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Focus tube too short for ASI533MC - extension needed?


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Posted (edited)

Hi all,

TL:DR - HOW CAN I GET 5-10MM MORE OUTWARD FOCUS? 

So, first night with my new ASI533MC Pro, and I have a problem.

I have a 130PDS, and I chopped the focuser tube so that my stars were nice and round. However, what worked perfectly with my previous camera, does not now work with this one. 

When I focus, the camera is moving out, and I get closer... then closer... then I'm just about there, and then the focuser tube pops out.  So it's close, very close, but no cigar. So I need the camera to go out a few mm more to achieve focus. The backfocus distance for the coma corrector is 55mm, which is why I need those spacers before it. 

Among my many mysterious bits and bobs lying around, I found an extender that looked like it might work, but it's too long.

So, this is too short (focus tube adapter then coma corrector then 55mm back focus):

short.jpg.b312a90c2a43597d52fd039992960233.jpg

And this is too long (focus tube adapter, then extender tube, then coma corrector then 55mm back focus):

long.jpg.4917de52afacd84c87336daa19b5dca1.jpg

So I think I probably need a shorter extension tube, to go between the focuser tube adapter and coma corrector? Or, something to go between the focuser tube, and the focuser tube adapter? Does this sound about right to you? Something like this maybe: https://www.firstlightoptics.com/adapters/astro-essentials-t2-m42-extension-tube-set-4mm-5mm-6mm-7mm-8mm-9mm.html, which would give me several options to play around until I get something that works.

If that isn't what I need, then if someone could point me in the right direction please, that would be great. I know exactly what I need to do, I just don't know how to do it or even what search terms to use to find what I need! I just need about 5mm-ish more outward room to achieve focus.

 Sorry, I know I'm probably coming across as a bit of a newb, but this is my first time with this problem so I'm not sure really what the best solution is.

Thanks, Brendan
 

Edited by BrendanC
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Posted (edited)

So, I just took a look at the primary mirror and noticed that, after collimating over the years, it seems to have traveled upward somewhat. So I adjusted it all the way back down and recentered it. This might give me a few more mm to play with.

I've also been recommended to get one of these - again, any/all comments welcome as to whether this might work: https://www.astroshop.eu/extension-tubes/ts-optics-extension-tube-m54-8mm/p,71884?fbclid=IwAR2Vks5Lrh7yP7CoZYCVoGM2Qfb2yxERu_2bdEhprrDDcJDelIFPuBMQ21w#tab_bar_0_select

Edited by BrendanC
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A baader click lock is approx 30mm deep so should push tube 15mm ish further into ota I used one on my 200pds but changed focuser to a steeltack using a threaded connection but I highly recommend the click lock very well made maybe someone using a 130pds can confirm this if using a zwo533mc with a click lock for NT 

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Posted (edited)
8 minutes ago, BrendanC said:

If that's like this, then I've been recommended this and have already bought it, so fingers crossed: https://www.firstlightoptics.com/adapters/astro-essentials-parfocal-rings.html

The parfocal ring will slide over the nose of the coma corrector and will 'shorten' the amount of the CC that gets inserted into the draw tube. As long as this doesn't leave the CC too short/unstable to hang the rest of the imaging train on, you should be ok.

The extension tube method lengthens the draw tube and so the length of the nose of the CC is unaffected. 

HTH

Ady

 

Edit: the click lock method also extends the draw tube, and as @bottletopburlysays, are an excellent addition if you have room in your imaging train - they self-centre and grip like **** 😉 

Edited by adyj1
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Just to tie this one off, I fixed it.

I was going through my mind to figure out what might have changed, for the focus to have moved out. "All I've done is collimate the scope," I thought. Then realised: OK, that might be all I've done, but it's something I've done that might change this.

So, I took the primary cell out, wound all the screws down so the primary was right at the very far end, and realised that, over the years, each time I've collimated it, it's moved up the tube a bit.

So, now, with the primary absolutely reset as far as it can go, and then recentered, everything is working again.

Boom shanka. :)

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I have a similar problem with my TS-PHOTON F4 200mm. Thankfully I am using the much longer TS-GPU coma corrector which I can pull out of the focuser barrel.
I have been considering asking my 3D-printer owning friend for a way to move my primary mirror cell about 4cm further away from the secondary, since this would solve the primary issue, as well as bring the camera closer to the secondary mirror, which means better illumination, and reduce the torque on the focuser.

I am worried 3D-printed plastic might not be tough enough for an 8" mirror+cell though.

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On 01/07/2022 at 23:51, BrendanC said:

Hi all,

TL:DR - HOW CAN I GET 5-10MM MORE OUTWARD FOCUS? 

So, first night with my new ASI533MC Pro, and I have a problem.

I have a 130PDS, and I chopped the focuser tube so that my stars were nice and round. However, what worked perfectly with my previous camera, does not now work with this one. 

When I focus, the camera is moving out, and I get closer... then closer... then I'm just about there, and then the focuser tube pops out.  So it's close, very close, but no cigar. So I need the camera to go out a few mm more to achieve focus. The backfocus distance for the coma corrector is 55mm, which is why I need those spacers before it. 

Among my many mysterious bits and bobs lying around, I found an extender that looked like it might work, but it's too long.

So, this is too short (focus tube adapter then coma corrector then 55mm back focus):

short.jpg.b312a90c2a43597d52fd039992960233.jpg

And this is too long (focus tube adapter, then extender tube, then coma corrector then 55mm back focus):

long.jpg.4917de52afacd84c87336daa19b5dca1.jpg

So I think I probably need a shorter extension tube, to go between the focuser tube adapter and coma corrector? Or, something to go between the focuser tube, and the focuser tube adapter? Does this sound about right to you? Something like this maybe: https://www.firstlightoptics.com/adapters/astro-essentials-t2-m42-extension-tube-set-4mm-5mm-6mm-7mm-8mm-9mm.html, which would give me several options to play around until I get something that works.

If that isn't what I need, then if someone could point me in the right direction please, that would be great. I know exactly what I need to do, I just don't know how to do it or even what search terms to use to find what I need! I just need about 5mm-ish more outward room to achieve focus.

 Sorry, I know I'm probably coming across as a bit of a newb, but this is my first time with this problem so I'm not sure really what the best solution is.

Thanks, Brendan
 

Baader click lock may work for you in this instance. 

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