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Using an st80 as a guide scope on a c8?

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#1 tomwall

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Posted 22 November 2020 - 11:06 AM

I want to use my st80 as a guide scope for my c8.

Does anyone have any advice on how to do this? 

 

Thanks!



#2 Jim R

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Posted 22 November 2020 - 11:35 AM

I am not an expert. I think probably doable, but OAG better?
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#3 alphatripleplus

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Posted 22 November 2020 - 11:39 AM

I agree,  you are probably going to do better with an OAG for a SCT, given the possibility of mirror flop even if you have the ST80 rigidly mounted on the C8.


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#4 wrvond

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Posted 22 November 2020 - 11:48 AM

IMG_3828sm.JPG


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#5 tomwall

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Posted 22 November 2020 - 02:34 PM

I do have an old OAG I can try. I may do that. My hope with mounting the st80 is that I'd have a dual scope setup. The st80 could do wide field and the c8 could do smaller targets. I'm running on a CGEM, so weight shouldn't be too much of an issue. I do agree that the OAG would end up giving me better guiding, but I'm just starting AP and don't want to bite off more than I can chew.

 

BTW: Those rings would offer me the adjustability I'm looking for. I'm currently running the st80 off of a piggy back camera mount on the rear of the c8. This doesn't allow me to accurately align the two scopes. 

 

Thanks! :-)



#6 alphatripleplus

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Posted 22 November 2020 - 03:41 PM

If you want to precisely point the ST80 and the C8 at the same target, for simultaneous imaging, the adjustable rings would definitely do that. I have an ST80 and only use it for narrowband imaging though; the chromatic aberration is severe for anything else.



#7 tomwall

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Posted 22 November 2020 - 11:35 PM

Thanks Errol,

The thought of getting a much nicer 80-100mm refractor is in my long range plans. Just waiting for my savings to build up. Do you know who sells adjustable rings that large?

Huxley is one of my favorite authors.

 

Tom



#8 alphatripleplus

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Posted 23 November 2020 - 09:48 AM

Scopestuff sells a range of  adjustable rings, that should handle a 80-100mm refractor.


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#9 tomwall

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Posted 23 November 2020 - 10:15 AM

Errol,

Thanks! That's exactly what I was looking for. :-)

 

Tom


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#10 Pauls72

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Posted 23 November 2020 - 12:12 PM

The problem with using an external guide scope on an SCT is that most SCT's have mirror flop unless you can lock down the primary mirror.

What this is the primary mirror is mounted very loose in an SCT. When the mount moves, the center of gravity moves in relation to the OTA and the primary mirror shifts around a small amount.

In visual observing, it's not a problem, but with AP it usually is.


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#11 *skyguy*

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Posted 23 November 2020 - 01:05 PM

I replaced the 50mm finderscope and bracket on my 12" LX200 with an Astro-Tech 90mm finder/guidescope bracket and an 80mm f/4.2 guidescope. I have no problems with differential flexure.

 

The system is capable of doing 10+ minute guided exposures (however, exposures are usually limited to 5 minutes since I use a NABG camera) with very minimal star elongation. Maybe, not absolutely perfect (stars can be fixed), but close enough for me.

 

With an 80mm guidescope on a C8, you'll need some counterweights under the OTA to properly balance the scope. Also, getting a f/6.3 focal reducer on the C8 will really help with the autoguiding.

 

You should also check out some the very good DSO images posted on "astrobin.com" taken through a C8 using a guidescope. However, many beginner astrophotographers seem to have a bad time starting out with an 8" or larger SCT using a guidescope. Some switch over to an OAG and others will drop the SCT in favor of using a smaller refractor for imaging.

 

Good Luck .....

 

LX200_Piggyback.jpg


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#12 t-ara-fan

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Posted 23 November 2020 - 06:01 PM

Losmand DVR 108/66 is a 3-ring set, that holds the camera steady in spite of the slightly floppy focuser.

 

http://www.losmandy....08_66wscope.jpg

 


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#13 WadeH237

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Posted 24 November 2020 - 03:35 PM

I have guided a C8 with an ST80.

 

It's already been mentioned that mirror movement in an SCT will limit the effectiveness of any guide scope, due to excessive flexure.  The other thing, specific to the ST80, is that the focuser is really cheap.  On my scope, there was noticeable slop in the focuser drawtube.  I'm not talking about slop in holding focus.  I'm talking about the fact that there is play between the draw tube and the channel that it rides in.  This is another source of significant flexure.  So much so, that I stopped using my ST80 to guide anything, even another refractor.


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#14 Pauls72

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Posted 24 November 2020 - 05:25 PM

I have guided a C8 with an ST80.

 

It's already been mentioned that mirror movement in an SCT will limit the effectiveness of any guide scope, due to excessive flexure.  The other thing, specific to the ST80, is that the focuser is really cheap.  On my scope, there was noticeable slop in the focuser drawtube.  I'm not talking about slop in holding focus.  I'm talking about the fact that there is play between the draw tube and the channel that it rides in.  This is another source of significant flexure.  So much so, that I stopped using my ST80 to guide anything, even another refractor.

You can fix the slop in the ST80 focuser. There are some small Allen/Hex screws you can adjust. Around the 10:20 time mark of the video, he explains how to make the focuser better.

https://www.youtube....h?v=Ylb7xnc_03U


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#15 ram812

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Posted 27 November 2020 - 10:07 PM

 Hello! This is also something I've done, took apart the ST80 focuser and cleaned all that goop grease that is packed in. When re-installing use little or NO derveenerslider in the rack itself (Stiction prevention). Tighten up the various Allen screws to support what gear you have on while maintaining smooth movement of the focus rack. I thread my SSAG directly to it and there is no tilt, slippage or flexure. Also, when I used it as I guider I had to just dial it in once at the beginning of summer and the beginning of winter, focusing and finding guide stars while mounted on my AT6RC or XT10  (EQ platform) was never an issue. I've recently downsized to Orion's 60mm Helical Focus Guide scope and that helped with weight, of course! But the ST80 is a very versatile little scope when properly attached and tuned. I had fun with mine, but alas, it's time to put it on the sale block and thin the herd outgrin.gif! Ralph



#16 CJW8

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Posted 28 November 2020 - 10:57 AM

I have guided a C8 with an ST80.

 

It's already been mentioned that mirror movement in an SCT will limit the effectiveness of any guide scope, due to excessive flexure.  The other thing, specific to the ST80, is that the focuser is really cheap.  On my scope, there was noticeable slop in the focuser drawtube.  I'm not talking about slop in holding focus.  I'm talking about the fact that there is play between the draw tube and the channel that it rides in.  This is another source of significant flexure.  So much so, that I stopped using my ST80 to guide anything, even another refractor.

For this reason I replaced the focuser in My ST80 with a 2 speed all aluminum focuser.




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