on the chain. The M42 portion fits to another spacer and the M48 portion will fit to the T-Adapter.

A little birdie told me the measurement was 4mm, hopefully you can confirm!

[jacko61]

Yep, on both ZWO extensions, the male thread is 4mm. Female end is 10mm deep.  On my SVBONY  SCT adaptor, the male M42 thread is 4.5mm deep but my SVBONY 10mm extension only has a 3.5mm deep female thread which is exactly the issue you're trying to avoid.  I'm currently shopping around for extensions with deeper female threads although I guess it would be easier to simply file the threaded end of the SCT adaptor back a mil or so. 

Graeme

[Maideneer]

Thanks so much!  Hey, one more thing...if I add a Barlow or ADC to my image train, does that increase the spacing or is that not taken into account?

Edited August 2, 2021 by Maideneer

[jacko61]

To be honest I have no idea. I've never used an ADC (although I could have done with one last night) and if I'm using a barlow I dispense with the reducer anyway as I'm only after close ups of the target.  As an example, last night I was hoping to image Saturn and Jupiter with my new ASI533MC pro so I removed the reducer and was viewing the planets through an 8.8 mm eyepiece and  barlow before hooking up the camera to the barlow with a 2" nosepiece.  I'm not sure (i.e. don't know  ) if the scope's focal plane is as critical when there's no reducer in place and In fact, that's probably a subject for a new thread which I might raise today. 

Graeme