Cannot Rotate OTA After PA Initial Step

Using SharpCap's Polar Alignment feature
GaryS
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Re: Cannot Rotate OTA After PA Initial Step

#11

Post by GaryS »

Well, not sure what that means since I don’t have a guide scope. I have a digital finder ASI290mmini and my main camera which is ASI294mc. I’ve set up plate solving with my main camera and it works perfectly for EAA gotos. I’ve never tried plate solve with the 290 mini. The 290 has a CS 8mm F1.5 lens directly attached so FOV is about 40x23 degrees. Would I need to set up different indexes for plate solving to work?

If I use the 294 my FOV is 82 x 56 minutes. Would that not work? Still not clear where the start point should be versus the first rotate-to point.
Gary
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oopfan
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Re: Cannot Rotate OTA After PA Initial Step

#12

Post by oopfan »

Gary,

I searched the net for "digital finder ASI290mmini" but came up empty. What is it?

Brian
GaryS
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Re: Cannot Rotate OTA After PA Initial Step

#13

Post by GaryS »

Brian
I’m sorry for the confusion and causing you to spend time on this.

I use a CS lens(security camera heritage I think), directly attached to the ZWO ASI 290 mono mini camera, as a digital ‘Finder’ camera. It gives me a great wide field view which works as a Finder, but also nicely captures views of entire constellations (Orion for example) and alerts me to approaching or dispersing clouds.
Regards
Gary

Attached Image of CS lens on ASI290mmini
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FF6717EB-D42D-4EA6-A691-09F2695DA58C.jpeg
FF6717EB-D42D-4EA6-A691-09F2695DA58C.jpeg (565.05 KiB) Viewed 1538 times
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Re: Cannot Rotate OTA After PA Initial Step

#14

Post by admin »

Hi,

the field of view of your CCTV lens will be too wide, but the FOV of your main camera at 82 x 56 minutes should be just fine - I'd suggest using that. You may want to use 2x binning with that camera - there is no need for massively high resolution for the polar alignment.

I have had success doing polar alignment with longer focal length CS lenses - for instance something like this : https://www.ebay.co.uk/itm/402660318506 ... Swrv1gDwHx at 35mm focal length. I've also seen a lens like that advertised as 50mm focal length ( I bought one, but it turned out to be 35mm focal length...). The ones like this with the variable aperture are great for testing cameras - I have quite a collection :)

cheers,

Robin
GaryS
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Re: Cannot Rotate OTA After PA Initial Step

#15

Post by GaryS »

Thanks Robin - timely advice. I'm hoping to test PA tonight if the sky stays clear.
Gary
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Re: Cannot Rotate OTA After PA Initial Step

#16

Post by GaryS »

So, I tried the SC PA routine for the first time last night. I was thrown off a bit looking for a star with an 'arrow' which I never was able to find. I also had no idea which way to manipulate the mounts AZ and Altitude adjusters but by trial and error and 2 'restarts', I managed to obtain the result shown in the first image attached below.

According to the text, it was 'excellent PA' but I'm confused because I never managed to center anything in any target. I could see arc'ed circles, suggesting a bullseye somewhere, but never saw the entire target. None the less, I kept manipulating the controls following only the written directions on which way to go. Eventually I ended up where you see in the image.

Is this right? I'm guessing I did something wrong but don't know what or how to do better next time. I was using the ASI 294 MC, as mentioned earlier, for the PA process.

Why did I never see a target and yet get excellent PA. Is following the Up/down and right/left instructions adequate? Is there a star with an arrow on it somewhere? What should I do differently?

thanks for your patience and help!
Gary

ps: When I use iOptron's built-in iPolar camera and their PA procedure, I get a different PA. I did this the night before so when I used SC last night, SC showed me to be way off from the NCP. Is such a difference common when using two different methods? Thoughts? Is the SC PA still in Beta?


Here's the screenshot from the PA process:

Screenshot (513).png
Screenshot (513).png (590.85 KiB) Viewed 1519 times

Here's another screenshot from SC drift graph. What is the message here relative to PA if there is one?
Screenshot (515).png
Screenshot (515).png (153.1 KiB) Viewed 1519 times
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Re: Cannot Rotate OTA After PA Initial Step

#17

Post by admin »

Hi,

the reason you only see partial circles is that your guide scope is pointing 'off axis' a bit - you could adjust this if you wanted using the adjustment screws, but it isn't important to the result.

When you have a big adjustment to make you should see an arrow like this

Image

Once you get closer the arrow changes to a pair of lines to try to get on top of each other (because seeing a really short arrow is hard)
Capture.JPG
Capture.JPG (16.53 KiB) Viewed 1518 times
It's perfectly good to follow the up/down/left/right instructions to get good results, or to follow the 'move the star to the end of the arrow' - both will give you the same result once completed.

If you have any flex in the mounting of the camera/guide scope then you can get incorrect results (the camera/scope may move due to the change in the direction of forces acting on it as you rotate). A dangling USB cable is enough to cause this in some cases. I expect the iPolar would also be affected by this if there is movement, but maybe not in the same way as SharpCap (different calculations).

You can read more on this at the polar alignment troubleshooting page : https://www.sharpcap.co.uk/sharpcap/fea ... leshooting

Robin
GaryS
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Re: Cannot Rotate OTA After PA Initial Step

#18

Post by GaryS »

Robin
I wasn’t using a guide scope. As I mentioned I was using the ASI294mc which is my main observing camera. So how could that be off axis?

Was the alignment I was getting from the numbers, ie, ‘Excellent’ to be believed not even seeing where my RA axis was pointing and not seeing the ncp as a target?

Is there anything you can say about what the second image is telling me?

Thank you
Gary
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Re: Cannot Rotate OTA After PA Initial Step

#19

Post by oopfan »

Gary,

This happens to me too, that is, rarely seeing the NCP in the image. This happens when my scope isn't pointing at exactly +90 declination. Let's say, for arguments sake, that it is 2 degrees off. I won't see NCP but SC still proceeds with PA. I try to keep my setting circles accurate, but in reality it doesn't bother me that my declination circle is off by a couple degrees. You probably have a more advanced mount. I imagine that you begin PA with the mount in the home position. Is there a possibility that it is not pointing at exactly +90 declination?

Brian
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Re: Cannot Rotate OTA After PA Initial Step

#20

Post by admin »

Hi,

yes, you can possibly have the scope in a position where home is not exactly +90 Declination, which will put the pole out of view. You can also have a misalignment called 'cone error' in the setup - basically where the front of the telescope is either pointing out slightly away from the dovetail or in slightly towards it. Neither will affect polar alignment, but cone error can cause a problem with goto accuracy and it's best to correct it if you can.

The second picture sadly doesn't tell you anything - if you look at the small print you can see that all but one frame has been ignored because alignment has failed (probably not enough stars detected to do the alignment), so the graph has no data in it.

Probably best to use PHD to do a drift graph if you are checking - it's set up for that and while you could do it with the live stacking it doesn't show you whether the error is in RA (possibly periodic error) or DEC (drift due to polar alignement).

Robin
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