Star Trails, Why?

[mballak]

Hi Folks,

I was hoping to image the Dumbbell tonight but I was having issues with star trails.  I polar aligned using the Drift method and was guiding using PHD2 but still got significant trail with a 300 sec exposure.  However when I switch targets to Andromeda I got no trails.  The only thing that is different is the Dumbbell is pretty much overhead and Andromeda is much closer to the horizon.  Someone mentioned before that stars appeared to move faster overhead but I thought guiding would take that into account.  Is there a setting in PHD2 that would help with this.  The examples below are both unprocessed 300 second exposures.  

Thanks,

Mike

[Knight of Clear Skies]

Just a thought, but could it be a balance issue maybe? The gears could be slipping when the mount is in one orientation but not the other?

I haven't used PHD, but have you checked the guiding graph to see what it was doing?

[Owmuchonomy]

Just a thought, but could it be a balance issue maybe? The gears could be slipping when the mount is in one orientation but not the other?

I haven't used PHD, but have you checked the guiding graph to see what it was doing?

Yes it could be. M27 is west of the Meridian and M31 is East currently at my location (not sure about Denver though). Therefore it could easily be balance. Same for Polar Alignment. I find it better to star align and polar align using target stars the same side of the meridian as my imaging target.

Mike, it us usually better to balance 'East heavy' in RA for imaging. Also if this run is unguided it is not unusual to get good subs followed by bad subs. Did you take several to see if the trailing was consistent?

[mballak]

Yes it could be. M27 is west of the Meridian and M31 is East currently at my location (not sure about Denver though). Therefore it could easily be balance. Same for Polar Alignment. I find it better to star align and polar align using target stars the same side of the meridian as my imaging target.

Mike, it us usually better to balance 'East heavy' in RA for imaging. Also if this run is unguided it is not unusual to get good subs followed by bad subs. Did you take several to see if the trailing was consistent?

 Thanks, I didn't think of that.  I waited until it passed the meridian on purpose so I wouldn't have to do a flip, plus it didn't get dark until about then.  I will definitely try the east heavy next time I have clear skies.  That isn't too often these days.  

[mballak]

Just a thought, but could it be a balance issue maybe? The gears could be slipping when the mount is in one orientation but not the other?

I haven't used PHD, but have you checked the guiding graph to see what it was doing?

Thanks, I'm going to try the east heavy balance next time.  I did look at the graph but I'm pretty new to this.  It looked ok to me.  

[Coco]

Always have weight slightly against the drive when guiding on the meridian, use a permanent sharpy marker to set your weights , this is what I do

[ollypenrice]

Firstly you need to work out which axis has the trailing. It looks as if only one axis was failing to track but is that RA or Dec? You can tell from the orientation of the camera. The lines are dead straight which rules out PA (which shows as rotation) and strongly suggests one axis misbehaving.

(By the way, you'll do yourself a big favour if you always align your camera to RA and Dec. There are several reasons but one is that you can identify a rogue trace at a glance.)

When the scope is on the zenith two things change. 1) Any front-back imbalance of the tube is neutralized because the scope is vertical. An imbalance might have been helping tame Dec backlash when the tube was more horizontal. 2) Any side to side imbalance of the tube (eg a finder or guider offset to one side) has its greatest effect and might become a problem.

Olly

[Leveye]

It's almost always an alignment and/or balance issue in most cases. When the mount does a meridian flip you should always re-balance. Don't forget that one.

[dph1nm]

Firstly you need to work out which axis has the trailing. It looks as if only one axis was failing to track but is that RA or Dec?

Looks like both to me - I think the cameras is aligned so constant dec is horizontal.

NigelM

[mballak]

Firstly you need to work out which axis has the trailing. It looks as if only one axis was failing to track but is that RA or Dec? You can tell from the orientation of the camera. The lines are dead straight which rules out PA (which shows as rotation) and strongly suggests one axis misbehaving.

(By the way, you'll do yourself a big favour if you always align your camera to RA and Dec. There are several reasons but one is that you can identify a rogue trace at a glance.)

When the scope is on the zenith two things change. 1) Any front-back imbalance of the tube is neutralized because the scope is vertical. An imbalance might have been helping tame Dec backlash when the tube was more horizontal. 2) Any side to side imbalance of the tube (eg a finder or guider offset to one side) has its greatest effect and might become a problem.

Olly

Olly,

Is there an precise way of doing this.  What I usually do Is,  when I pick my southern star to do my drift align for Azimuth, I orient the camera so the star moves horizontally and vertically with the corresponding buttons on my controller.  

[ollypenrice]

Olly,

Is there an precise way of doing this.  What I usually do Is,  when I pick my southern star to do my drift align for Azimuth, I orient the camera so the star moves horizontally and vertically with the corresponding buttons on my controller.  

That will do it. Just take a sub of about 5 seconds on a slow slew and note the orientation of the trail.

If the trails in your posted image are really a composite of RA and Dec thenI 'm mighty surprised and will stand corrected.

Olly

[mballak]

That will do it. Just take a sub of about 5 seconds on a slow slew and note the orientation of the trail.

If the trails in your posted image are really a composite of RA and Dec thenI 'm mighty surprised and will stand corrected.

Olly

Oh...got it.  will definitely do that.  

[m37]

With a good polar alignment and sturdy physical setup with good balance I would think PhD should suffice without drift aligning.

Are you using an oag or separate guide scope?

Also, is polar scope perfectly aligned with mount?

Level your tripod and tighten legs firmly but don't over tighten

Attach head and ensure tightened down

Balance accurately by using loosened clutches through both axes, one at a time

Once balanced, move weight a couple of mm away from the scope to seat the gears consistently

Carry out a rough polar sighting i.e. just on the cross hairs

Now maybe try a polar alignment method I had success with...

Find an accurate 12 o clock position by lining polaris perfectly on the central cross hairs, then move it up in the polar scope by turning the altitude bolts. Polaris should now be on the large circle directly above the cross hairs

Rotate through RA until the smaller polaris target circle is directly over polaris at 12 position. Set your setting circle to 12.

Now use a polar finder programming and find out what polaris position is

Rotate through RA using the setting circle to position the smaller target circle where polaris should be AMD lock your clutch

Now use the alt az bolts to position polaris in the correctly located smaller target circle

Rotate through RA with loose clutch and observe whether polaris sticks exactly to the larger circle

This should be good for unguided shots of 300 seconds, when you get this nailed maybe try drift alignment if you feel you need to. Its not the dark art people make it out to be but I never needed it while guiding up to about 10 minute subs

There is also a very cool bit of software called align master which automates to some extent the polar alignment routine. Works very, very well. Cheap too, although it chooses pairs of stars and if your view is restricted it can be tricky.

Different sky positions can be more/less for giving of poor polar alignment but it could be as simple as balance or a loose physical connection.

Lots of variables in imaging and only when they are all spot on will you get your nice round stars consistently.

Maybe push the unguided as far as you can and when you're able to polar align in your sleep move on to guiding? It's a whole other area of variables. Lots and lots of variables!!!

Oh, and they are nice images, good stuff!

Good luck mate

Chris

[glowingturnip]

Now use a polar finder programming and find out what polaris position is

Hi Chris,  that's a very useful PA technique.  What's the polar finder program you're using in the above step, is it AlignMaster ?

I try to get the little circle in the right place on the 'clock' by trying to get the angle between alkaid and mizar to look the same in the polar scope and in real life, but yours is clearly more accurate, I shall use it.

[m37]

Hey there. Polarfinder is OK and is Windows freeware.

If you have an android device I like polar finder https://play.google.com/store/apps/details?id=com.techhead.polarfinder&hl=en_GB which is great for 73p !!! I think iOptron are even punting this as their official android app now.

The approach with the 12 o clock position worked well for me, even if you have a polar app you would still be guessing roughly where the small target circle should go. It takes a little longer but is pretty much bang on. Can't remember where I heard about it, probably on SGL somewhere.

Alignmaster is cool enough but my views are restricted to 2 directions here so I struggled to find decent star combinations that worked well and were accessible.

All the best, let us know how you get on.

Chris

[glowingturnip]

yeah, I've got a restricted western view too, and quite high (hilly) horizons, so 3-star align is usually very frustrating, I just do 2-star.  Suspect it'd be the same with alignmaster then

Apologies OP for the thread steal !  I'll pipe down now