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polar alignment spot on but star trails?

alcol620

By alcol620
in Getting Started With Imaging

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alcol620

alcol620

Posted
Posted

Hi folks

Grateful for any ideas of what might be going on.

Kit as follows AZ EQ 6 mount, 80ED scope with reducer/flattener, fixed pier.

My polar alignment is pretty much spot on, as far as I know! But when I image without PHD guiding running I get star trails with relatively short subs, less than 120 seconds. However, with PHD running I can take 600 sec subs that are excellent with round stars all over the image.

Anyone like to offer a reason the unguided star trails? You could say - forget about it if you get good subs with PHD running, but I am interested to find out what might be going on.

Thanks for looking - seasonal greetings and a healthy 2017

 

 

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cjdawson

cjdawson

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Star trails are caused by several things.

1. Inperfect polar alignment.

2. Astmospheric refraction.

3. Changes in air pressure

4. Changes in temperature

5. Periodic error

6. Incorrectly calibrated tracking motors

 

I would be willing to put money on you having all of the above, to a greater or lesser degree.  Polar alignment has to be perfect to stop trailing, though, close should be good enought for 2 min subs. 2,3 and 4 from my list are all bascially "The weather". 5 & 6 are down to the mechanics of the scope.   Whilst each on their own could be a small thing.  They can all happen at the same time and could cause a big issue.   The answer to this is the feedback and corrective loop of auto-guiding, but you already know that's the solution.  Hope this gives some insite into the why.

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ronin

ronin

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Difficuylt to say without full idea of the data you use, Colchester is 51.9N, suspect the best the scale gives is 52N and even that is questionable. Really besides a polar scope a datum flat is required that is machined parallel to the motor axis so that people can set a digital angle meter on to get an accurate setting.

Is a polar scope "calibratable" ?  and if so is yours right.

The rate is Sidereal, as in you were not recently looking at the moon and changed it. Has happened.

Almost standard question - what is the power source ? Have you checked the voltage it is supplying. I woud say that if it reads 12v then it is insufficent, from previous it seems that around 13v is better, so 12.5v may be a more realistic minimum.

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Andyk93

Andyk93

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How do you polar align? I found that when I used phd2 for drift alignment I couldn't get unguided subs over a minute without it moving slightly. I use sharpcap now and can get PA to about 4 seconds of error. I can easily get 2 minutes unguided and 30 minutes guided now

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D4N

D4N

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You should be able to see from your guiding graph what PHD is having to correct for.

 

When running unguided you need to correct for PE, you may be able to teach your mount to correct for it but I'm unfamiliar with the Az-eq 6 so can't say for sure.

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wimvb

wimvb

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+1 for Dave's suggestion. Improper balance can cause trailing. Periodic error can be checked by examining your guiding graph. The period is about 8 minutes for the az eq6, I believe.

With good polar alignment and balancing, you should be able to get 4-5 mins unguided subs with your setup, with reasonable quality.

Cheers,

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Davey-T

Davey-T

Posted
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1 hour ago, alcol620 said:

Hi Dave, not sure I didn't take much notice. Next time I have the kit out I will note the info and get back. What's your thinking?

If you note which way the star trails are going you can see if it's in RA or Dec.

Dave :icon_santa:

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wornish

wornish

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I had a similar problem with mine.  I found the reticle in the polar scope was slightly out .  The manual for the mount shows you how to set it up its really fiddly adjusting the three grub screws but worth getting it spot on in the end.  In theory you should only have to do it once but its worth checking.

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alcol620

alcol620

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Dave

Can't actually remember if I checked the polar scope when the mount arrived some time ago. Another on my list of things to do. Thanks Dave.

As I said in my post, whatever errors are occurring PHD is taking them out, so in practice I don't need to do anything BUT following your inputs I will make an effort to sort out what is causing the set up to star trail with no guiding.

Thanks for your feedback everyone. I will post again if I get to the bottom of it.

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D4N

D4N

Posted
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2 hours ago, alcol620 said:

Dan

 

I thought I read somewhere that there is some sort of conflict when correcting for PE and running PHD guiding?

Yeh it is possible that both end up correcting for it and you get an overcompensation wobble.  But for unguided it could be part of your problem.

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cjdawson

cjdawson

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4 hours ago, alcol620 said:

cjdawson

Thanks for your input. "Incorrectly calibrated tracking motors" what is this about?

Some mounts have operations for calibrating the drive motors.  I don't know what you are using, so I don't know if your mount has such a feature.

I know that my Meade mount has this function.  Drive training is a option in the menu.  The operation is to focus on a distant object in the day time - with the tracking turned off.

Then I start the training.  The scope will move the motor off in one direction.  You then use the handbox, to bring the scope back to the correct spot.  The scope will move the other way, you then do the same thing.   Do it for both RA and Dec, and the drive it trained.

You mount might not have this feature, but it might have.

 

There are also settings in some mounts to adjust the speed of the motors, effectively adjusting how the scope understands sidereal time.

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alcol620

alcol620

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3 hours ago, cjdawson said:

Some mounts have operations for calibrating the drive motors.  I don't know what you are using, so I don't know if your mount has such a feature.

I know that my Meade mount has this function.  Drive training is a option in the menu.  The operation is to focus on a distant object in the day time - with the tracking turned off.

Then I start the training.  The scope will move the motor off in one direction.  You then use the handbox, to bring the scope back to the correct spot.  The scope will move the other way, you then do the same thing.   Do it for both RA and Dec, and the drive it trained.

You mount might not have this feature, but it might have.

 

There are also settings in some mounts to adjust the speed of the motors, effectively adjusting how the scope understands sidereal time.

I'm using a AZ EQ 6 mount

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wimvb

wimvb

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... which, I'm pretty sure, doesn't have this training functionality. Only permanent PEC training. If your Periodic Error is large, it will put a low limit on your unguided subs. The guiding log will tell.

If you do PEC training, you can set tracking mode to sidereal + pec to improve (un-)guided tracking performance.

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ollypenrice

ollypenrice

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A periodic error of around 30 arcseconds is par for the course with the EQ5/6 mounts. That's a big PE and would be fatal if it didn't guide out well, but it usually does. If you look up your imaging scale in arcseconds per pixel on here  http://www.12dstring.me.uk/fov.htm or on FLO's site you can easily work out how many pixels of elongation you'd expect with a PE of 30 arcseconds. If you were imaging at 1 arcsec per pixel you'd expect a star to trail by 30 pixels. At 2 arcsecs PP that would make a trail 15 pixels long. If your trails are in this order of magnitude you're in typical EQ5/EQ6 country with nothing to worry about. You can test your PA by stacking just a first and last sub together, registering on the stars. You do this and then look at how misaligned the outer frames of the two subs are in the combined image. If PA is poor the frames will be rotated.

I wouldn't trust a polar scope alignment from the SW polar scope unless it had been very carefully collimated with the mount.

Olly

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