Why Guide

[Grierson]

Perhaps the following is naïve comment. My current setup is a SkyWatcher ED80 on a belt modified HEQ5 with either a ZWO 294 or ZWO 1600. (Both CMOS cameras) Having watched SharpCap's Robin Glover's presentation where he recommends lots of short subs rather than fewer longer subs I get to thinking that running a guide 'scope and software on my rig does not contribute significant benefit.  I typically take 30-40 x 180sec. subs.

All the control gear, ASIAIR, power distribution, focuser and guide 'scope are mounted on the ED80 and I WAS giving some thought to lightening the load by fitting an OAG. But then I hesitate and wonder if guiding on my setup is worthwhile. I should say that clear nights in recent times have not allowed any experimentation to answer my own question but on the occasions when I have not guided I see reasonably round stars. I wonder what more experienced imagers think.    

[Dr_Ju_ju]

Are those "reasonably round stars" in the same place within the frame? if not, then guiding should ensure they are, as well as getting round stars... 

[Grierson]

41 minutes ago, Dr_Ju_ju said:

Are those "reasonably round stars" in the same place within the frame? if not, then guiding should ensure they are, as well as getting round stars... 

I didn’t take much notice at the time but I assume Star Alignment in PI would have taken of the issue!

[kens]

There is a floor on practical sub length. Once they cease being shot noise dominant and become read noise dominant you need more integration time to reach a given SNR. And at some point the individual subs may not have enough SNR to stack. And you also get to a point where download time can be significant leading to more wasted imaging time.

So let's say that lower limit is 30s and your mount has a worm period of 600s and PE of 30" p-p. So every 300s it moves 30" and in 30s it moves around 3" which would be noticeable. 

[Grierson]

Thanks Ken. I’m afraid I will need a bit of time to digest your comment. John

[tomato]

I was wondering the same thing when imaging with an ASI 178. The smart histogram in SharpCap recommended 30 sec subs, so that was what I was using. However, as imaging time was precious I didn’t have the nerve to turn the guiding off for fear of losing the data.

[ollypenrice]

Round stars: what do you think round stars are telling you about your captured resolution?  They might be telling you that you have perfect tracking or, at least, tracking so good as not to impact on the final resolution. But they might just as easily be telling you that your tracking errors are about the same in both RA and Dec.  Our first Mesu gave round stars from the outset once we fired up PHD. However, they were large round stars and only when we had found the right guiding parameters, giving an RMS of about a third of an arcsecond, did they become small round stars, meaning that the mount was guiding well enough for the image scale we were using. (0.62"PP.)

Have you measured the PE of your mount? It needs to be less than about half the image scale at which you are working. Given that a standard HEQ5 might be expected to have a PE of about 20 arcseconds your belt mod is going to have to have had an almost supernatural effect for it to bring the mount anywhere near the accuracy you need. Thinking very roughly and making the crudest assumptions, 3 minutes is about a third of a turn of the wheel which might equate to a third of the total PE, so about 6 seconds of arc. This would support a finest image scale of about 12 arcseconds per pixel. With the ZWO 1600 you're at about 1.6"PP so you actually need an RMS of about 0.8 arcseconds. If the quick calculation above is anywhere near correct (my calculations never come with a guarantee!) you certainly need to guide.

Olly

 

[alan4908]

12 hours ago, Grierson said:

Perhaps the following is naïve comment. My current setup is a SkyWatcher ED80 on a belt modified HEQ5 with either a ZWO 294 or ZWO 1600. (Both CMOS cameras) Having watched SharpCap's Robin Glover's presentation where he recommends lots of short subs rather than fewer longer subs I get to thinking that running a guide 'scope and software on my rig does not contribute significant benefit.  I typically take 30-40 x 180sec. subs.

All the control gear, ASIAIR, power distribution, focuser and guide 'scope are mounted on the ED80 and I WAS giving some thought to lightening the load by fitting an OAG. But then I hesitate and wonder if guiding on my setup is worthwhile. I should say that clear nights in recent times have not allowed any experimentation to answer my own question but on the occasions when I have not guided I see reasonably round stars. I wonder what more experienced imagers think.  

It is an interesting question.

When I started astrophotography I noticed that with my SW ED 80 on an NEQ6 mount (non belt modified) I found that imaging at 1.4 arc seconds/pixel that I could get acceptably round stars (eg star aspect ratio < 25%) with up to 120s seconds exposures.  However,  I also discovered that for my site taking lots of short exposures significantly limited me in imaging faint objects.  After a bit more investigation, I discovered that my optimum broadband exposure sub frame length for my site/set up was around 600s.  A detailed discussion this subject can be found in the book The Astrophotography Manual by Chris Woodhouse - basically more light pollution lowers this figure.   I 'd therefore suggest you determine your optimum exposure time for your particular site/set up. 

In terms of improving your mounts performance, I'd suggest you quantify the situation by firstly determining your maximum load weight for imaging and then weighing the sum of all your attachments (dovetail bar, guide scope, camera etc).  If you well within the mounts load limit, then you are unlikely to see any performance improvement. 

Alan 

[Grierson]

Food for thought. Thanks for the contributions. John