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Astrophotography and Sketching >> Beginning and Intermediate Imaging

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Ned Smith
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Reged: 04/10/10

Loc: NW Georgia, USA
Guiding: OAG vs Guide Scope
      #5509947 - 11/08/12 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


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Phil Sherman
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Reged: 12/07/10

Loc: Cleveland, Ohio
Re: Guiding: OAG vs Guide Scope new [Re: Ned Smith]
      #5510267 - 11/08/12 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

Edited by Phil Sherman (11/08/12 05:15 PM)


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*skyguy*
Carpal Tunnel
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Reged: 12/31/08

Loc: Western New York
Re: Guiding: OAG vs Guide Scope new [Re: Ned Smith]
      #5510336 - 11/08/12 05:56 PM

I'm staying out of this one!

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SMigol
sage


Reged: 07/30/10

Loc: California, USA
Re: Guiding: OAG vs Guide Scope new [Re: *skyguy*]
      #5510391 - 11/08/12 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.


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TxStars
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Reged: 10/01/05

Loc: Lost In Space
Re: Guiding: OAG vs Guide Scope new [Re: Ned Smith]
      #5511011 - 11/09/12 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".


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agmakr
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Reged: 12/14/07

Loc: Agrinio, Greece
Re: Guiding: OAG vs Guide Scope new [Re: TxStars]
      #5511030 - 11/09/12 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.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5502330/page...

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


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Ned Smith
super member


Reged: 04/10/10

Loc: NW Georgia, USA
Re: Guiding: OAG vs Guide Scope new [Re: TxStars]
      #5511110 - 11/09/12 07:27 AM

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


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Guiding: OAG vs Guide Scope new [Re: Ned Smith]
      #5511263 - 11/09/12 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.


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JoseBorrero
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Reged: 09/04/09

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Re: Guiding: OAG vs Guide Scope new [Re: Jared]
      #5511603 - 11/09/12 01:04 PM

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

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TxStars
Pooh-Bah
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Reged: 10/01/05

Loc: Lost In Space
Re: Guiding: OAG vs Guide Scope new [Re: Ned Smith]
      #5512112 - 11/09/12 06:28 PM

Quote:

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



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


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guyroch
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Reged: 01/22/08

Loc: Under the clouds!
Re: Guiding: OAG vs Guide Scope new [Re: TxStars]
      #5512135 - 11/09/12 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

Edited by guyroch (11/10/12 03:04 PM)


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Ned Smith
super member


Reged: 04/10/10

Loc: NW Georgia, USA
Re: Guiding: OAG vs Guide Scope new [Re: guyroch]
      #5513262 - 11/10/12 02:28 PM

Thanks for your comments.

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jgraham
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Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: Guiding: OAG vs Guide Scope new [Re: Ned Smith]
      #5513332 - 11/10/12 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.

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wolfman_4_ever
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Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: Guiding: OAG vs Guide Scope new [Re: jgraham]
      #5514708 - 11/11/12 04:08 PM

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

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TxStars
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Reged: 10/01/05

Loc: Lost In Space
Re: Guiding: OAG vs Guide Scope new [Re: wolfman_4_ever]
      #5514799 - 11/11/12 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.


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jgraham
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Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: Guiding: OAG vs Guide Scope new [Re: TxStars]
      #5514818 - 11/11/12 05:19 PM

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

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Patrick
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Loc: Franklin, Ohio
Re: Guiding: OAG vs Guide Scope new [Re: jgraham]
      #5589102 - 12/26/12 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


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Footbag
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Reged: 04/13/09

Loc: Scranton, PA
Re: Guiding: OAG vs Guide Scope new [Re: Patrick]
      #5590008 - 12/26/12 05:12 PM

Quote:

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.


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hcsceo
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Reged: 10/14/09

Loc: Austin, TX
Re: Guiding: OAG vs Guide Scope new [Re: Footbag]
      #5591276 - 12/27/12 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.

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jgraham
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Re: Guiding: OAG vs Guide Scope new [Re: hcsceo]
      #5591606 - 12/27/12 06:47 PM Attachment (13 downloads)

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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