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Astrophotography and Sketching >> Beginning and Intermediate Imaging

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jgraham
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Re: Guiding: OAG vs Guide Scope new [Re: jgraham]
      #5591615 - 12/27/12 06:53 PM Attachment (18 downloads)

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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wolfman_4_ever
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Re: Guiding: OAG vs Guide Scope new [Re: jgraham]
      #5591951 - 12/27/12 11:10 PM

Just do the best of both worlds..

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



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JoseBorrero
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Re: Guiding: OAG vs Guide Scope new [Re: wolfman_4_ever]
      #5593187 - 12/28/12 07:28 PM

my best guiding is so far with tandem setup shown here: http://www.flickr.com/photos/70224675@N06/7989767360/in/photostream

lately I experiment with 50mm mini guide scope and got same good guiding results as the tandem. http://www.flickr.com/photos/70224675@N06/8273999430/in/photostream


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CounterWeight
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Re: Guiding: OAG vs Guide Scope new [Re: JoseBorrero]
      #5593495 - 12/28/12 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.


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hcsceo
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Re: Guiding: OAG vs Guide Scope new [Re: CounterWeight]
      #5593575 - 12/28/12 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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groz
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Re: Guiding: OAG vs Guide Scope new [Re: hcsceo]
      #5594386 - 12/29/12 01:14 PM

I think one thing folks tend to overlook in this discussion of oag vs separate guidescope, is the mount. It is after all, the mount that guiding is intended to make corrections for.

With a long focal length and small field that's typical of most oag setups, you need relatively long exposures to get a star, bright stars are rare in the oag field. If you do 30 second exposures on the guide camera, finding a star will never be a problem. It also means, corrections can only happen once every 30 seconds, a big problem if your mount has an 8 second cycle on the PE that you are trying to correct for.

So, if you are trying to correct a low end mount, with a first order PE cycle on the range of 8 to 12 seconds, you need to get corrections running at roughly 1 second intervals. This is a challenge for many oag setups. On the other hand, if you are correcting a mount that has 4 or 5 arcseconds of PE, over 8 or 10 minute timescales, with no noticeable period on shorter timeframes, using an oag with 15 second exposures is going to provide plenty of opportunity for corrections at the rate needed.


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hcsceo
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Re: Guiding: OAG vs Guide Scope new [Re: groz]
      #5594695 - 12/29/12 04:28 PM

Groz has a very good point here. Part of this is helped with a sensitive guide camera but the combination of focal length and mount is critical. On my CG5 with C8 my max exposure is 4 seconds and my average is around 2 seconds. At 4 seconds I'm getting a lot of drops. Any more than that then there is no point with the PE I'm dealing with. This means that I must use the .63 reducer to be successful. I've been trying it at f10 but haven't had much luck yet with the OAG. The big advantage of the separate guidescope is you can move around the sky a little to find a suitable star. On an SCT you have to decide if losses to mirror shift with a separate guidescope or losses to longer exposures on the OAG due to lack of bright stars is worse for you. Like almost everything in this hobby there is a balance to be found, but everything comes down to the mount.

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jgraham
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Re: Guiding: OAG vs Guide Scope new [Re: hcsceo]
      #5594844 - 12/29/12 06:23 PM

I can see where some would have problems finding guide stars with an off-axis guider. I'm fortunate to have a very sensitive guide camera with a fairly large chip and so far I haven't had any problems.

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jgraham
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Re: Guiding: OAG vs Guide Scope new [Re: jgraham]
      #5595301 - 12/29/12 11:13 PM Attachment (24 downloads)

I was cleaning images off of my DSLR's memory card and came across this picture of my SC8 with its off-axis guider. I was really pleased with how this turned out. It is very solid and compact.

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ChubbyNinja
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Re: Guiding: OAG vs Guide Scope new [Re: jgraham]
      #5597781 - 12/31/12 11:31 AM

I tried out my Hutech OAG5 last night for the first time since I received it 3 or 4 weeks ago (it has been so cloudy here!) I use a DSLR for imaging and a QHY5 for the guide camera. I was able to achieve focus on the guide camera by aiming the scope at the moon. Even with the bright moon it took a while to focus so I couldn't imagine doing it on a dark night. Now that it's done I don't have to worry about it later so that's good.

Finding a guide star took some time because of the sensitivity of the QHY5 I think, so I may consider an upgrade to a lodestar or something.

I was able to take a single shot of M42 for 15 minutes and the stars looked great -- even with my fast and sloppy polar alignment. I probably could have taken a lot longer exposure but it was cold and the moon was too bright to make any resulting pics worth it. Plus, I'm not so sure long exposures of that magnitude are really necessary for a DSLR. With my previous setup using the QHY5 and a Celestron 80mm piggyback I was lucky to get 3 minutes before flexure kicked in causing my target to slowly shift out of the frame over the course of the night.


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CounterWeight
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Re: Guiding: OAG vs Guide Scope new [Re: ChubbyNinja]
      #5597797 - 12/31/12 11:39 AM

Wayne - just curious what that flexure was from, were you able to isolate the source? Glad to hear the OAG solution worked for you How long of guide exposures were you using? I'd be interested to see a 15 minute sub from a DSLR - I don't visit that forum.

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Peter in Reno
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Re: Guiding: OAG vs Guide Scope new [Re: CounterWeight]
      #5597871 - 12/31/12 12:18 PM

Wayne,

I have Hutech OAG-5 with Lodestar and they work great with my C-8 EdgeHD. I have an optional Helical focuser at the guide port and it's very handy to precisely focus a guide star. I parfocal both Lodestar and imaging camera during the day. I aim for a distant object like street light about a mile away. Once parfocaled during the day, it should be very close under the dark sky. You might have to tweak a bit.

I will never go back to guide scope. I typically have to throw out up to 50% of the subs with guide scope. With OAG, I usually never throw out a single sub unless an airplane flew by. I can easily guide up to 30 minutes with OAG.

See my setup and images in "Peter's Galleries" in my signature.

Peter


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ChubbyNinja
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Re: Guiding: OAG vs Guide Scope new [Re: CounterWeight]
      #5598133 - 12/31/12 03:02 PM

Quote:

Wayne - just curious what that flexure was from, were you able to isolate the source? Glad to hear the OAG solution worked for you How long of guide exposures were you using? I'd be interested to see a 15 minute sub from a DSLR - I don't visit that forum.




Hi there,

No I never isolated the problem. I tried tightening everything down as much as possible but no matter what I did, the image would eventually slip out of the frame over the course of the night even when I took short enough exposures to keep the stars tight. (Typically 60 to 120 seconds) It was so frustrating.

Given the moonlight and cold last night I didn't get to perform a similar test but I am going to assume that since the stars stayed round in 15 minutes then the image isn't going to move on me once I start taking lots of subs - fingers crossed.

The picture I took last night was hot with light pollution but the stars looked good. I'll post the image if I get a chance to boot up the laptop later.


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ChubbyNinja
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Re: Guiding: OAG vs Guide Scope new [Re: Peter in Reno]
      #5598139 - 12/31/12 03:04 PM

Quote:

Wayne,

I have Hutech OAG-5 with Lodestar and they work great with my C-8 EdgeHD. ...
Peter




Hi Peter,

So what is your process for finding the guide star? Do you locate your target then rotate the OAG around until you see something on the display? I notice that there are some additional adjustment screws on the OAG but I'm not sure what they may be used for and was wondering if you have to use them at all in your experience?


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Peter in Reno
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Re: Guiding: OAG vs Guide Scope new [Re: ChubbyNinja]
      #5598190 - 12/31/12 03:28 PM

Hi Wayne,

I have been very fortunate because I have not had to hunt for guide stars after framing an DSO with imager camera. Thanks to Lodestar's high sensitivity, at least one guide star is always in Lodestar's FOV. I highly recommend to get a Lodestar. My scope is C-8 EdgeHD at 2000mm focal length.

I orient my OAG so the guide port is parallel to Dec axis. I first focus on a bright star with imager camera and Bahtinov mask, then move the mount's Dec axis until the same star is in OAG's guide port, focus the star using Helical focuser. The sharper the focus of guide star, the brighter the stars are and the easier to find guide stars. If you are imaging with C-14 (from your signature) which is a very long focal length scope, you might not have as good luck as my C-8 due to much narrower FOV from C-14.

First loosen the two screws at the top of OAG, move around the OAG guide port until you find a guide star and tighten the screws. If you can't find a guide star, then slowly rotate the OAG at the scope side by loosening three screws until a guide star is found and tighten the three screws.

Peter


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Peter in Reno
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Re: Guiding: OAG vs Guide Scope new [Re: ChubbyNinja]
      #5598245 - 12/31/12 03:51 PM

Quote:

No I never isolated the problem. I tried tightening everything down as much as possible but no matter what I did, the image would eventually slip out of the frame over the course of the night even when I took short enough exposures to keep the stars tight. (Typically 60 to 120 seconds) It was so frustrating.




60 to 120 seconds guiding exposures?!?!?!? No wonder the image drifted off. Typical guide exposures should be less than 5 seconds.

It's usually next to impossible to find the source of the flex between guide and main scopes. If you are trying to guide a C-14 EdgeHD even with mirror locks engaged using guide scope, you will likely to have differential flexure not because of mirror might flop, but the focal length is very long and OAG is usually the the best method.

Peter


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ChubbyNinja
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Re: Guiding: OAG vs Guide Scope new [Re: Peter in Reno]
      #5598303 - 12/31/12 04:24 PM

Quote:

60 to 120 seconds guiding exposures?!?!?!?




Oh sorry I meant my images were 60 to 120 second exposures. The guide camera was set to 2 or 3 second exposures. : )


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Peter in Reno
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Re: Guiding: OAG vs Guide Scope new [Re: ChubbyNinja]
      #5598313 - 12/31/12 04:29 PM

That's more like it.

Peter


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Peter in Reno
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Re: Guiding: OAG vs Guide Scope new [Re: Peter in Reno]
      #5598608 - 12/31/12 07:17 PM

Hi Wayne,

There is a cool feature in some planetarium software called Field of View Indicator or "FOVI" which allows you to plan ahead for suitable guide stars when using OAG. Take a look at the old CN thread at:

The Sky FOVI

Do a search for "FOVI" in Google and you will find useful information.

Peter


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mewmartigan
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Reged: 07/02/08

Re: Guiding: OAG vs Guide Scope new [Re: Peter in Reno]
      #5610198 - 01/07/13 01:00 PM

The Lodestar and the SBIG STi seem to be the top guidecameras for sensitivity.

I had switched from piggyback to OAG just to help reduce weight. A side affect was apparently better guiding even though I didn't think I had flexure.

I usually average about 30 10 minutes frames in a night. With the guidescope, my stacking software threw out 5 or 6 frames...first time with the OAG it threw out 1 so I was happy.
I have the SBIG STi and like it....I bought it withe the SBIG OAG for the 8300 cameras. It's a nice package but a much better deal if you buy the bundle with the camera.


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