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Guiding: OAG vs Guide Scope

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#1 Ned Smith

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Posted 08 November 2012 - 01:08 PM

I am building a GEM for my new VRC8. Can someone tell me the pros and cons of OAG vs a separate guiding scope with guiding camera.
TIA

#2 Phil Sherman

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Posted 08 November 2012 - 05:11 PM

OAG:
1. limited FOV to find guide stars
2. Heavier equipment for the focuser to support.
3. Requires camera with two imaging chips or an attachment between the camera & focuser to divert the guide camera light. An attachment will increase back focus.

guide scope:
1. Can use different focal length and f/ ratio than imaging scope
2. Subject to differential flexure - imaging scope & guide scope bend (sag) a little differently.
3. Can be aimed at different stars.
4. 3. above can cause more field rotation because you can image further away from the guide star.
5. More weight for the mount to carry
An 80mm APO makes a great guide scope and, if it's a short focal length compared to your main scope, you can swap the imaging and guide scopes to get a wider FOV.

Phil

#3 *skyguy*

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Posted 08 November 2012 - 05:56 PM

:gotpopcorn: I'm staying out of this one!

#4 SMigol

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Posted 08 November 2012 - 06:40 PM

One con to an OAG that may not be easily tested:
Be sure that your scope has enough in-focus travel to handle the extra length required by the OAG. I recently tried this on a scope of mine and found this problem.

#5 TxStars

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Posted 09 November 2012 - 03:46 AM

If I'm not mistaken the VRC-8 is the same scope as the AT-8RC from Astro-Tech.
If it is then you have several options open to you including:"OAG" "On axis guiding".

#6 agmakr

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Posted 09 November 2012 - 04:27 AM

Phil made an excellent analysis on the pros and cons.

I used a guidescope for some time and almost always had problems
with differential flexure.So i ended up throwing away about 30% of the fits i got.

I recently changed to an OAG and i managed to get 15,20,30min subs without any trail.
However, it may be a bit tricky to focus the OAG at first.
You may check here:
http://www.cloudynig...5502330/page...

If you choose the guidescope path, make sure you mount it
very compactly with the main scope.

#7 Ned Smith

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Posted 09 November 2012 - 07:27 AM

Yes, that is the scope.
What are the options?

#8 Jared

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Posted 09 November 2012 - 09:15 AM

If you are going to be imaging with a scope greater than about 1,500mm focal length, you will likely find that an OAG provides a noticeably better result since it isn't subject to differential flexure. If you are going to use a scope with a moveable primary mirror, especially one without mirror locks, you will likely need to use an OAG. At shorter focal lengths, guide scopes are easier to use since it is easy to find a guide star in virtually any part of the sky. As mentioned, OAG's will also use up some back focus, so make sure you have enough room.

I think OAG's are the only way to go if you want tight, round stars above (approximately) 1,500mm focal length, despite the drawback of a limited field of view.

Oh, one other possible advantage to guide scopes is that you can guide on the center of the field. If you want to image a comet, this can be critical.

#9 JoseBorrero

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Posted 09 November 2012 - 01:04 PM

It seems that if you fail to set an OAG will frustate you and go guide scope

#10 TxStars

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Posted 09 November 2012 - 06:28 PM

Yes, that is the scope.
What are the options?

There are several off-axis guiders that can be used.
One example:
http://www.optcorp.c....aspx?pid=17671
You can also use an On-axis guider which makes finding a guidestar easy:
http://www.astromart...?article_id=836

#11 guyroch

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Posted 09 November 2012 - 06:44 PM

Once you get the OAG going you'll never go back to a guide scope imo.

Phil forgot to mention that a OAG takes care of flexure, whereas a guide scope may cause flexure depending on the sturdiness of your gear.

Hope this helps,

Guylain

#12 Ned Smith

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Posted 10 November 2012 - 02:28 PM

Thanks for your comments.

#13 jgraham

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Posted 10 November 2012 - 03:33 PM

I use both; a guide scope on my SN6 and an off-axis guider on my SC8. Personally, I would never use an off-axis guider on a moveable focuser, way too much weight, though for an SCT with its fixed eyepiece tube the weight is far less of an issue. I've also found that an off-axis guider benefits from a guide camera with a large chip (I use a Pro III as my guide camera). Sooooo, a Newtonian or any other scope with a focuser that uses a moving draw tube I'd suggest a guide scope. For telescopes with a fixed draw tube an off-axis guider is worth considering if you also have a nice guide camera.

#14 wolfman_4_ever

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Posted 11 November 2012 - 04:08 PM

Is flexure an issue if the guidescope is setup in a side-by-side configuration?

#15 TxStars

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Posted 11 November 2012 - 05:02 PM

When using a guidescope there is always a chance for flexure to cause elongated or strange shaped stars.
The longer the focal length you image at the more any flexure will show up.

#16 jgraham

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Posted 11 November 2012 - 05:19 PM

I don't think side-by-side is any more or less susceptible to flexure. I've done it both ways.

#17 Patrick

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Posted 26 December 2012 - 12:21 AM

With SCT mirror locks is the flexure issue as much of an issue with a side by side or guide scope arrangement? It seems like it would not be. I do not like off-axis guiders because I've been unable to get mine working to date.

Patrick

#18 Footbag

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Posted 26 December 2012 - 05:12 PM

With SCT mirror locks is the flexure issue as much of an issue with a side by side or guide scope arrangement? It seems like it would not be. I do not like off-axis guiders because I've been unable to get mine working to date.

Patrick


Flexure comes from more places then the mirror. I'm limited to 8m subs due to flexure in my setup despite the mirror locks.

I understand what you are saying about not getting an OAG working. I never got my Celestron Radial Guider working with my SSAG(Needed more inward focus). Then I tried the Older model Celestron OAG(Need more outward focus, but thumbscrew doesn't hold then camera stable enough)

I just purchased a TS9 OAG and will be getting a Lodestar. I'm fairly confident this setup will work, but won't know until it all shows up.

#19 hcsceo

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Posted 27 December 2012 - 02:30 PM

A couple of notes from my OAG experience. First thing is that I love it! Much better with my SCT than a guide scope ever was. However, getting there was not easy and I gave up on it at least once. A sensitive guide camera is a must. The longer the focal length the more important this is. Don't expect to put a webcam on there and be guiding. Dsi pro mono is minimum. A lodestar would be awesome. Next finding focus is where I'm guessing most people fail. It was a horrible experience for me. In the end I measured distances with some calipers from imaging chip to the front of the pick off prism OAG and again from the pick off prism to the guide chip. These need to be the same. I ordered a bunch of spacers from scoffstuff to get this perfect. It seems to find stars a little easier if focus is a little soft. Take it all outside during the day and focus both before trying at night. Watch out for the prism casting a shadow on your imager during this phase. You might need to play with the prism location. On mine it is located on the long side of my imager ccd. One final note is to get the oag as close to the imager as possible to reduce vignetting. Because it is picking off light from the edge of my SCT light cone I have some coma in those stars. This doesn't seem to affect the performance however.

#20 jgraham

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Posted 27 December 2012 - 06:47 PM

I chose to use a guide scope on my SN6 as its tube rings made a natural attachment point for the scope and it's nice to be able to align the field of view of the guide camera with the imaging camera.

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#21 jgraham

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Posted 27 December 2012 - 06:53 PM

However, for my SC8 I ended up with an off-axis guider. This was a nice option for me as my SC8 doesn't have tube rings and the fixed focuser hardware made an effective attachment point for the combined weight of the imaging and guide cameras. I'm using a DSI Pro III as my guide camera which gives a generous field of view. I was also able to keep this system very compact which helped a lot in making this such a nice system to use. An unexpected bonus is that I don't have to set the focus of the imaging an guide cameras separately. After the initial setup I've never had to fiddle with the focus of the guide camera.

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#22 wolfman_4_ever

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Posted 27 December 2012 - 11:10 PM

Just do the best of both worlds..

Side-by-Side.. and an OAG with an AO!

Posted Image

#23 JoseBorrero

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Posted 28 December 2012 - 07:28 PM

my best guiding is so far with tandem setup shown here: http://www.flickr.co.../in/photostream

lately I experiment with 50mm mini guide scope and got same good guiding results as the tandem. http://www.flickr.co.../in/photostream

#24 CounterWeight

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Posted 28 December 2012 - 10:52 PM

I've never experienced differential flexure... I use piggyback exclusively. I think any flexure is bad, anywhere, for imaging.

Smeared stars due to mirror movement I don't consider to qualify as the term often defined (by that I mean purely mechanical issues elsewhere)- but it's an obvious reason to use an OAG.

Good rings and saddle plate/DTP are important, focusers and collets as well. OK, everthing is. Using two screws vs. one in ring attachment, making sure and certain all well seated and square to tube and there is no play if your rings have felt or other mat'l to avoid marring scope finish. I often state that IMO other than optics alone, it's a guality focuser assy that makes the difference, this includes the IMO often neglected collet type and being certain CCD is is 'ortho' or seated squarely/securely.

One thing I've been curious about that I see mentioned rarely and that is if you are remote and using a tripod (possibly allpies to porta-piers too)- that adds a world of things IMO as far as the tripod legs, load, tensioning, spreaders, surface type/hardness.

If you are going portable and setting up every time, there is lot to get right. I've been using a permanent setup (or somewhat permanent anyway) and can take my time - but I've had good stars all the way with various piggyback setups using refractors and being very careful about focuser CCD setup and critical focus.

Seems each setup has it's own things to be certain of, and this without considering the portable pier or tripod.

#25 hcsceo

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Posted 28 December 2012 - 11:47 PM

My setup is completely portable and setup every time at different locations. It is on the standard CG5 tripod. I leave the OAG, FR, spacers, DSLR, and Meade DSI Pro all together in a camera bag and take them out and screw them on as a complete assembly when ready. This keeps dust and dirt out of the image train and I only have to clean he front of the FR if needed. I've had no issues. I also automate it with CCD commander and have no issues acquiring stars with any target throughout the night.






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