Attaching ADC to image train

[Maideneer]

I suddenly found myself in an equipment predicament that I might not have thought through and now I can't figure it out.

For my C9.25HD, I successfully put together my needed146.05mm back-focus with the following:
1) T-Adapter - 91.05mm
2) Sensor to flange = 12mm
3) 5mm spacer ring
4) 16.5mm spacer ring
5) 21mm spacer ring

That equals 145.55mm, which is only .5mm off from 146.05 so that works just fine, nothing noticeable. Now the problem I didn't account for is attaching my ADC.  The spacer rings don't have thumbscrews and the T-Adapter doesn't either and is larger than 1.25" in size...so I have nowhere to put this thing.  On one side of the ADC, I can thread the camera onto the barrel end but the nosepiece side is a different story.

I'm thinking I now have to re-think my whole train because I can't think of an easy fix.  The 21mm ZWO Filter Drawer *looks* like it has a thumbscrew but that's just a little pull tab to open the actual drawer, so probably a no-go.

Really could use some fresh thinking here since I'm kind of stuck.

[CraigT82]

Don't get too hung up on the 146.05mm spacing. That is just the point behind the scope where the spherical aberration is fully corrected. If your sensor isn't exactly there the image isn't going to suddenly start to break down. You will have a bit of room to play with - not sure exactly how much as I've never imaged with an SCT.

If you have the ZWO ADC the nosepiece should unscre revealing a T2 thread, so you can thread it in-between the 5mm spacer and the camera (gender changers may be required) and it will act like a 37mm ish spacer (or whatever the ADC body length is) so you can get rid of the 21mm and 16.5mm spacers.

Your barlow ideally wants to go in front of the ADC if you're using one. But as the scope is f/10 anyway it shouldn't be too much of an issue if you can't get it in front.

[Maideneer]

1 hour ago, CraigT82 said:

Don't get too hung up on the 146.05mm spacing. That is just the point behind the scope where the spherical aberration is fully corrected. If your sensor isn't exactly there the image isn't going to suddenly start to break down. You will have a bit of room to play with - not sure exactly how much as I've never imaged with an SCT.

If you have the ZWO ADC the nosepiece should unscre revealing a T2 thread, so you can thread it in-between the 5mm spacer and the camera (gender changers may be required) and it will act like a 37mm ish spacer (or whatever the ADC body length is) so you can get rid of the 21mm and 16.5mm spacers.

Your barlow ideally wants to go in front of the ADC if you're using one. But as the scope is f/10 anyway it shouldn't be too much of an issue if you can't get it in front.

Am I really that thick that I didn't try unscrewing the nosepiece lol?  Looking at the specs, the nosepiece accounts for 27.5mm, so if I remove that then I'm left with 55.9mm of ADC.  Combined with T-Adapter and flange distance, that's 158.95 or about 13mm over recommended (and that's with ditching all the spacers).  Do you think THAT much difference will make a change or is it gonna be like a trial and error thing where I just have to get out there and give it a shot to know?

Totally forgot about barlows...that's gonna complicate things further isn't it?  I haven't even attempted it yet.

Here is a recent one I just did of Saturn. No barlow, no ADC, and I think it turned out alright.  Anyway, thanks for the insight, that REALLY helps.

 

Edited August 23, 2021 by Maideneer

[Cosmic Geoff]

4 hours ago, CraigT82 said:

If you have the ZWO ADC the nosepiece should unscre revealing a T2 thread,

I never knew that. 🙂.  Actually both sides of the ZWO ADC can be unscrewed, one revealing a male T2 thread, the other revealing a female T2 thread, the central active part bring quite short. (Useful to know if you have to clean dust out of it.)

For what it's worth, here is the configuration I use for imaging Jupiter & Saturn with a C8 SCT:

Standard visual back, standard 1.25" prism diagonal, ZWO ADC with 1.25" nosepiece, bubble level of ADC facing rearwards, ASI224MC camera with 1.25" nosepiece.   Using the diagonal is mainly for convenience and I can un-flip the image in processing.

If I have to use a flip mirror diagonal, the nosepiece of the ASI224MC unscrews to reveal a T2 thread which mates with a T2 thread on the back of my flip mirror diagonal.  (I didn't know these were T2 threads - confirmed it just now with a T2 extender tube).

[Maideneer]

7 hours ago, CraigT82 said:

Don't get too hung up on the 146.05mm spacing. That is just the point behind the scope where the spherical aberration is fully corrected. If your sensor isn't exactly there the image isn't going to suddenly start to break down. You will have a bit of room to play with - not sure exactly how much as I've never imaged with an SCT.

If you have the ZWO ADC the nosepiece should unscre revealing a T2 thread, so you can thread it in-between the 5mm spacer and the camera (gender changers may be required) and it will act like a 37mm ish spacer (or whatever the ADC body length is) so you can get rid of the 21mm and 16.5mm spacers.

Your barlow ideally wants to go in front of the ADC if you're using one. But as the scope is f/10 anyway it shouldn't be too much of an issue if you can't get it in front.

It turns out I need one of those M48 to M42 adapters. I played with every conceivable piece I own and that was the missing link, so I ordered it.  So now I'll have a train setup with and without an ADC. I won't worry about Barlow setup yet but I imagine that's another hurdle but confident I can solve it now thanks to you.

Since I was shopping I also picked up an auto-focus motor lol...man I really fell in the rabbit hole here!

[CraigT82]

Nice one glad it works... the motor focusers are great bits of kit, and really make life easier. 

[Cosmic Geoff]

20 hours ago, CraigT82 said:

Don't get too hung up on the 146.05mm spacing. That is just the point behind the scope where the spherical aberration is fully corrected. If your sensor isn't exactly there the image isn't going to suddenly start to break down. You will have a bit of room to play with - not sure exactly how much as I've never imaged with an SCT.

I have never worried about this issue at all. It may be relevant if you are doing full frame imaging with a DSLR etc but with a tiny planetary image, I really don't think so.

[CraigT82]

1 hour ago, Cosmic Geoff said:

I have never worried about this issue at all. It may be relevant if you are doing full frame imaging with a DSLR etc but with a tiny planetary image, I really don't think so.

Yes you're right, I think the specified 146.05mm is where the edge HD lens correction of the field flatness is optimal and so it's not overly important for on-axis planetary imaging.

However I could be wrong but I think there is also a back focus distance where the spherical correction of the corrector plate is optimal, and you'd think they would design that to be at the same point. Deviating from that may introduce spherical aberration which effects the whole field?