Drifting image while guiding is working

[acarleton (Aidan)]

Hi all

I have bought a televue 2X power mate and attached my DSLR and it to my 250 f4 newt. i have also been guiding with a separate guide scope. i discovered a bit of an issue and wanted to see if anyone else had this problem. as my scope would track a target over the night it would drift in my cameras field of view. the guidescope was still dead on, just the image in my DSLR. the only thing i can think of is that the weight of the rig is causing it to slightly move as the scope becomes more vertical. i have checked out the equipment and it seems very solid but i guess it would only need to move a little to have an impact on the image. has anyone else had this issue? if so how have you overcome it? is it because of the poor quality of GSO focussers ?

FYI, weight of the powermate and camera is 1.1 kg

Thanks for the help

[astro744]

The drift is due to not being 100% polar aligned. The reason you are seeing it in your main telescope and not your guide scope is because the effective focal length (E.F.L.) of the main telescope with 2x Powermate attached is much greater than that of the guide scope.

You have 250 x 4 x 2= 2000mm E.F.L. for the main telescope. What guide scope and eyepiece are you using?

[Amaranthus (Barry)]

Could be differential flexure too? http://astronomy.mdodd.com/flexure.html

[acarleton (Aidan)]

I think the issue is differential flexure (i just had no idea what it was called). i was not looking for long exposures, i am trying to capture saturn for a full night and track the progress of the moons, last nights test run make me think this will be a good time lapse. so even though i am using a small guide scope (orion 50mm) if the issue was polar alignment then i would expect blurry long exposures, however, what i am seeing is a consistent drifting over time in a single direction. I did a test of connecting my powermate and then my laser collimator and watching it as i lifted the scope to a vertical position. the laser did move slightly, so with the camera it will be a bigger issue. i will look at trying to brace the camera and see if that fixes the issue. an OAG would also solve the issue i guess, but i will wait till i get a QSI ccd with it in-built

[BlackWidow (Mardy)]

If you are guiding on a star, but looking or imaging a planet, you will get drift. This is due to the fact that the planet is moving and the stars are not.. Not totally true but best for this explanation. You are able to track relative to earths rotation but tracking another planet changes this. In some scopes you can change the tracking to allow for planet work and you then need good polar alignment... I don't use guiding with my guide scope for planet work...



Regards
Mardy

[acarleton (Aidan)]

last night i aligned my guidescope to Saturn and looked through the main OTA and could not find it. it looks like the explanation for the drifting image is the fact that the guidescope has significantly twisted during the course of a session. i did not expect that.

[Nico13 (Ken)]

Hi Aiden,
There is also another thing that may be in play here if you are using EQMOD.
This was a drift issue that was only ment to be a problem with earlier version of the EQ6 mount but I have been plagued with it and my mount is a later only two years old mount. My symtoms were the same as you describe and I'm guessing the drift was mount slow, images drifting up when in the East or down when imaging in the west.
EQMOD went to the trouble of putting a drift compensation adjustment into their software, you will find it in the bottom right hand corner of the expanded setup window.
The recommendation was to set it at +3 for the older mounts and that seems to work to improve things for me so until you go off axis guiding try this setting and see if it helps.

Nico.