Guided short focal length refractor - drift alignment and PEC needed?

[Andyb90]

Hi all,

I'm starting guiding with my 80ed refractor and found this post on SGL, looks like a very comprehensive post.

http://stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/

However, based on other astrophotography articles I've read and other posts I was a bit surprised by some of the statements about imaging with a short focal length refractor.

"Personally, I skipped the middle bits and went straight to guiding! Polar alignment of my NEQ6 mount consists of putting the tripod on some painted marks on the patio and spending five minutes using EQMOD and the polar scope to do a rough alignment. I don’t drift align and I don’t use PEC; I rely on my guiding rig entirely."

"With a short focal length scope for (say) ten minute exposures, a polar alignment done with just the polar scope will be fine. If you’re imaging with a long focal length scope and going for hour-long exposures, then you may well need to drift align to avoid field rotation."

I'm interested in starting with say 5 minute exposures, but thought you need very accurate polar alignment and PEC to achieve good results even when guiding with a shorter focal length.

Would be good to get peoples experiences on this topic and whether you use drift alignment and or PEC for a guided setup with a short focal length refractor.

Andy.

[Andyb90]

Forgot to add, I'm particularly interested in this as I don't have a permanent set-up, so have to assemble everything and have been trying to get very accurate polar alignment every session.

Andy.

[johnrt]

Neither, I polar align with just the polar scope in my AZEQ6, no drift align or PEC, as I tear down and set up each session. That is good enough to image 20 min+ guided subs at a focal length of over 900mm and under 1arcsec/pixel, a longer focal length and tighter ratio than you are proposing.

I'm not dismissing the importance of good polar alignment, but there is no point in wasting time going over the top. Experiment see what works, if the polar align is good enough straight from your polar scope then get on with the imaging!

[Matthew Beardsworth]

Same for me. I use the excellent Eqmod polar scope alignment routine which only takes a few minutes. 30 minute exposures are no problem at 480mm.

[ollypenrice]

I find that it's the sub length which becomes the limiting factor with an imperfect polar alignment. Five minutes is not demanding but half an hour certainly is. If your sky doesn't allow very long subs anyway, and you use a stacking software which operates x-y plus rotation (most do) then you may even find that you get noise reduction benefits from slight misalignment at the cost of slight cropping of the edges.

Olly

[D4N]

It will depend how good your alignment is and where the guide star is in relation to your image.

I had an issue at one point where I had knocked the PA out by a few degrees without realising it. I got on target fine because Astro tortilla just compensated for it and the guiding worked fine too. Short subs were also fine but then I went to 20 mins I had a lot of field rotation, the stars on one corner were perfect but the other corners were all arcs.

At least it was an easy fault to diagnose though as field rotation is fairly obvious.

/Dan

Sent from my iPad using Tapatalk

[Pompey Monkey]

I did a test guided 20 minute Ha exposure with my new camera last week. The mount was just polar aligned using the time (to within an hour, I suspect) and the polarscope. I had just the beginnings of field rotation at 675 mm FL and an image scale of about 1.3" per pixel.

My (slightly) limited experience suggests that you will run into other more significant issues before field rotation becomes an issue with

your set-up.

[Andyb90]

I must admit the thought of having to drift align every session was a bit daunting, so everyone's responses are encouraging.

Will have a look at the EQMOD routine. I also downloaded Alignmaster to give that a try too as it looks quite quick to use.

Andy,

[Owmuchonomy]

My personal understanding from this forum and also as a result of testing a mount by means of a detailed protocol received directly from Synta is that PEC should be disabled during guiding.