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HEQ5 tracking issue


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I posted recently about my problems setting up polar alignment. It didn't work 1st time I tried, and went through the process again - and realised that while I'd aligned the polar scope to the mount the previous month, I'd missed out setting the setting circles (a month between setting the scope up and getting a clear enough night to try the alignment). So, I managed to push the subs up to 3 minutes, with M42 staying in position with only slight drift, and had the joy of pointing the scope at Mars and have it stay in the centre of the eyepiece/TV monitor (even with a 5x barlow for 2 mins).

However, I noticed that at 3 minutes,there was some obvious drift on my subs. When I went to process these, I had a closer look, and what I saw was that there was drift - but the frames drifted in two separate directions - some would be going 'up', some at 90 degrees to 'up', and some showing a slight curve between 'up' and 90 degrees. Using sigma-clipping, these subs are usable, but I'm wondering what is causing this.

Is this 'periodic error'? Or is it my alignment? Or is there a fault with the mount/motors? It is an HEQ5, bought second hand and about 2 years old. I do have a synscan upgrade, which may help - but is this the point at which I need to start looking at autoguiding?

M.

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Mechanically the HEQ5 is a very sound mount.

Although I have little experience everything that have read suggests that tracking is ok for exposures of a minute or two but anything beyod that needs auto guidance.

If there is no slipping in your mount it is probably just a tracking/guiding issue.

Is this 'periodic error'?

I think I am right in saying that if you use EQMOD then that comes with Periodic Error Correction.

There is my pennies worth but I am sure someone with a little more knowledge will be able to give you a more detailed answer.

Regards

Marc

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'I do have a synscan upgrade, which may help - but is this the point at which I need to start looking at autoguiding?'

Yes!

I continue to insist that the wording of most blurbs on PEC are dishonest in implying that it will allow long exposure astrophotography. It won't - unless you have bought yourself a six grand mount like a Tak or an AP.

It strikes me that you have done extremely well to get as far as you have without autoguiding and you won't get much further. The errors sound like a mixture of periodic, polar misalignment and backlash.

Time to autoguide!

Olly

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OK, thanks. I'll do the synscan upgrade, let that bed in, then start looking into how I can get to autoguide. I have an Opticstar PL150C, which when coupled with an EQMOD box is supposed to be capable of autoguiding. I wanted to bypass the EQMOD box, and hook the synscan handset up to the laptop and autoguide that way - but I'm not sure how to do that with the Opticstar. I've been putting off getting to grips with ASCOM and autoguiding, I must admit.

I have to say that getting the alignment done proved very useful with planetary imaging - I could place Mars or Saturn in the middle of the CCTV image on the TV, using a 5x barlow, and it would stay put for at least 5 minutes. However, I did notice that nudging it back to centre did result in a bit of backlash. I don't have much scope for balancing with the ED80, as the dovetail is a fixed foot-type, which just sits in the shoe without much leeway in either direction, but seems pretty balanced, although I guess the addition of the DSLR could throw the balance out - while one counterweight right next to the mount seems to be the closest I can get to the counterbalance. Maybe I need to try without a counterweight - or get a smaller weight? All I have are 2 x 5kg, and the Onyx weighing appx 3 kg I'd have thought one 5kg close to the mount head should be sufficient.

Thanks again,

M.

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