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Guide scope for 127mm Refractor question

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#1 Kevin G

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Posted 08 December 2019 - 01:31 AM

Hello All,,

 

Just checking with you who know: Would the Orion 60mm Multi Use Guide Scope be a good fit for my ES 127/952 telescope?

 

SW EQ6 R Pro mount.

 

I'm thinking of using a ZWO ASI290mm mini guide camera.

 

Thanks in advance!



#2 james7ca

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Posted 08 December 2019 - 01:52 AM

I think you'd be okay in terms of image scale (with the ASI290mm), but at that focal length and with a separate guide scope your're going to have to be pretty careful about differential flexure between the two optical systems. Where do you plan to mount the guide scope and what kind of image scale will you be using (camera pixel size at 952mm)? Also, any guess as to your typical exposure times?

 

Frankly, this might be a case where you want to consider an OAG.


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#3 Kevin G

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Posted 08 December 2019 - 02:07 AM

james7ca: I was planning on mounting it on the top of the handle on the tube rings of the ES scope. OR, I suppose I could also mount it on the finderscope shoe. DSLR T7i,  was thinking of a lot of 1 min. exposures, fair amount of light pollution where I am unless I travel. 



#4 james7ca

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Posted 08 December 2019 - 05:57 AM

Okay, keeping it to one or two minute exposures should help. But, you're imaging at a scale of 0.81 arc seconds per pixel which is fairly demanding. Given that you're using a DSLR I guess you won't be trying any long exposure, narrow-band DSO work.

 

I use a separate guide scope with my 660mm focal length refractor and since I use the ASI183MM (2.4um pixels) that means I image at 0.75 arc seconds per pixel. However, that's with an A-P Mach1GTO mount and even then it's not particularly easy to get drift-free guiding for over a few minutes. Four or five minutes is okay, but I don't think I could go to 10 minutes without some problems. I always see some differential flexure in the guiding system even though I'm using a very rigid guider and mounting with a pretty decent focuser on the imaging scope.

 

Then again, you'll be using a one-shot-color camera with a color filter array which may make things even a bit easier.


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#5 jerahian

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Posted 08 December 2019 - 07:47 AM

Hello All,,

 

Just checking with you who know: Would the Orion 60mm Multi Use Guide Scope be a good fit for my ES 127/952 telescope?

 

SW EQ6 R Pro mount.

 

I'm thinking of using a ZWO ASI290mm mini guide camera.

 

Thanks in advance!

You should be OK with the 60mm scope for guiding.  BTW, I have the exact same setup as you, and I’ve been using a used 80mm Orion ST-80, with a 290MM, as my guide scope, mounted in a 3D printed clamshell on my ES127 handle bar.  Works like a charm.  If you don’t already have the 60mm, you can get a used ST-80 on CN Classifieds usually for pretty cheap.


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#6 Kevin G

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Posted 08 December 2019 - 02:07 PM

Jerahian, thanks.  If you have a picture handy, I'd love to see it.



#7 jerahian

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Posted 08 December 2019 - 09:05 PM

Sorry this took so long to come back to you...was out with the family all day.

 

Here are a couple of shots of the fully assembled scope, with the ST-80 attached with the clamshell I designed and 3D printed.  If you have access to a 3D printer, the clamshell itself can be found here on thingiverse.

 

gallery_298242_11004_286363.jpg
 
gallery_298242_11004_636731.jpg

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#8 Stelios

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Posted 08 December 2019 - 09:36 PM

Hello All,,

 

Just checking with you who know: Would the Orion 60mm Multi Use Guide Scope be a good fit for my ES 127/952 telescope?

 

SW EQ6 R Pro mount.

 

I'm thinking of using a ZWO ASI290mm mini guide camera.

 

Thanks in advance!

Since you are thinking of the ASI290 (the biggest expense) I'd skip the guidescope and go straight to an OAG. The ZWO one is a great match for that camera.

 

With an OAG, say goodbye to differential flexure, and hello to round small stars. 


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#9 SonnyE

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Posted 08 December 2019 - 10:06 PM

I can't recall the ratio, but think is was something like 3 to one, or 5 to one. Somebody should know.

It's one of those things I considered and forgot.

Or you could wade through here: https://www.cloudyni...main-telescope/

 

I image with an 80 mm ED80T CF, and use an Orion MMAG scope (50 mm), and an Orion SSAG camera.

And I did it on an AVX mount, and got some good images. The atmosphere was more trouble than the guiding.

In my Opinion, a guide scope only is asked to pick a star and stay on that bugger. A star.

If your PA and alignment are good, not rushed, you can enjoy a long trip guiding without worrying. As is evidenced with nice round stars and pinprick stars.

If your guiding is "hunting" is when you (I) get irregular or fuzzy stars.

So my 50 mm guide scope, was more than sufficient for my telescope. I'd bet a dollar to a donut a 60 mm would do you fine with your 127 mm.


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#10 james7ca

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Posted 08 December 2019 - 10:42 PM

I think the Orion Multi Use Guide Scope would be a better solution than the ST-80, mainly because of the long overhang on the ST-80 focuser which could result in differential flexure (you want as rigid of a focusing arrangement as possible, on both the guide scope and the main imaging scope). Of course, you could add a set of rings to directly support the focuser drawtube, that's what I've done with my setups (see images below).

 

First image the A-P/Baader Vario Finder (comes with a third set of rings for the focuser). The Vario Finder is a 61mm f/4.1 objective with a focal length of 250mm (but, it's very expensive, I got mine used at a reduced price). Second image, a Stellarvue SV50ED with three sets of rings. The Stellarvue is a 50mm f/6.6 ED objective with a focal length of 330mm. The optics on the SV50ED are probably "wasted" when used as a guider/finder and the scope has a long overhang on the focuser, but it worked well with the three ring setup although it's kind of heavy for a guide scope.

 

I also use a second set of rings to support just the guide scope, since the main optical tube can shift in relation to the ring set that connects everything else to the mount. Thus, if you mount the guide scope to the standard rings you are guiding to the rings and not necessarily to the main optical tube. That said, flexure in the focusers may be the biggest issue of all and as always YMMV.

Attached Thumbnails

  • Vario Finder.jpg
  • SV50ED Guide Scope.jpg

Edited by james7ca, 09 December 2019 - 01:32 AM.

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#11 Ballyhoo

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Posted 08 December 2019 - 11:04 PM

 

Sorry this took so long to come back to you...was out with the family all day.

 

Here are a couple of shots of the fully assembled scope, with the ST-80 attached with the clamshell I designed and 3D printed.  If you have access to a 3D printer, the clamshell itself can be found here on thingiverse.

 

gallery_298242_11004_286363.jpg
 
gallery_298242_11004_636731.jpg

 

do you mind me asking, what type of bolts are you using to mount your guide-scope to your ES 127?  I have avoided your method because it makes it harder to put everything back in the case, versus me sliding my guide scope in and out of the native guide dove-tail.  Yet your method would do more for flexure issue.



#12 Kevin G

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Posted 09 December 2019 - 01:42 AM

Thanks for the input! All very helpful ideas to consider!



#13 jerahian

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Posted 09 December 2019 - 11:19 AM

do you mind me asking, what type of bolts are you using to mount your guide-scope to your ES 127?  I have avoided your method because it makes it harder to put everything back in the case, versus me sliding my guide scope in and out of the native guide dove-tail.  Yet your method would do more for flexure issue.

I used three M6 x 50 bolts and hex nuts.  If you look at the 1st image below, you can see that inside the clam shell, I allowed spacing for the bolt heads to fit without touching the scope.  The other end of the bolt extends out the bottom (2nd image), through the ES127 handle, with hex nuts securing it in place.  It's definitely not meant to be broken down every time.  I just leave my guide scope attached, even when I move it in and out or travel to a dark site with it (by car).

 

2019 12 09 11 09 18 Orion ST 80 Telescope Clamshell By jerahian   Thingiverse

 

2019 12 09 11 09 36 Orion ST 80 Telescope Clamshell By jerahian   Thingiverse

 

GL&CS,

Ara


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#14 jsrj98

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Posted 11 December 2019 - 06:14 PM

+1 to skip the guide scope and go directly to an OAG. The combo of ZWO OAG, their optional Helical Focuser, and the ASI290 is very nice and solid. It's super easy to tweak the focus of the 290 in PHD2. Once you get the OAG perfected, there's no looking back. At your FL you will NEVER have an issue with finding a suitable guide star and ZERO differential flexture.To me this is the best path to getting the tightest stars.


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