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Quote:I admire others who have had the steady nerve and patience to do long exposures this way. I don't think I could do it.
Quote:Hi Tim,When I drift aligned using a reticle eyepiece I used 200X (10mm reticle eyepiece, 2X barlow, 1000mm focal length). These days though I drift align with my guide camera and scope, using PHD. I first make sure the guide scope is aligned with the imaging scope, then focus the guide scope on a star, turn on the fine grid overlay in PHD, start looping exposures and rotate the guide camera until one axis of the grid aligns with the RA motion of the mount. So I'm drift aligning with the same FOV that I will be guiding with, and using the same software. PHD's guiding precision of 1/4 pixel is better than my visual precision, but if I can see no drift in 5 minutes, I am satisfied that there will be no perceptible field rotation in 5 minute subs.
Quote:Your CGEM mount has a fine polar alignment routine that is good enough for at least 10 minute guided exposures. My suggestion is, donâ€™t guide with a SCT. It is a bad, bad idea. Also use a scope with larger aperture to shorten the exposure time and to increase S/N. 66mm is too small. It is like imaging with binoculars. It is almost funny.
Quote:Good guiding starts with very good polar alignment. Without it, the best guider in the world won't be worth a hoot. Second is balance. You've got to ensure that the RA axis worm gear is continually engaged. Third is flex. If your system has inferior rings and/or dovetails that are overtaxed by the eqipment, it's not going to guide well.David
Uncle Rod Uncle Rod's Astroblog: http://uncle-rods.blogspot.com/
Quote:That's interesting. I may have to try that using PHD. Another question for you (or anyone) if you don't mind. Do you typically guide with Declination guiding on auto or do you guide in one direction only? If in one direction only, what's the best way to determine which direction you need to guide in (North or South)?
Quote:............1) Software that didn't make me think to hard. That's why I wrote PHD in the first place.............
- Jared Willson
Quote:What got me over the hump of consistently good guiding? I'll give you two answers--one long and one short.First the short answer: Money. I bought a better mount and virtually all of my autoguiding problems disappeared.Now the long answer: I struggled with a mount that was at the limits of its capacity for a little over a year. I had no problems with differential flexure or mirror flop since my camera is internally guided. Ultimately, what it came down to for me was that my mount, when loaded with 22 pounds of imaging gear, simply didn't track that well. There were shifts of 1-2 arc seconds in RA that occurred over time frames that were too fast for an autoguider to keep up with. I was able to make incremental improvements through adjusting the worm/wheel meshing, adusting the couplings between the drive and the worm, making sure the mount was well balanced (meaning a few ounces East heavy), etc., but I never got the results I thought my scope was capable of. FWHM numbers simply weren't where I wanted them to be. Then I received my ridiculously expensive, very high quality mount and all my guiding problems went away instantly. When using my old mount, I typically got RMS errors between guide exposures of 0.4 pixels to 0.6 pixels with my refractor (1.2 to 1.8 arc seconds). I put that same scope on the new mount, and the first night out--a night of particularly good seeing--I got RMS errors between guide exposures of 0.02 to 0.1 arc seconds. The high end mount took away the struggle.Can you successfully autoguide with a mid priced mount? Sure. Can you autoguide well with a mid priced mount? I never did, but I have seen plenty of evidence that others do. How can you get truly good autoguiding results with a minimum of effort? Buy a better mount and don't overload it.
Quote: I learned after the fact that I shouldn't expect low PE out of the box with a mount in this price range. I confess I was disappointed to learn that but I see plenty of folks getting good results out of this mount class and hopefully I'll get there (with a few frustrations thrown in to keep me humble I'm sure).
Quote: I thought that polar alignment wasn't important for a mid-range mount
Quote:PE is highly overrated since, for a mid-range mount, there are other noise sources that have to be chased
Quote: Could you please quantify how the good polar alignment should be. For a good autoguider, the 10' or 15' polar misalignment error is not a problem.
Quote:A mount with a low PE will most likely have 'other noises' low too and the opposite is true as well.
Quote:....Could you please quantify how the good polar alignment should be. For a good autoguider, the 10' or 15' polar misalignment error is not a problem.....
Quote:Here's an applet that can help you answer that question.
Quote: Quote: I learned after the fact that I shouldn't expect low PE out of the box with a mount in this price range. I confess I was disappointed to learn that but I see plenty of folks getting good results out of this mount class and hopefully I'll get there (with a few frustrations thrown in to keep me humble I'm sure). I wouldn't worry too much about PE, but I would aim to train the PEC using the PECTool and 8-10 turns of the worm. You should be able to do well with this mount, and things will be much more forgiving with a shortish refractor.Keep an eye on the shape and size (arc-seconds) of the stars you get in raw, 5-15 minute exposures. If stars are oblong, determine if it is flexure or a guiding problem. Make sure your focus is tight.A mount like this can be guided well, but it does involve "chasing the mount" - which is a good thing to do. This means low-latency and short guide exposures, with prompt, well-tuned corrections. Keeping the weight down may help, and a lightweight OAG with a lightweight SX camera has an advantage over guidescopes and much heavier cameras. I don't think you need OAG to guide a refractor, though.Frank
Quote:How does PECTool work?
Quote: Quote:How does PECTool work? Hi-You can get it (free) from the celestron site. It should work for your cgem. You just hook it up and tell it the number of worm periods you want to average. Then you start autoguiding and it logs each period, then averages them and loads them into the mount. It's very simple, but effective for my cge. I do 8-10 worm periods on a night with decent seeing. I find the resulting PEC log lasts a long time.You do want to have your guiding tuned pretty well when you do it, though.Frank
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Quote:I tried more aggressive guiding Friday night starting with 2 sec exposure times and 50 percent guide rates and going down to 1 sec exposures and up to 80 % guide rates. Didn't seem to make any difference.