Help needed please - ZWO ASI1600 plus EFW, with Sky-Watcher 130PDS and coma corrector

[BrendanC]

Hi all,

I recently decided to take the plunge into mono, after two fun years with my trusty EOS1000D and 130PDS combo. So, I've recently invested in a (second-hand) ASI1600 MM Cooled with 8-position electronic filter wheel and LRGBHSO filters.

Everything seems to be hooked up and working fine, and I can achieve focus. However, in order to do so, I'm noticing that, to my slight dismay, the focuser tube is actually further in than it was with the DSLR, and intrudes on the primary mirror even more. I did not expect this. I think it's because of the combination of camera plus EFW plus 0.9x coma corrector.

I think the backfocus is correct because my understanding is, if this wasn't right, I wouldn't be able to achieve focus at all. Am I right?

I've looked into removing the coma corrector from the chain, and it seems essential.

So, my options are:

• Live with it - which I might be able to do, don't know yet
• Move the primary up the tube - which I'd rather not do
• Chop the focuser tube - which I'd really not rather do
• Get a different scope or camera - ditto

Any takers?

Thanks, Brendan

Edited March 15, 2022 by BrendanC

[Varavall]

6 minutes ago, BrendanC said:

Hi all,

I recently decided to take the plunge into mono, after two fun years with my trusty EOS1000D and 130PDS combo. So, I've recently invested in a (second-hand) ASI1600 MM Cooled with 8-position electronic filter wheel and LRGBHSO filters.

Everything seems to be hooked up and working fine, and I can achieve focus. However, in order to do so, I'm noticing that, to my slight dismay, the focuser tube is actually further in than it was with the DSLR, and intrudes on the primary mirror even more. I did not expect this. I think it's because of the combination of camera plus EFW plus coma corrector.

I think the backfocus is correct because my understanding is, if this wasn't right, I wouldn't be able to achieve focus at all. Am I right?

I've looked into removing the coma corrector from the chain, and it seems essential.

So, my options are:

• Live with it - which I might be able to do, don't know yet
• Move the primary up the tube - which I'd rather not do
• Chop the focuser tube - which I'd really not rather do
• Get a different scope or camera - ditto

Any takers?

Thanks, Brendan

Hi

I have the same camera and filter wheel plus a coma corrector but on the 200pds. I assume the focuser is the same on both models (could be wrong). On mine when in focus the focus tube withdraws about 10-15mm to achieve focus. I have not noticed any problems with the focus tube impairing imaging. Perhaps on the narrower tube the focus tube could be more intrusive, so I would suggest take some images and flats and see if there is some artefact produced by the intrusion of the focus tube. That's my two penny worth. Hope it helps.

[BrendanC]

Thanks - I don't know about the 200PDS, but for the 130PDS it's definitely a noted problem that, especially with a DSLR and their own coma corrector (which is a 0.9x corrector, so also a reducer, which I didn't make clear in the original post and have just added), the focus tube intrudes on the primary mirror and causes little bitemarks out of stars, also known as 'pacman stars'. I mitigated this to an extent with a primary baffle, but I was very much hoping that a change to a dedicated astro cam would finally cure this. Instead, it's made it worse.

[Varavall]

Maybe it's natures way of telling to get a bigger tube 🤔

[BrendanC]

I'd rather examine plans A, B, C etc before I change the scope!

[BrendanC]

Just realised another option could be to get a new coma corrector, one that isn't also a reducer. Hmmmm........

[Laurin Dave]

Do you have the correct items in your imaging train?  they should give 56mm back focus (55mm plus 1mm  for the filter) which is the same as a DSLR plus camera adapter so the focus point should be the same as with your DSLR.  You should have the camera (6.5mm)  plus EFW (20mm) plus T2-T2 adapter (2mm) plus 11mm ring plus 16.6 mm extender..  as per this link

The best solution of 55mm back focal length | ZWO ASI (astronomy-imaging-camera.com)

 

[BrendanC]

Thanks, yes, I have the correct backfocus, exactly as per that very diagram. My understanding is that, if I didn't have that, I wouldn't be able to focus at all, is that right?

The tube focus point is very similar to the DSLR, but slightly further in (I think). Either way, I was hoping it would be further out!

[teoria_del_big_bang]

What about a low profile focusser similar to THIS with a much shorter tube ???

Would that be an option ?

Steve

[BrendanC]

Absolutely no idea! I guess I'm looking for a shorter tube, so if that's what that is, then yes! Thanks, this might be an option too.

[teoria_del_big_bang]

Just now, BrendanC said:

Absolutely no idea! I guess I'm looking for a shorter tube, so if that's what that is, then yes! Thanks, this might be an option too.

I fitted one to my Explorer 150PL so assume your tube is very similar just shorter slight different diameter  and it was a straight forward fit and has a very short tube from what I remember (not used this scope for ages but can get it out and take some pics if that helps) or even give FLO a ring and ask their opinion.

Steve

[BrendanC]

This is a very good idea, the more I think about it. It's pretty much exactly the same as chopping the tube... without chopping the tube. And it's a lot less than buying a new coma corrector which might not even work. Thank you, this is actually a completely new idea I hadn't thought of, which might just do the trick!  Nice one.

[teoria_del_big_bang]

Ah I might be lying there, I think this is what I fit to my 200 Dobsonian so this may not fit your diameter tube

Steve

[BrendanC]

Bum

Looks like there aren't any for such a small Newt. However, it's given me another idea: buy a full-size spare, ideally second-hand, and cut that down instead, then replace it. That would still work, cost about the same, and would be reversible.

Edited March 15, 2022 by BrendanC

[Adam J]

2 hours ago, BrendanC said:

Hi all,

I recently decided to take the plunge into mono, after two fun years with my trusty EOS1000D and 130PDS combo. So, I've recently invested in a (second-hand) ASI1600 MM Cooled with 8-position electronic filter wheel and LRGBHSO filters.

Everything seems to be hooked up and working fine, and I can achieve focus. However, in order to do so, I'm noticing that, to my slight dismay, the focuser tube is actually further in than it was with the DSLR, and intrudes on the primary mirror even more. I did not expect this. I think it's because of the combination of camera plus EFW plus 0.9x coma corrector.

I think the backfocus is correct because my understanding is, if this wasn't right, I wouldn't be able to achieve focus at all. Am I right?

I've looked into removing the coma corrector from the chain, and it seems essential.

So, my options are:

• Live with it - which I might be able to do, don't know yet
• Move the primary up the tube - which I'd rather not do
• Chop the focuser tube - which I'd really not rather do
• Get a different scope or camera - ditto

Any takers?

Thanks, Brendan

Had exactly that combo and chopped the focuser. 

[teoria_del_big_bang]

An interesting thread HERE also mentions the intrusion of the focusser into the lightpath.

Steve

[BrendanC]

1 minute ago, teoria_del_big_bang said:

An interesting thread HERE also mentions the intrusion of the focusser into the lightpath.

 

Yep, and I'm reading similar things. Darn it, Sky-Watcher, why couldn't you just sell products that work properly?

[BrendanC]

2 minutes ago, Adam J said:

Had exactly that combo and chopped the focuser. 

That's another conclusion I'm rapidly coming to, if I can't source a low-profile focuser. How much did you take off? Also, did you completely remove the focuser first? And what did you use to cut it?

[Adam J]

3 hours ago, BrendanC said:

That's another conclusion I'm rapidly coming to, if I can't source a low-profile focuser. How much did you take off? Also, did you completely remove the focuser first? And what did you use to cut it?

I have a band saw but you could just use a dremel or a hacksaw in a vice (dont deform the tube).

I would not like to try and recal how much a took off, just focus it while imaging then place a black line with a marker pen where is exits the tube holder aperture, leave an extra 10mm longer than your mark, then cut it.

You then need to glue a tiny peice of plastic or similar onto the flat at the bottom of the tube to act as a stop so that you cant move the tube out past the internal mounting bearings. Simply place it inside the current lip by the same distance as the thicknes of material removed.

To me its a little crazy to buy a 150-200 pound focuser to save you from having to cut the tube on a 189 pound scope.

Adam

 

Edited March 15, 2022 by Adam J

[BrendanC]

Thank you.