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PA with smaller FOV?

Posted: Tue Aug 06, 2019 5:34 am
by RazvanUnderStars
I currently use an 8" SCT with a 0.63x reducer and an ZWO ASI294 MC Pro camera (no other camera). FOV is 0.86 deg x 0.58 deg, smaller than the required FOV listed as between 1 and 2.5 deg on each side as per https://www.sharpcap.co.uk/sharpcap/fea ... -alignment. It still works very well.

I'm considering changing the OTA to an EdgeHD 11" with the 0.7x reducer which gives an FOV of 0.56 deg x 0.38 deg. Smaller than the above. I'm concerned it will really prevent me from doing a PA with SharpCap.

Has anyone managed to do PA with a similar FOV? Also, the fact that it already worked with a smaller FOV than formally required, has the PA routine improved since that requirement was written and therefore can work with a smaller FOV?

Thanks!

Re: PA with smaller FOV?

Posted: Wed Aug 07, 2019 3:27 pm
by admin
Hi,

The polar alignment routines have been improved since the initial recommendations to have at least one degree in each direction. There is now a very good chance of it working properly with .5° field of view in each direction. However, as the field of view shrinks there is an increasing chance of ending up with the camera pointing at an area where there are insufficient bright stars in the polar alignment database for the plate solving routine to work. As the field of view decreases this problem – which will show up as a failure to plate solve – becomes more and more likely.

Cheers, Robin

Re: PA with smaller FOV?

Posted: Thu Aug 08, 2019 12:37 am
by RazvanUnderStars
Thank you. I also heard from another observer that it works very well with just about 0.5 deg.

Re: PA with smaller FOV?

Posted: Sat Sep 14, 2019 8:45 pm
by M Thomas Rolfsmeyer
I have a Celestron C8 with 6.3 focal reducer and ZWO ASI 294 pro camera. However I can not get this setup to solve for polar alignment.

Sharpcap PA works very well at F2.0 and hyperstar with the above camera but will not solve at F6.3. I have tried various setting for star min max gain etc with no luck. Can some one out there give me a few pointers to help me resolve this issue.

Tom

Re: PA with smaller FOV?

Posted: Sun Sep 15, 2019 12:48 pm
by admin
Hi,

The field of view calculator at https://astronomy.tools/calculators/field_of_view/ indicates that your field of view would be about 0.85 x 0.58° which is getting on towards the minimum value that is likely to work. I think some people have had success with fields of view like this, but you're starting to get into the zone where there may not be enough stars in SharpCap's polar alignment database within the field that you are looking at for the plates solving to work.

You could certainly try making some adjustments to the star detection settings to ensure that you are getting 20 to 30 stars being detected and then rotate the telescope gradually around the RA axis to see if you pick up any plates solving solutions at any point during the rotation.

Of course the 'check the plug' thing to do is to make sure that you are already within 5° of the pole – if you happen to be pointing at a point that only 82 to 83° north then you won't get a plate solving solution because you are out of the range of SharpCap's internal database.

Cheers, Robin

Re: PA with smaller FOV?

Posted: Sun Sep 15, 2019 4:23 pm
by M Thomas Rolfsmeyer
Hi Robin,

I always start my polar alignment sequence with putting Polaris in the cross hairs of the FOV before starting Sharpcap. Does putting Polaris at this point exceed the 5 degree rule?
I also have a guide camera that is closely aligned but not perfectly with the main scope. Can I use that for PA? It is a Loadstar X2 attached to a Orion short tube 80 with a 400 MM focal lenght.

Thanks

Tom

Re: PA with smaller FOV?

Posted: Mon Sep 16, 2019 7:44 pm
by admin
Hi,

Certainly by having Polaris in view you will be within the area covered by the polar alignment database. However you might be better off trying to use your guide camera – the lodestar camera combined with the 400 mm focal length give you a slightly wider field of view, so that would definitely be worth a try. You could also potentially put a cheap 0.5x reducing lens on the end of the lodestar camera to give you a wider field of view – some people have taken this approach.

Hope this helps, Robin

Re: PA with smaller FOV?

Posted: Wed Sep 18, 2019 3:49 pm
by M Thomas Rolfsmeyer
Thanks Robin,

I will try using the guide camera to do the polar alignment and let you know how this works.

Tom