Do these video's show PA issues?

[swag72]

I have recently changed my original finder guider for an AA mini guider package. It's a great piece of kit, but since using it I have developed some serious issues that I THINK are PA, but I'm not sure. So, there's a link to a couple of .avi's that were quickly done in Maxim just to show the amount of movement I am getting.

The first .avi shows how my drift was prior to changing the guider and how it is now. Does this look to you like dodgy PA?

http://dl.dropbox.com/u/41385123/PA%20Abell%2021.avi - little drift

http://dl.dropbox.com/u/41385123/PA%20IC1396.avi - Masses of drift!!

If this is identified as PA, then I will need to take the scopes off the mount and do it at my earliest time.

Thanks for looking.

[swag72]

Any thoughts guys? I've got the scopes off the mount, so hope that it will be clear enough to PA tonight using EQMOD.

[swag72]

:embarrassed: No one want to look at my quick videos and see if they think the issue is PA? They're less than 5s to watch, I promise!! Then a quick yes or no one here would be fab!!! Please!!!!

[Shibby]

Is it me or is the forum less active than it used to be?

Not sure what to make of your videos Sara... I don't think they tell us much other than there is some drift going on. What time frame do they cover? One thing I have noticed is that the star "trails" don't seem to match the direction of the drift (particularly with IC1396) suggesting that the star shapes are an optical issue.

It's always worth checking your PA anyway. Check every step carefully, particularly the alignment of the reticle. I always PA with the scope on the mount, because I worry about it shifting when I start to attach everything!

[callump]

Sorry, i'm not sure what you mean by PA - do you mean PE ? - Periodic Error ?

Callum

[Shibby]

Polar Alignment

...but that has made me realise. The star shapes could be PE, the drift PA.

[callump]

On the other hand (now having looked at the video), if you mean Polar Alignment - then almost certainly.

You would be best to do polar alignment using drift method to get it spot on.

Callum

[Shibby]

Although it is worth getting the PA as perfect as possible, the guiding *should* in theory be able to cope with errors in PA, resulting in a rotation around your guide star. What did you guide graph look like Sara?

[swag72]

Cheers for the answers guys - Seems no one wanted to look for a while!!

Anyway, regarding guide graph, nice and flat in Maxim.

Time scale - The IC1396 was about 13 x 25min exposures and the Abell 21 would have been 20min exposures and there was I think 11 or 12 of them. I'm not even going to think about the star shapes being an optical issue - The Pentax 75 is a real beauty with built in flattener. With short exposures (3s) the stars all look nice and tight.

Not able to Polar Align last night because of the clouds. Hope to get it sorted soon though. Can't work out how it got so bad though as my setup is in an obs. The only change has been the guider scope now pointing at the imaging target, whereas before it was considerably out and pointing off towards the right somewhere - Certainly no where near the target that's for sure.

[Spikey]

I think that you have just cured your own problem there Sara.

You will get a lot of image roation if the guidescope is pointing elsewhere. It happened to me when I had my guidescope misaligned and I scratched my head for ages, blaming flexure, PA and all sorts.

Pointed the guidescope to the same place as the imaging scope and voila no more rotation.

[Black Knight]

I think that you have just cured your own problem there Sara.

You will get a lot of image roation if the guidescope is pointing elsewhere. It happened to me when I had my guidescope misaligned and I scratched my head for ages, blaming flexure, PA and all sorts.

Pointed the guidescope to the same place as the imaging scope and voila no more rotation.

Now this makes sense to me. However, I thought the whole point of guidescope rings was to allow you to point the guidescope somewhere other than where your imaging scope is pointing precisely in order to find a guidestar. So does doing this increase the risk of image rotation?

I is all confuzzled again!

[swag72]

I think that you have just cured your own problem there Sara.

You will get a lot of image roation if the guidescope is pointing elsewhere. It happened to me when I had my guidescope misaligned and I scratched my head for ages, blaming flexure, PA and all sorts.

Pointed the guidescope to the same place as the imaging scope and voila no more rotation.

That's the point, it's working opposite for me!! When my guidescope was misaligned, there was minimal drift - Now that it's aligned to the target, I am getting real drift issues. That's why I just don't get it.

@Andy - As I understood it, you want your guide star as close to the imaging target as possible. Now mine is bang on target, I am getting issues that I never had before

[Spikey]

As I understand it guiding is used to correct polar alignment errors, the further away you guide from you're target then the angular rotation is increased and field roation is more pronounced.

I have misunderstood what you meant Sara and have just re-read that your excessive drift is caused post guider alignment.

In that case I would make sure everything is bolted down tightly and your polar alignment needs double checking. Are you using EQMOD to PA?

[swag72]

Yes Simon, excessive caused post guider alignment!! Everything is all bolted down tight and no movement that I can determine. I am looking at checking the PA as soon as the clouds clear. I do use EQMOD to polar align, but as I am in a permanent setup, I absolutely fail to see how the alignment can have changed so drastically. But I will check it anyway, scopes all off and waiting!!

[Whirlwind]

Hi

It might be worth just stacking the images without aligning them so you can see what the overall trailing picture looks like. Although there is obvious drift the movie makes it difficult to see what is actually going on. It could be a number of factors all combined so you might iron out one only to find another. For polar alignment I find PEMPro really useful (not sure if it works with your mount though), as it it allows you to drift align with the main camera, hence avoiding differences in pointing between the main and guide cameras. The other thing to check is that you have no loose cables dragging as that can cause issues. Also have you checked the balance on your scope - if each of the images are on different sides of the meridian when you imaged them then you may have set up your mount for one side which if you have crossed over result in the 'weighting' being opposite to what you need (for me this seems to be more of an issue as you approach pointing straight up).

Thanks

Ian

[swag72]

Thanks for your thoughts Ian. I can answer a couple of those questions straight off. Both images were on the same side of the meridian - I don't often 'do' flips!! No cables dragging at all. I may have a go at stacking them without aligning and see how that goes. Will look at PEMPro - At the moment I'm using EQMOD for alignment.

[Starflyer]

It could just as easily be differential flexure, but I'd rule out PA error first and confirm / fine tune the PA with a drift alignment in PHD. Once you know for sure that your PA is good if you still get drift then it can only be flex and you'll need to track down the cause; side-by-side / piggyback mounting, focuser slop, cable drag etc.

I really struggled with flex and always had some drift / oval stars, I ended up going the OAG route.

Cheers,

Ian

[Shibby]

It still doesn't make much sense attributing the drift to PA... If your guide star was indeed one within the FOV of your imaging camera and you did really get a good guide graph, then PA errors should not create this sort of drift - at worst, you would see a rotation around the guide star. Cable drag or flexure would make sense. Another idea (and forgive me for suggesting this) - you weren't guiding on a hot pixel by any chance? I have nearly done this before! :-O

[swag72]

No guiding on a hot pixel as I can always see clearly in Maxim what it has guided on - definitely a star. Far too big to be a hot pixel!!

Well, it's going from bad to worse to be honest.

I have done the following now.

1) Cheacked PA

2) Checked all cables - Absolutely no sign of cables dragging, tight or wrapped in others.

3) Removed the heavy scope in order to ascertain whether weight was the issue - Still a problem

4) Returned to my original guider than never gave me issues - No change, still masses of drift and star elongation as well now.

In desperation I downloaded PEMPro as it was recommended as being a good PA tool. DId the PA, but never got the chance to do any more subs for checking as it was 0430 by this time and I'd lost the will to live.

I checked backlash using PEMPro and I definitely have some, but when I changed the backlash settings in EQMOD it made no difference whatsover to the Backlash.

Today everything has all been rebalanced for the umpteenth time. The big scope had to go back on and the dual bar as there was no way at all that we could get any decent balance with only the small 'frac on the mount. Everything has been checked for tightness, nothing is moving. I am hoping to try again tonight, but I am really not holding out any hope. I am expecting masses of drift still and elngated stars.

Hey ho.

[Shibby]

How frustrating! I feel for you, but at least this is a good time of year to be ironing out issues rather than wasting the good nights!

Given the good guide graph and guide scope alignment, common sense says there has to be some sort of differential between your guide camera and imaging camera; flex, sag, tug... Sorry I can't be of any more help than that. Perhaps someone else has a similar setup to yours and can comment on the possibility of this?

[themos]

I have done the following now.

1) Cheacked PA

2) Checked all cables - Absolutely no sign of cables dragging, tight or wrapped in others.

3) Removed the heavy scope in order to ascertain whether weight was the issue - Still a problem

4) Returned to my original guider than never gave me issues - No change, still masses of drift and star elongation as well now.

When you say "checked PA", have you done a calculation of the error? Can you get your hands on MaximDL and MaxPoint? They offer an automated method (you can use 100 stars or more) to get numbers for every mount misbehaviour possible (including flex, pa etc).

No4 suggests to me that your PA is off.

Also, can you verify that your guider corrections are getting through to the mount? You can use manual control in PHD and send a long guiding pulse and confirm that the star is moving when you do that. The elongation that I have seen in the videos seems to be along the drift direction and that must be the declination direction. You could try biasing the weight distribution in the Dec axis so that the gears always have something to push against, in case it's excessive dec backlash that is causing the elongated stars.

[swag72]

Hi Themos, I'm glad you came along

I did the PA last night using PEMPro - I shall see if there's some kind of checking measurement within the programme. So far I've not taken a measurement of the error. I've got MaximDL, but not Maxpoint.

I'm certain that the guider corrections are getting through to the mount as it sounds like it's guiding, but I will check that as well.

When you say about biasing weight on the DEC axis - I really struggle here to know what is DEC and RA - I always have the mount East heavy and then lighter on the counterweights if I am imaging towards the west and downwards again.

I will look at calculating the PA error. Should get a few stars with the 75 and the Atik 314L I hope!

[themos]

You should be able to use the free PoleAlignMax utility with MaximDL. Use Maxim's help, search for "polar alignment" and see the info there. For a permanent setup like yours, you definitely should be using something like that.

I see you do bias the RA axis and that's good. The same principle applies on the DEC axis. But let's sort out the drift question first before we worry about elongated stars.

[swag72]

Looks like Polealignmax needs a motorised focsuer to work - http://users.bsdwebsolutions.com/~larryweber/ in which case I'm back to the drawing board as I definitely don't have one of those!

[Helen]

That's focusmax you need polealignmax

Helen