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Advice needed on tracking issue

I have been using a bit old eq5 mainly for observing. But I want to develop my astrophotography skills.I think I manage fairly well to align the scope and level the mount. But I have run in to this problem that I would appreciate advice on:

* I adjust the scope with drift alignment so that it looks like I can keep a star centered + 5minutes.

* But when I look closely it seems the star I have centered is first drifting slightly, very slowly, up to a certain point and then it drifts back to the original position.

* This is confirmed when I take a photograph. If the exposure time is around three minutes the star will trail and form a V- or U-shaped path.

I am not sure what kind of error this looks like. I hope it is that I am simply not careful enough when balancing or that the polar alignment needs to be improved. But I fear it has something to do with the mount and perhaps the quality of the worm gear.

I would be glad for any advice on this. Best Sten
Posting the picture and scope used may help. Also if you are looking for 5' plus for subs, you'll need to use an autoguider.
Blessings and clear skies, Bill Celestron CPC 1100 DX EdgeHD Celestron DX Pro Wedge w/tripod Celestron x.7 Reducer Starizona Hyperstar 3 Meade 10"@ f/6 Dob Celetron 80mm w/ Nexguide Orion SSAG w/ ST 80 Cannon EOS Rebel T2i
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Hi Sten:

This sounds like periodic error in the gears. This is common to one degree or another in nearly all mounts and you need to use autoguiding to correct for it. Autoguiding is where you use a second camera and scope (usually a smaller one) to allow computer software to monitor a guide star and send correction commands to the mount if the star drifts.

How old and what brand is your EQ5? It may or may not support autoguiding.


- Dan 



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Sten -

I agree with Dan. Definitely periodic error. You have two options. Like Dan, I recommend autoguiding if your mount supports it because it handles periodic error as well as other tracking issues very nicely. The other option is to use periodic error correction (PEC). But if your mount doesn't support autoguiding, you probably can't do PEC either.

So, that's the next question: Does your mount support autoguiding or not. I think the best way to tell is to look for some RJ plug holes in the mount (i.e. a place to plug something like this in: your mount has a few of those, there's a good chance it supports autoguiding.
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Oh, and there's actually a third option I forgot about: Shoot lots of frames and just throw away the ones where you have an objectionable amount of periodic error.

That's not an ideal fix, but it might allow you to get some decent images without having to buy any more gear.
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It sounds like you are suffering from periodic error combined with some residual error in polar alignment. There are lots of threads here that discuss both issues.

Periodic error is due to imperfections in the right ascension drive gears. They make the mount move faster or slower at different times rather than steadily. Since this behavior repeats every time the gears go through a complete cycle, it is called periodic error. There are two ways to reduce periodic error to an acceptable level.

Many mounts have PEC (periodic error correction). This allows you to store in the mount's controller correction signals to offset the mount's intrinsic periodic error.

The other way is autoguiding. This requires a guide scope and camera attached to your main scope. When the camera on the guide scope sees drift, it sends correction signals to the mount. Of course, the mount has to be able to accept and act on these signals. Autoguiding will also correct for small errors in polar alignment. Orion Telescopes and Binoculars sells a good autoguiding setup called the Magnificent Mini. Note that it requires a computer to detect drift and generate the appropriate correction signal.

Did you use a reticle eyepiece during drift alignment? An eyeball estimate of center isn't good enough. I like Celestron's CrossAim. It has good eye relief and is battery powered.

If your mount is old and relatively inexpensive, it probably has neither PEC nor an autoguiding port. I am in that predicament. Using a 300 mm telephoto lens as my astrophotography "telescope", I cannot expose for longer than 90 seconds without getting oval stars.

Shoestring Asronomy sells a cheap kit to add an ST-4 port to a popular hand controller made by Synta and sold by Celestron and Orion. Part number is GPKIT-EQ. Download their instructions to see if you have a suitable controller and are up to installing the kit. They have suggestions for other controllers that can't use this kit.
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Your description of the image sounds exactly like what I get when doing a photographic polar alignment. The direction of the return trail for the V, relative to the starting trail will tell you which way the mount needs to be adjusted. The size of the V's opening is an indication of how far off the polar alignment is.

Unfortunately, you're not trying to do a polar alignment. The V shape is a direct result of tracking error. You can verify this by taking a series of exposures and combining them WITHOUT aligning. You'll need at least an hour's worth and your star trails will make zigzag patterns. The width of the zigzag (E-W) is the magnitude of the periodic error, while the height (N-S) shows the polar alignment error. Starting a picture like this just W of the meridian is the best place to get an image of your polar azimuth error.

The best way to run your mount is using the ASCOM platform and EQASCOM (aka EQMOD). If you have a webcam, you can use it and the other tools that come with EQMOD or PhD to generate a PEC curve.

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Alan has written a 'How To...' here
it shows how to diagnose and measure drive errors, skip the maths if you like.


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