SharpCap 2.9 - Polar Alignment

[Vox45]

3 minutes ago, rwg said:

All of the camera settings are saved in a .CameraSettings.txt file next to the actual capture file, so the data is there if you want to use it.

cheers,

Robin

ok I'll find a way to be able to use those field as variables in a post-processing software. Thanks for this info

[johnpd]

Is there a "How to use Sharpcap" .PDF or Tutorial? I cannot find one on the website or at AstronomyShed.

Thanks,

  JohnD

[rwg]

Hi,

sorry, no official documentation I'm afraid beyond the web site, the tips at startup and whatever you can find on the forums. When there's a choice between adding a new feature and writing documentation I pretty much always choose adding the new feature!

I did have a couple of people volunteer to help bring together some of the info in various forums into documentation, but it all fizzled out I'm afraid

cheers,

Robin

 

[davidgr1976]

Hello,

I like very much this piece of software. I always have done PA with drift method since i didn't have polaris view, and I usually leave PA error in some arcsec (15-30").

Now i have moved to another house where I can see polaris but can't point to neither west or east at 10-20 degrees altitude in the backyard.

I was considering on buying polemaster but i discovered this.

I've been testing it. I get 20" error but when i check it with PHD2 the PA error after 10min is 4 arcmin... What could be the cause of this big difference with sharpcap?

 

Thank you!

[rwg]

This is a good question and one I don't yet fully understand. The plate solving to find the position of the pole is accurate - just tested with a star field from Stellarium with the equatorial grid still in place and you can see how well it does - SharpCap ignores the blue equatorial grid from stellarium easily and puts its own NCP (green) spot on in the right place.

If you test when you have finished polar aligning in SharpCap by turning the mount on the RA axis, you should see the stars in view rotate around the point marked as NCP - this confirms that you are correctly polar aligned as the center of rotation is matching the NCP.

So... what does that leave? The only thing I can think of is physical flexing of the tripod/mount/pier as the load moves from the 'home' position with scope up and weights down to whatever position you run a drift align from. To be honest, I'd have thought that this shouldn't happen if you are have properly balanced the mount payload, but I'm not 100% sure. If this is the problem then you'd expect to see less change with a lighter load on the mount.

Any further thoughts on this welcome!

cheers,

Robin

 

 

[davidgr1976]

14 minutes ago, rwg said:

This is a good question and one I don't yet fully understand. The plate solving to find the position of the pole is accurate - just tested with a star field from Stellarium with the equatorial grid still in place and you can see how well it does - SharpCap ignores the blue equatorial grid from stellarium easily and puts its own NCP (green) spot on in the right place.

If you test when you have finished polar aligning in SharpCap by turning the mount on the RA axis, you should see the stars in view rotate around the point marked as NCP - this confirms that you are correctly polar aligned as the center of rotation is matching the NCP.

So... what does that leave? The only thing I can think of is physical flexing of the tripod/mount/pier as the load moves from the 'home' position with scope up and weights down to whatever position you run a drift align from. To be honest, I'd have thought that this shouldn't happen if you are have properly balanced the mount payload, but I'm not 100% sure. If this is the problem then you'd expect to see less change with a lighter load on the mount.

Any further thoughts on this welcome!

cheers,

Robin

 

 

Hello,

I like very much this piece of software. I always have done PA with drift method since i didn't have polaris view, and I usually leave PA error in some arcsec (15-30").

Now i have moved to another house where I can see polaris but can't point to neither west or east at 10-20 degrees altitude in the backyard.

I was considering on buying polemaster but i discovered this.

I've been testing it. I get 20" error but when i check it with PHD2 the PA error after 10min is 4 arcmin... What could be the cause of this big difference with sharpcap?

 

Thank you!

 

I haven't thought about pier flexure.... This could be an isue due to the weight in 3 points on the grass (about 110kg?).

 

I am going to put under de bases some wooden parts to avoid pressure n the ground.

 

Thank you very much!.

 

[themos]

13 hours ago, davidgr1976 said:

Hello,

[...]

I've been testing it. I get 20" error but when i check it with PHD2 the PA error after 10min is 4 arcmin... What could be the cause of this big difference with sharpcap?

Hello, whenever I have verified the PHD GuideLogs after a session, I find an estimated PA error that's quite similar to what PhotoPolarAlign (same method that Robin has put in SharpCap) measured. I use PHDLab for analysing PHD GuideLog files. 

A couple of things to check: It could be that PHD's Drift Align procedure is not as robust as PHDLab's analysis (remember that PHDLab can see the entire session, not just 10 minutes, and tell how much Dec correction was applied). The other thing is refraction. If you are imaging fairly low in the sky, there will be significant and varying refraction. The path of the star will not be a perfect circle and you will get apparent drift no matter how perfectly polar aligned you are.

I image at up to a metre focal length and up to 10 minute subs and consider any polar error under 5 arcmin as good enough. 

[darrenl]

Hi all,

I've been using SharpCap for the last month or so and it has become a game changer for me with the HEQ5 and a 150PDS. I've been capturing very good (for me) unguided 60+ seconds exposures after alignment whereas before this I was failing to correctly align and only managing about 10 seconds before the inevitable trailing and lack of detail!

I use my ZWO 120 mono mounted at the back of the tube through a 50mm Orion guide scope, the guide scope is very roughly aligned and yet SharpCap manages this alignment process effortlessly and very quickly. Generally I'm aligned after a couple of minutes with an indicated error of sub 20 arc seconds. I then move the camera to prime focus knowing that I'm sorted for the evening followed by a simple one star alignment on the handset. Next step for me is sorting out the HEQ5's very horrible bolts and learning PHD to get longer exposures (need to dig out my very old Philips SPC900 and see if can get it to work under windows 10).

The only gripe that I had with it was that I failed to follow the very clear instructions for the final step, only one wasted nights imaging! I was adjusting the RA and DEC before clicking forward to the final step. The software shows the adjustment offset indicator after rotating the RA through 90 degrees and because I saw this, I automatically started adjusting bolts without thinking.  I could not understand why when I repeated the steps again to confirm the alignment that I was way off!

Thanks to Robin for a great piece of software, will have to cross the PoleMaster off the Christmas list and think of something else to add to it!

Darren

 

[DavM]

I have tried PA using Sharpcap a couple of times. I have a question regarding the "moving" of the RA axis. Can this be rotated by disengaging the clutch and rotating, with the mount turned off, or with the mount turned on and "slewing with the Hand Set, or doesn't it matter which!

Thanks in advance for replies.

Dave

[rwg]

Either way will work fine - take your pick.

cheers,

Robin

[Horwig]

I'll add my experiences of trying PA with Sharpcap for the first time tonight.

I already have a QHY5L-II-C, a CS to Nikon adaptor, and a selection of old manual Nikon lenses, so everything needed.

My mount is a Skywatcher AZ EQ 6, so mounted the scope on it, with its Telrad, and the Nikon /QHY combination.

Started out with a 200 f4 nikkor and four second exposures at 100% gain. Just placed the mount roughly north and starting the PA routine, no luck. So, had a look through the polarscope, no Polaris in sight, adjust the mount in AZ and DEC to bring Polaris into the polarscope and try again, still no luck. Powered up the Telrad, no wonder I was getting nowhere, the polar axis might have been pointing Northish, but the Dec axis was way off, so point the scope to get Polaris roughly in the telrad and start again. BINGO, fist plate solve took seconds, so rotate the RA axis a good quarter turn and continue, again a few second later another solve. Adjusted the RA and Dec bolts, brought the star to the mark, jobs a good one, well pleased.

Then had another play, tried a 105mm f2.4 Nikkor, dropped the exposures to one second at 100% gain, now this lens with the QHY is just outside the guide FOV for a solve at 2.6 x 2 degrees, so wasn't hopeful, but it was giving near instant results, very easy to move the mount, and got the error down to near single figure seconds of arc.

I tried another iteration to see if I could get the error even smaller, but this time it was off by about a quarter of a degree, but on looking up, cloud was starting to obscure stars, so I assume it was getting well confused.

Not perfect, but as a first tryout, this was a very impressive start, next clear night will try again, but I think the QHY/Nikkor 105 will live alongside the Telrad as a permanent fixture, and this will be how I find North from now on.

Robin, I salute you.

Huw

[rwg]

@Horwig glad to hear it is working for you. The early versions (2.8) had some problems with large fields of view and too many stars as well as with too few stars. That's fixed in 2.9, so you will only get problems if the FOV is too small and there are not enough stars that SharpCap recognises in the field of view.

cheers,

Robin

[Crose01]

I used the latest version last night and had sub 30 Arc Secs of Polar Alignment error with my HEQ5 and Altaiir Astro GPCAM. Looking now how I can get it to work on my Astrotrac. Will this work with the Meteor Lens that you get with the GPCAM?

[rwg]

No, the field of view is going to be too wide with that lens. You might be able to find a 40-50mm CS lens on ebay that would might get you (just) to a small enough field of view. The 2.1mm meteor lens is just too wide angle.

cheers,

Robin

[Horwig]

Interesting second attempt with SharpCap last night.

Solving with the 105mm lens was near instant, and aligning the mount a breeze, however, on trying another iteration, it was a few minutes of arc out, and  again on a third iteration, this might be down to flex/sag in the mount when rotated a quarter revolution, will investigate again.

I'm going to build a bracket to mount the polar finder on the mount body itself, not on the scope, should make life easier, and see if the problem remains.

Also, interestingly, after aligning, I had a look through the polar scope, on rotating the polar axis, Polaris moved on a perfect arc around the alignment circle, though not ON the circle, looks like the scale of the markings on my polar scope are a bit too large.

 

Huw

[Horwig]

 I should admit, as another poster did above, that I did not follow the procedure correctly, and did not go to the final screen before adjustment!

Now that I have overcome the RTFM fault, and also mounted the camera solidly on the body of the AZEQ6, I'm consistently getting alignment to about 10 arc seconds, reckon that might be good enough...

Thanks Robin for a superb piece of software

 

Huw

[rwg]

I have been trying to work out what the problem is with adjusting before moving on (both in practice and theoretically), and I think I have finally worked it out...

If you start adjusting before you move on to the last phase then you notice that as you adjust, the position marked in the frame for the RA axis starts to move. This is wrong... The position of the RA axis within the frame is determined only by the alignment of the scope and camera you are using to the mount RA axis. Moving where the RA axis points in the sky should have no effect on the position of the RA axis within the frame.

So, basically, by adjusting before the final stage you are confusing SharpCap's calculation of where the RA axis currently is, which in turn throws the alignment off.

cheers,

Robin

[Bobby1970]

Can anyone suggest what sort of c mount lens would be good to use with Altair Gp cam for polar alignment within sharpcap? I only have the small meteor lens at the moment. I assume this will be no use?

[rwg]

The meteor lens is much to wide angle (too short a focal length). Recommended focal length for a GPCAM or equivalent sensor size is 200mm (finder guider) - this gives a 1.2 degree FOV. 100m should work (~2.5 degree FOV). 50mm has a chance of working (~5 degree FOV) but is untested as far as I know.

If you don't want to go down the finder-guider route, it might be worth looking into cheap 2nd hand manual focus zoom lenses for SLR cameras (say a 70-210 zoom) and an appropriate adapter to attach to a C/CS mount. I have a Pentax 70-210/4 zoom that I use for camera testing and 3d printed a GPCAM adapter for it (https://www.thingiverse.com/thing:1838091)

cheers,

Robin

[Bobby1970]

Robin, thanks for the info I was struggling on a previous attempt to get my celestron 70mm travel scope to work. I think it was possibly due to thin and patchy cloud cover not allowing a decent view of the polar area. 

I tried again last night using the same scope and it worked fine. 

Until the front element dewed up. Lol. 

Many thanks for the info again. Much appreciated. 

[alcol620]

 

I have a fixed pier and have been taking 10 minute subs with no visible star trailing

Downloaded the software and tried out this evening. Followed the process and the program came up with the following results.Polar Align Error 00 06' 50" and told me to move the Polar Axis Right 00:05:36 and Up 00:03:55 however it didn't show anything else on the laptop screen to show where to make the moves to. Any ideas?

Thanks for your help

[rwg]

Could you post a screenshot showing the issue. It would be very helpful if you could capture a pair of frames (one in each orientation) and share those too.

cheers,

Robin

[alcol620]

Hi Robin

I seem to have had better luck a second time and could see all the info on the screen. Played around with the Alt and Az and got the Polar Align Error down to about 10". Took a 30 minute image with no noticeable star trailing. Will check against PHD polar alignment in due course. Provided my alignment is as accurate as the software says, then it is an excellent way of getting accurate polar alignment. Many thanks for your efforts.

Alec

[Bobby1970]

I used it again last night. Seemed to work well. Had the 70mm travelscope on top of the st120. Got to within 30 seconds and left it at that. Seems like an excellent solution in my limited experience

[alcol620]

Hi Robin

Do you have any suggestions of how this software might be used to polar align a Skywatcher Star Adventurer? Presumably you would need a light weight guide camera and guide scope? Or is there another way?

Thanks for your help

Alec