Hi folks
I'm experiencing a lot of periodic error.
When using an illuminted reticle i notice that when the scope is pointing lower in the sky i get worst drift than when i am pointing closer to zenith and South Celestial pole.
My mount is a CG5 with dual axis motor drives installed (non computer goto) and drift along the east west line in general is not that good even after having the mount striped and modified. Is there a way to improve on the error for astrophotography without me having to buy a losmandy mount?
Carl,
I'll kick this off then.
You are certain it is PE, and not simple poor polar alignment?
Can you explain the "drift".
Lastly assuming it is PE, and that you autoguide, maybe the guiding will correct most or all of it. Yes, better is better, but if the autoguider does it's thing then PE shouldn't be the issue some feel it is.
Gary
Great circles are larger in diameter near the celestial equator, so celestial objects appear to move more rapidly. Nearer the SCP, the circles are smaller so objects appear to move smaller distances in the same amount of time. This is why tracking and auto guiding is usually “better” nearer the SCP and “worse” near the celestial equator, regardless of mount.
PE used to be a problem that is now solved by autoguiding. Autoguiding will solve even bad polar alignment, but at longer exposures will still lead to image rotation. Autoguiding used to be expensive so the PE of a mount was important.
What are you trying to do?
Please don't get too hasty in condemming your mount. Stars within about 15 degrees altitude can cause inaccurate settings due to atmospheric refraction. To start the alignment process I usually choose stars within 10 degrees of transit that are very close to the celestial equator (eg. 34 Alp Aquarii, 55 Zet Aquarii, 62 Eta Aquarii). To aid in my polar alignment I have compiled a list of about 60 bright stars with right ascensions from 0Hrs to 23:59 and within 0.5 degrees of the equator - if you like I could post the list here. Some stars are perfect for fast checking polar alignment. (eg. TYC 4766-1369-1 in Orion RA = 5h 34m 29.3s, Dec = - -0° 0' 44" , mag = 6.4)
Carl have you mapped the error using something like K3CCDTools? Is the error cyclic or or oscillating or is it more or less continuous drift in one direction? If the error is greatest near the east or west horizon, I would be starting to think it might be incorrect altitude adjustment.
I have the same mount. Its the 2 brass drive gears that mesh thats usually the problem, their just not made well enough. My RA axis speeds up & slows down, so its a rhythmic thing. Manual guiding & corrections might be the only answer perhaps.
Thanks for the replies
Gary, Bert and Paul i notice your replies include Autoguide ad K3CCD tools.
Unfortunately my scope does'nt have computer control. It's all manual guiding in my neck of the woods.
Any further suggestions welcome?
Regards Carl
You can still use programs like K3 (I'm not sure about others) to manually guide. It has a graph that shows the movement of a star you select to guide on. To use it manually you just adjust your telescope drive axis to keep the moving graph centered on the 0 line. Easy as! When I was having difficulties with my autoguiding I spent a couple of weeks guiding like that. Sure beats looking through a reticle.
Sounds interesting
How do I get hold of K3CCD Tools?
I'm also noticing that after a while when guiding watching my star go east and west along the RA line with periodic error the star will simply keep going west and has sometimes gone out of the eyepiece view. This happens at different locations in the sky. Any idea what's happening and how to fix it?
You can pick up K3CCDTools here . Got to "My Software" and select "K3CCDTools3. If you scroll to the bottom of the page you will see there are two versions. V1 and V3. V1 is free but you have to get a new key every couple of months. It's not a drama but just so you're aware of it. V3 has a 35 day free trial but to use it after that is $49 US. I have found it to be worth every cent.
You can use V1 np, just make sure your webcam is fairly square in the eyepiece holder. It doesn't have a lot of the features of V3 (naturally ) but it is still very usable for your situation.
Sounds interesting
How do I get hold of K3CCD Tools?
I'm also noticing that after a while when guiding watching my star go east and west along the RA line with periodic error the star will simply keep going west and has sometimes gone out of the eyepiece view. This happens at different locations in the sky. Any idea what's happening and how to fix it?
Carl
If the star goes west, it means the RA drive has slowed down or stopped. Have you checked connections? You could also try switching the RA and Dec motors--I think they are the same once you get them off their mounting plates, but you'll need to check this.
Just as encouragement, I have one of these CG5 mounts and it works quite well after stripping and cleaning. I can take 30s unguided subs with 1200mm fl and keep about 60-70% of them, but I needed to get polar alignment spot on and also do a lot of work in getting the worms adjusted; if they are too loose, backlash is terrible and if they are too tight you get strange movements when you use the hand pad for guiding--the scope moves in the wrong direction for a tad and then reverses and goes in the right direction. The best compromise still leaves annoying backlash, but not so bad that you can't live with it.
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