Polar alignment with AVX. Target never centred

[joecoyle]

Hi

Perhaps it's me, but no matter how many times I polar align my AVX in a session, I can never get my target fully centred. It's always off to one side.

A few nights ago I tried 3 times. Each time my alignment error was 0"0"0". It got closer each time to centre but still not there yet.

There is also still some movement over the course of a 3 /4 hour session. The target will move gradually out of the fov.

Is this my accuracy in polar aligning ? I can't do any better than 0s or can I ?

Is it poor guiding?

I thought the whole point of PA was that the target didn't move! The drift isn't so bad that PI can't align them, but I do end up with some pretty hefty black edges on my final images.

I feel I'm missing something. Some key step along the way.

I'll see if I can upload a video showing the movement.

Cheers

Joe

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[Owmuchonomy]

PA and GoTo are separate matters.  I think if your software is telling you that you have 0 error on your PA there is something wrong with that aspect.  However, setting up GoTo is separate to your PA in some respects so consider it such.  However, a rough PA first will help your GoTo set up.  If you are getting drift of an object over an evening session then your PA is not accurate.  When you say drift, is it field rotation i.e., is the trend in frames to move in a circular fashion?  I'm not familiar with your mount so somebody may be along soon with more detailed info.

[joecoyle]

Hi I'm hoping this video of a recent M76 illustrates the issue. The target wasn't great but you can see the definite "movement".

Cheers

Joe

Ps apologies for the hand shot video. I can't get PixInsight to output the video from Blink.

https://vimeo.com/141988579

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[PeterCPC]

For a start - after you do the first PA, if you ask to display align, it will always say 0.0.00 because you haven't callibrated it. After the first PA you must update the star alignment as is stated in the manual by unsync etc etc. Do you use an illuminated reticule to get the stars dead centre? You need to.

Do you adjust the diagonal at all so it's easier to see - if you do that will affect the PA I have found. Also, even after 2 PA iterations you will not get an error of 0.0.00 but more like up to 5' but PHD should be able to cope easily with that. Last night I had an error of 2' and 5' (not my best PA) but PHD coped with 400sec exposures no problem at all.

I don't know why the target is moving but maybe the Ra and Dec controls are not tight enough after your PA.

Peter

[joecoyle]

That's what I thought I have been doing as far as I know - turn off, re-align back to index marks and do it all again.

Is that not right?

I use my CCD and nebulosity with cross hairs to do the alignment.

I had been thinking 0.0.0 was too good to be true.

I tighten them fairly tightly.

Joe

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[PeterCPC]

Personally I never turn it off. I just unsync and then go back to the original alignment stars as per the manual but I think that turning it off should also work.

Maybe someone else can suggest something else. I don't think that you will ever get 0.0.00 but Celestron say that 10' should be ok - I always aim for less than 5' if possible.

Peter

[Thalestris24]

Hi Joe

I take it you weren't dithering?? I understand that imaging at longer focal lengths can be challenging. However, the stars in your video don't look particularly eggy - good sign! I would recommend using the drift align tool in PHD2 for PA. Also, skip the handset star alignment and just use either Astrotortilla or APT/PointCraft for platesolving and centering your target. Once set up, platesolving saves a lot of time and makes life much easier. You can make use of the PHD2 Guide Assistant tool to check whether you have any excessive drifting after you've polar aligned.

Louise

[joecoyle]

I will try this then. I think I'm introducing error back into the mix because those index marks are so broad I'm never going to get it lined up again perfectly. I will unsync and try that method.

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[joecoyle]

Hi Louise, I use a Mac at home. As much as it pains me, I might have to invest in a Windows laptop just for astrophotography.

I was trying to do everything Mac but there are some things that just plain don't work. I do fancy the ASCOM stuff too.

I may have lost the battle on this front. Bite the bullet!

I will look into plate solving though. Anything for time saving.

No dithering yet either. That's on the hit list too.

Cheers

Joe

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[Thalestris24]

Hiya

Tut! There is some stuff for Mac - phd2 and DSS, at least, I believe. See here: http://stargazerslounge.com/topic/176721-mac-osx-software/ there is mention of a basic plate solving app - post #10. Dunno anything about it.

PCs are definitely better supported software wise...

You can still make use of the PHD2 tools Handset for star aligning, I suppose...

Louise

[Davey-T]

Hi Joe, looking at your video you look to be drifting south, from a PA point of view if you equate it to drift aligning on a star in the south you need to adjust east  / west to correct this, you don't necessarily need to go the whole PHD drift aligning tool, just turn off guiding in PHD and watch to see which way the guide star goes in north south (ignore east / west) and then adjust in az to move the star back whence it started, just make a note when calibrating so you know which is north and south on the PHD screen, a few iterations should get close, if you go too far the star will start to drift in the opposite direction. Hope this makes sense and it's not more confusing 

Dave

[joecoyle]

I will give it a go. Because I'm running PHD on my Mac, it doesn't feed back accurate scope pointing info through ASCOM so I think I need to do certain bits manually.

I will get my hands on a Windows laptop to test it properly.

Is a drift alignment worth doing when I have to set up each night ?

Joe

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[Thalestris24]

Hi

Yeah you obviously need to PA each time you set up. It shouldn't take too long. You might consider installing Parallels - I think it works quite well though I've no direct knowledge of it. Maybe ask around on here about it. I'm sure I've come across others on here that use it. You obviously need to meet the system requirements and have a Windows installation disk. It might be better to get a pure Windows laptop - I don't think you need anything especially powerful/expensive.

Louise

[Davey-T]

I use Bootcamp on MB Pro and all astro stuff works OK and it's free as opposed to Parallels, though you still need a Windows disc.

Dave

[joecoyle]

I use parallels currently for controlling my NexImage. It works fairly well. Do have to unplug / replug the usbs a few times sometimes to get it to recognise. Boot camp might be the way to go

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[Thalestris24]

I use parallels currently for controlling my NexImage. It works fairly well. Do have to unplug / replug the usbs a few times sometimes to get it to recognise. Boot camp might be the way to go

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USB's are always a pain! Um, if you already have Parallels, can't you install Ascom, APT, Astrotortilla etc?

Louise

[joecoyle]

I could do. I've held off purely because I wanted everything native.

I'm used to my Mac workflow.

I'll have to get to grips with APT etc on parallels then maybe buy a dedicated laptop in future.

Joe

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