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Polar alignement precision for mobile setups


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Hello all,

i am curious as to what degree you optimize your polar alignement with your mobile telescope setup. My "problem" is somehow related to the limited time i have for imaging. Obviously in order to improve the polar alignment, drift alignment is the best tool. However usually it is very time consuming as you record the tracking deviation through multiple worm cycles and therefore i find that it is not really feasible for my mobile setup. At the moment i`m imaging with a Nikon D5300 and a 8" F5 Newton telescope (0.8" / pixel ). To reduce star bloat by pixel saturation, i`m experimenting with "low iso" astro photography. In order to avoid discretization noise and to get a reasonable histogram i therefore have to push exposure times.

After belt modding my EQ6 the guiding performance of the mount is good enough to allow for an exposure time longer than one worm cycle (imo). However now the effects of field rotation are visible which means i typically get slightly elliptical stars at longer exposure times (i think cable drag is sorted out, there is also still a change that this is due to differential flexure... but i would like to focus on the PA precision here). So the question is how could i improve on that?

Solutions would be:

- Smaller exposure times

- Higher ISO (star bloat)

- Get a faster focal ratio (=smaller exposure times)

- Change to smaller focal length 

I would be interested on how you handle the polar alignment on your mobile setups! Please let me know :)

best regards

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I use a the EQ3-2 setting circles to get a very accurate position of my polar scope (polaris in the small circle is aligned with the 12 oclock position on the RA circle) I have added some friction with a spacer so that it all rotates together then all I have to do is dial the RA axis to match the polaris hour angle given in stellarium wrt to zero position on the mount.

I figure that this method gets me within 3 mins of the position of polaris and only takes a min or so to do.

Alan

Edited by Alien 13
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Hello,

thanks for your responses. I use guiding, but i always thought that guiding doesn`t prevent field rotation. Actually while googling for the topic i found a field rotation calculator.  I plugged in my numbers and for a deviation of 10 arc minutes(i hope i`m better than this) of the celestial northpole the field rotation is only 0.1 pixel / minute at declination 0° at the sensor border. But what i see in my pictures is much more than that. So i think i missinterpreted my images.

My stars are actually elongated uniformely on the whole frame, so maybe i have to start looking in the "differential flexure" direction?

What do you think? Thanks again :)

best regards

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Bad PA will manifest itself as a rotational distortion in your stars.

What you have described is either dodgy guiding or severe flex. If your guide graph is good but your stars are still eggy (consistently across the frame) then its most likely going to be flex. Make sure you veclro up all your cables, and tighten up your guidescope (a lot). If your guidescope is in rings and you dont want to dint the paintwork by severe tightening, use an old cut-up leather belt andd use it to protect the OTA from the guidescope thumbscrews..... then you can get some finger-shredding tightness going on. Also take care to remove any cable strain from your guide camera (and your imaging camera).

Or you can bolt the guidescope directly to the top of your imaging scope (using standard rings), they dont really need to be pointing at exactly the same patch of sky to get good guiding.

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Thanks for your input! My guiding graph typically looks something like this.

Guiding near M1:

guide graph

I guess thats more or less ok?  
From all what you have said, i think i will have a look at the cabling and the guide scope mount. At the moment my guidescope is attached via this holder:

best regards

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That guide graph looks a bit too busy, here is what im normally faced with:

post-5513-0-71174800-1423084203.jpg

That is using a 50mm finderguider, rigidly attached to the top of my imaging scope tube rings using some mini guidescope rings (part of a Baader fiinder bracket). Ive checked out your guidescope, tbh you need to get away from using the finder shoe and think about bolting it to a dovetail bar, then mounting that on top of your tube rings.

Your guidescope is held in by three screws only, so that may also be a flex source. Taking the cable strain off the guide cam might help there until you can find a way of making it more rigid (I used to pack my old 9x50 finderguider out with masking tape).

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Hello,

this is from yesterday. I uploaded the image in full resolution in the gallery. I`m not sure how it will show in this thread.

I just converted the raw files to jpg and did nothing to them. The images were taken with a D5300 through a 12nm O III filter ( i know it doesn`t make much sense :) ),

Here is a 900s sub:

900s sub with O III filter and D5300

And here is a 360s sub (taken a few minutes before the above):

360s sub with O III filter and D5300

The collimation of the telescope was not perfect which results in the distortion you see in the upper left corner.
best regards
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That guide graph looks a bit too busy, here is what im normally faced with:

attachicon.gifphd.jpg

That is using a 50mm finderguider, rigidly attached to the top of my imaging scope tube rings using some mini guidescope rings (part of a Baader fiinder bracket). Ive checked out your guidescope, tbh you need to get away from using the finder shoe and think about bolting it to a dovetail bar, then mounting that on top of your tube rings.

The guidescope is held in by three screws only, so that may also be a flex source. Taking the cable strain off the guide cam might help there until you can find a way of making it more rigid (I used to pack my old 9x50 finderguider out with masking tape).

Thanks. I will try your suggestions and also try your guiding parameters. At the moment i still have the standard PhD parameters.

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Just looked at the sub, and there are multiple issues. In the central area the stars are showing signs of guiding or flex issues, while in the corners the issue is more to do with the corrector you are using (its not delivering a flat field across your chip). That is apparent with the stars in the bottom right, when compared with those in the top and bottom left - so your camera/corrector could be either mis-spaced or not sitting square.

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Thanks for your help! I will try to find a way how to tightly attach the guider to the scope (most likely looking for rings now :) ) and then see what happens. How did you modify your EQ6? Your guiding curve looks fantastic :)

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Sorry, forgot to add: I use a coma corrector (Baader RCC1).

I found the Baader to be extremely sensitive to spacing, a fraction out and the field is no longer flat and the stars elongated. The spacing is also very system specific for the Baader so you may have to experiment with spacing as for the finder/ guider make sure that is as rigid as possible.

A.G

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Great advice here! For polar alignment I use, as Rob does, Jason Dale's Polar Finder app, from my laptop, or PolarAlign by George Varros on my iPhone. I ensure my mount is accurately parked in RA before I start. Check out Dion Heap's tutorials for this on YouTube (his site is Astronomy Shed). I can't post a link as the site is blocked here at work.

Then I polar align by putting Polaris on the large circle where the app shows its position to be. I don't bother with the little circle at all. I then rotate the mount in RA to ensure Polaris doesn't drift from the large circle (its orbit). If it does, then the alignment's wrong.

Alexxx

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I found the Baader to be extremely sensitive to spacing, a fraction out and the field is no longer flat and the stars elongated. The spacing is also very system specific for the Baader so you may have to experiment with spacing as for the finder/ guider make sure that is as rigid as possible.

A.G

Thanks for the hint. To be honest, i read about this issue before i bought the RCC1 but i somehow neglected it :)

I think whenever i have a non-uniform image field its due collimation and corrector mounting. I ordered new guide rings now and will see if it influences the guiding/star shape. 

@Alexxx: Thanks for your PA advice, i will give it a try!

Best regards

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How about using the PA routine from Eqmod? That gets you close enough if you guide and you might argue that a tiny bit of field

rotation is good as it acts as some form of dithering. It's what I use and I have to set up every time,

E.

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  • 2 weeks later...

Hello,

i just wanted to report back. I changed the way my guide scope was mounted acording to Uranium235`s suggestion:

Guide scope mounting

This has now resolved my elongated star problem as can be seen in this 900s frame:

900s light frame

On the lower right is a 100% crop of the framed area.
I`m quite happy now about how the mount performs. I even would be happier if the frames where a tad crisper. I think the guiding performance could still be improved, although i`m not sure if already seeing limitations start to kick in since i`m imaging at 0.8"/pixel.
What do you think?
Best regards
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