Drift Align Problem

[dave1944]

Hi,

I have an EQ6-R pro mount with an 8" SCT, mounted on a solid pillar in my observatory. I am also using "The Sky Pro" and not the handset. I have ensured that there is no backlash and that the assembly is properly balanced. However, no matter what I have tried I cannot get good polar alignment. I am using the main camera and following the correct drift align procedure. When pointing south I have no N or S movement for at least 10 mins, and when pointing east the same applies. However, in both these positions I have easterly movement which I have been unable to erradicate, and therefore startrails when imaging unless I use PHD2 guidance.

Any help would be very much appreciated.

Many thanks

Dave

[dunc]

Hi Dave, 

What software are you using to polar align? I use PHD2 v2.6.5dev5 for guiding but it also has three polar alignment routines built in which are very accurate , one is drift alignment but the other two work on the stars around the pole. SharpCap 2.9+ also has a really good polar alignment tool also working on the polar star field. I get PA of around 1" to 0.5" using SharpCap 3.x and my main camera, well worth £10  and I don't have to slew the mount east and south so its dead quick and fairly foolproof (which is good for me!).

HTH

Duncan

 

[kens]

Polar alignment error causes drift in declination. If you are getting movement East-West in right ascension then it is most likely periodic error in your mount. Autoguiding is the best solution to that.

[dave1944]

Thanks Duncan and Ken.

I've tried the PHD2 align and I get the same result.

I don't think it's periodic error because I always get the same drift regardless of what position the mount is in.

[Laurin Dave]

Maybe the mount is running slow... my AZEQ-6 does when heavily loaded. Try measuring PE with PHD2 and see if the trace remains centred or drifts.

Dave

[newbie alert]

How are you guiding, with the normal guidescope or oag?

[dave1944]

To guide I use a normal guide scope and it keeps the guide star within a couple of seconds which is fine. However, because of the drift the stacked images need to be aligned. I know my final images are usually ok but it should be better.

If/when I get another clear night I'll measure the PE as you suggested.

[ollypenrice]

Is your mount set to track at sidereal rate? Thee are other tracking rates to which it might default.

However, and this is a big 'however,' there is nothing abnormal about having to guide. All mounts need guiding though some expensive ones are 'encoder guided.' (Whether this adds to the complications or subtracts from them is a very moot point.) An 8 inch SCT has a long focal length and will certainly need guiding - and guiding well.

Uniform star trailing does not indicate bad PA, it indicates bad tracking (probably due to some periodic error.)

It is also absolutely normal to need to align your sub exposures when stacking. It would be a miracle if you didn't have to.

Here's how to test your PA properly: do a long run of subs while autoguiding. We'll assume that you're getting round stars in these subs, guided. Take just the first and the last of these subs and ask your software to align them using the stars, as is the normal method. Now look at the position of the frames of the first and last subs. Are they still aligned or are the image edges rotated relative to each other? Any misalignment in the edges arises from field rotation which can only arise from inaccurate PA. It has nothing at all to do with tracking accuracy.

I have a mount which is extremely accurate with excellent PA, low periodic error and responds superbly to guiding. None the less, there is a very small field rotation between subs taken five hours apart. This is not really avoidable, given the size of a pixel!

Olly

 

[dave1944]

Thank you Olly that's something else I can try when it's clear.

Dave

[newbie alert]

Sorry just re-read what you said, are you saying you get drift when guiding or drift when not using a guider.

With a normal guider you may get flexture,mirror flop which will disagree with each other causing trails in the image..in my opinion,the only way to guide a sct is with a oag,which might cause issues in its own right setup wise..

[dave1944]

I get drift when not guiding.

I always use a guide scope to track and the images are usually acceptable, albeit there is slight misalignment between images.

I'm probably asking too much from the system.