Quick guiding test - m63

[JohnP]

Popped out on Monday night for a few hours primarily to do a guiding test. I have had issues with drift for the last couple of years that has been really bugging me. Basically, my guiding all works OK but the whole image tends to drift in a certain direction so when I look at first & last sub of a set there might be as much as 10-15 pixel shift over a 2hr session. This isn't a big problem but as I only image with a 640x480 sensor it does tend to make stars mis-shapen which pee's me off...!!! Anyway, I have tried everything to fix problem including different scopes, mounting method, rings, guide progs.. Anyway on Monday I decided to give drift alignment a go. So I downloaded WCS for webcam & gave it a go. Seemed to do the job really well & although my altitude (up/down) polar alignment was pretty much spot on my East/West (azimuth) adjust was off & took a few iterations to adjust. I always just used polar scope & whilst I get close I think the problem was my scope was still having to make corrections in Dec when guiding which has been causing me probs.

Anyway after adjust tried following image for quick test. 15x300secs (crap skies) - CLS filter - ED80+Atik16ic. Although, I still did get some drift it was a lot less than normal & the thing I am impressed with is stars are really round I think with a few more practices it'll only take me another 15mins or so to do the drift align so I will start to make it part of my standard setup as worth it to get nice round stars. By the way I also used Guidemaster software for guiding. It worked faultlessly, was a breeze to setup & works great with my modified webcam (much better than PHD does)..

Cheers,  John.

[Greg]

Just spotted your image on the forum. Why is there a bright 'star' at the galactic centre. You haven't captured a supernova have you?

[JohnP]

Hi Greg... I wish  I think it's just the core of the galaxy maybe stretched a little too far - Cheers,  John

[MarkS]

John,

Glad you got the hang of drift alighnment - I have never taken the time to do so!

However, look at this problem from a different angle: suppose your guide star was in the middle of the image, then there's no way the image could drift even if your polar alignment was not precise - the guiding would not let the guidestar shift by 10-15 pixels so none of the other stars could drift either.  Unless of course there is a differential shift between imaging scope and guidescope (flexure).

In my opinion your problem is either that the guidestar is well outside the imaging area or you have flexure.  Or a combination of both.  Accurate polar alignment will certainly reduce the former.

As part of my routine when doing my 1-star alignment, I always align the guidescope precisely onto the same star.  Then when imaging I try to choose a guidestar that requires little or no adjustment of the guidescope.

Mark

[JohnP]

umm thks Mark I can see what you are saying. I am sure it is not flexure - everything is really solid. I have scopes mounted side by side on a thick dual mount bar like this http://www.scopesnskies.com/magnify/1056.1.html. I use non adjustable scope rings. I am pretty sure both scopes point to same part of sky (very close) but will double check next time but like I say I have no way to independently adjust one against other. The drift align has certainly helped though.

Out of interest do your images always stack straight on top of each other... (excluding your dither settings)

[MarkS]

Out of interest do your images always stack straight on top of each other... (excluding your dither settings)

Nowadays, yes, except that sometimes there may be a very slight field rotation which will be due to polar alignment being slightly off.  Before I bought the QHY5 guide camera I had to search further afield for a bright enough guide star and this did cause a bit of drift combined with rotation.

You are quite right to tackle polar alignment.  When the polar alignment is off then the whole image will rotate around the guidestar (and this is without any flexure).

[JohnP]

Cheers Mark - I appreciate your help/ comments - I have a few more things to check but think I am moving in right direction...

[Mac]

Have you also had a look at Iterative alignment?
its a bit quicker then drift.

Select a star,
Sync on it
slew to polaris,
Polaris should be dead centre, if not reduce the distance error to about half
by using your adjusters,

go back to the first star, resync and repeat,
the error should disappear after a few goes.

Use a barlo for better accuracy.

Then have a look a pole align max,
its a nice bit of free software that does roughly the same.

Mac.

[Rocket Pooch]

Its impossible to get polaris on my EQ Mount would not work for me.

[JohnP]

I've been talking with Mark & analyzing my images & it is evident that there is NO rotational content to the drift so definitely pointing towards flexure although I still can't see where the flexure can possible occur myself. Anyway MR. 'Logical Head' Mark came up with a deceptiively simple test to see if it is flexure. Just point/centre imaging scope at a star then make sure guide scope is centred on exactly same star. Then use hand control to move scope forward a couple of hours in RA centre the imaging camera on another star & see if the same star is in the centre of guide scope camera again. If not then I have a flexure problem & it'll be time to test an OAG.

Will let you know result which I will hopefully test tonight.

Cheers,  John.