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Equipment Discussions >> Mounts

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ADBjester
professor emeritus


Reged: 09/27/10

Loc: Reading, PA
Re: Mount For AstroPhotography new [Re: locod1]
      #4554091 - 04/30/11 08:36 PM

Quote:

I was actually wanting to order the "Orion Awesome Autoguider Refractor Package" this weekend. Dont really know how it comparse to other systems in its price range tho.




Reading the older posts in this thread, I noted this one. Since your scope is a Schmidt-Cassegrain, I would advise against any form of piggyback guide scopes.

First of all, both products you mention have simple tube mounts with two tight screws and one loose spring mounted one -- just not stable enough. It will move microscopically in relationship to the imaging scope as the scope moves. (This is one form of differential flexure.) While some have tried with moderate success to use six-point Stellarvue guide rings to minimize this, I've got a KWiq, mounted in rings, with an 8" SCT... and I still had flexure.

Second, the inherent nature of an SCT is that the focusing is done via a mobile primary mirror, which means it's just not rigid enough inside the tube to retain its relative position to the tube (not microscopically, anyway). This is the other form of differential flexure, which simply cannot be overcome with better gear (unless you have a high end SCT with a mirror lock, like the Meade ACF or the Celestron HD lines).

To overcome flexure, you want an off-axis guider. Then, you're using the same light path for the guiding and the imaging -- the OAG can't move relative to the camera, and if you experience "mirror flop", the camera and the guider both see it.

I may throw another reply on after reading other posts, but I've seen some people suggest a CG5. Don't do it. For astrophotography, I saw you comment that you wanted to buy more mount than you need. You also saw the correct advice that (unless you're dealing with a $10,000 Astro-Physics mount), you shouldn't expect to exceed 50% payload for photography. With a 10" SCT in the wings, and likely some additional gear like an off-axis guider ($100 for entry level, up to $700 or more for Cadillacs like the Moag or the Taurus), your camera, your finderscope, dew shield if the dew point calls for it, etc, etc... my guess is that even the Atlas EQ-G or the CGEM may be "pushing" it (if indeed you want to "overbuy".)

Either is a great first mount for AP, but each has their quirks and learning curves and issues to overcome.

If you can stretch your budget into the low $3000 range, though, you absolutely cannot go wrong with the Losmandy G-11. I wish I'd saved up a bit longer and bought that instead of my CGEM.

Jester


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ADBjester
professor emeritus


Reged: 09/27/10

Loc: Reading, PA
Re: Mount For AstroPhotography new [Re: locod1]
      #4554122 - 04/30/11 09:03 PM

Quote:

Thank you all for the great input. I ordered a autoguider with the atlas mount I haven't been able to use it as of yet because I am still waiting on one more counterweight. After last night I also have a dislike for an EQ mounted Newt, the eyepiece seems to always end up in a totally impossible viewing position. On the other hand for AP it seems to be a dream compared to the LX 50. I used the autoguider package on my LX 50 (as it shipped almost a month before the atlas) and after 3 hours of drift alignment, plus autoguiding I couldn't do more than 25 sec exp. at F6.3. I assume the PE for the LX 50 is just more that can be guide out? Now last night i tryed the new atlas out for the first time, attached is a image I took. I am really excited with this capture, as I only did a quick alignment thru the polar scope. I did not drift or autoguide and this is a 60s exp. M51




That's not a bad 60 second unguided image of M51, especially for your relatively southern latitude.

Take it from someone that just went through all this.... the problem is not in your periodic error. It's probably not even in your polar alignment, though you can bet that if you're doing pure visual drift as a first-timer, you may be making errors. I still can't drift unassisted, and am happy that:

1) The CGEM has a built-in All-Star Polar alignment routine that will get you within one degree of the pole.

2) With a CCD there are ways to "drift" computer-aided, including EQAlign, PHD's companion, PEMPro, and "Straight line exposure".

If you're within a degree of perfect polar alignment, good auto-guiding will overcome the PE of your Atlas easily, for exposures of 5 to 10 minutes. However, if you're experiencing differential flexure (as I detailed in my last post), this can kill your session.

I've also had recent experiences with the mount not being adjusted for backlash (not familiar enough with the Atlas to know where it's settings for that are, but it needs to be done), and having backlash bite you even when adjusted because of balancing issues. You have to have a *slight* imbalance to keep a slight but steady pressure on the gears. If it's told by the guider to slow down or stop, you don't want momentum to carry the scope to the other side of that gear tooth. That motion isn't much -- about 3 small star widths on that mount.... but it will wreck your image.

Since the Right Ascension motion is always to rotate west, to "push back" you'll want a few ounces of "push" toward the East. This differs depending on where the scope is. If the scope tube is on the west side of the mount, move the counterweight more to the east (away from the scope) -- just enough to ensure that that's the ever-so-slightly heavier side. If the scope tube is on the east side of the mount, move the counterweight closer to the scope (again, to the east). Always move the weight toward the east, from perfectly balanced.

I suspect your inability to get beyond 30-60 seconds is either or both of the differential flexure in the standard Orion Easyguide package (mirror flop doesn't matter on your Newt), or a misbalanced scope, or both, and possibly could be to incorrect backlash correction settings as well.

Check all three, and upgrade the mount on your EasyGuide at once. The Stellarvue 50mm rings listed here are cheap (~$20) and far more stable than the spring-loaded junk that shipped with the Orion:

Stellarvue 50 mm finder mount from Skies Unlimited

Good luck!

Jester


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locod1
member


Reged: 03/12/11

Re: Mount For AstroPhotography new [Re: ADBjester]
      #4555986 - 05/01/11 09:40 PM

here is a image from the first time I used the auto guider Friday night
M 65 and 66
It is just one exposure of 4 minutes. I adjusted the white balance, but other than that it's just a raw image.


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