Oval Stars

[astro mick]

I have recently started using a 8x50 finder scope as a guider,and since adjusting the settings in the Brain,to around 2000 ,i was getting good results the other night.However last night i tried for high up targets in Draco,the calibration usually completes within 27 steps,and kept the guide star bang on the cross wires.After a 10 min exposure,my image was really dissapointing.The stars were slightly oval,and spoilt the image.I have an omni 120mm refractor with a 80 ed piggybacked ontop,well within the cababilities of the EQ6.Since i have a lot of cables,dangling about,although i have tried to secure them,although difficult with a long refractor,as some slack is required by the nature of the mount.

Do you think the Finder is not upto the job,of guiding a long scope at high altitudes.Or do you think there is flexture creeping in some where.

Sorry for the long post,just trying to explain things.

A 5 min exposure just starts to show the stars becomming elongated.

Cheers. Mick.

[Ad Astra]

Hi Mick,

Sounds like an alignment error to me. If the alignment of the mount is slightly off in either altitude or azimuth from true polar alignment, then tracking over a period of time would begin to show image drift... hence oval stars.

Check your polar alignment before trying anything else. It is the easiest to fix and usually the first thing to check.

Dan

[brantuk]

Don't forget that even if you are guiding the Earth is still rotating under the mount. So it really needs accurate polar alignment for a 10 mins exposure or you'll get star trailing, or in this case, slightly oval stars.

[astro mick]

Thanks for the replys guys.

I thought my alignment was pretty good,because guiding at F7.5 produced round stars,its only since using the finder,that oval stars have crept in.

However i will check again.

Cheers.

Mick.

[Earl]

Thanks for the replys guys.

I thought my alignment was pretty good,because guiding at F7.5 produced round stars,its only since using the finder,that oval stars have crept in.

However i will check again.

Cheers.

Mick.

I thnk i have that also, im going to look at the images i took last night in better detail later.