Helpful tip

Using SharpCap's Polar Alignment feature
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oopfan
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Helpful tip

Post by oopfan » Sun Jul 09, 2017 12:50 pm

My experience with SharpCap Polar Alignment is generally good although sometimes it just wants to fight me and can take up to an hour. Last night was one of those great nights where I got within 5 arc-minutes in about 10 minutes.

Here is a handy rule that I hope helps others:
SharpCap begins by asking you to point your scope within 5 degrees of the pole. (It is not clear from the instructions if this should be from the home position.) When you achieve that and then click "Next" it will attempt to do a plate solve. Then it will ask you to rotate 90 degrees.

Anyone who has used SC Polar Alignment knows that plate solving can be agonizing. Sometimes you are lucky and are positioned in a rich star field that is solved quickly, and other times the field is sparse and cannot be solved quickly.

My recommendation is that you should consider restarting the alignment process several times in order to find a rich star field in your last movement. That is where you will spend the bulk of your time making tiny adjustments to your mount. You want to be in a rich star field so that you get a plate solution on every frame update.

In other words, do not place much importance on the speed of the initial plate solve when you begin the procedure. That is a one shot deal for the algorithm to plant its feet. Concentrate instead on the solution speed after you make the 90 degree rotation. You want it to be fast.

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Re: Helpful tip

Post by admin » Sun Jul 09, 2017 9:31 pm

Yes, this is entirely true - it's the plate solving during the final stages that needs to work every frame. Ideally though, you want *every single frame* to solve correctly. I find this is achievable without too much trouble with the following equipment/settings

* 200mm F/4 guide scope
* Altair GPCam Mono (AR0130M sensor)
* Max gain
* ~2s exposures

Problems might be caused by

* Longer focal length means reduced field of view. If there are not sufficient stars in SharpCap's database within the field you are looking at then it will never solve. The database goes down to Mag +13.5
* Slower F ratio - this reduces brightness and needs longer exposures to compensate
* No gain control for camera - try turning up the digital gain control in the star detection instead and turn down the black level too.
* Colour camera - less sensitive than mono, so may require longer exposures
* Lots of hot pixels being detected as stars - try turning up the minimum star width or the noise reduction options.

SharpCap shows you the status of the most recent frame it has processed in the UI
Capture.PNG
Capture.PNG (8.75 KiB) Viewed 214 times
If this shows 'Could Not Solve' for more than about 2-3 frames in a row then there's no point waiting in the hope that a frame will solve by luck - it's time to try adjusting things to get more frames to solve.

Things to do include

* Turn up the exposure (might be necessary particularly for colour cameras that are less sensitive overall than mono ones)
* Turn up the camera gain
* Play with the star detection settings (noise reduction and digital gain seem to have the most effect)
* Rotate the RA axis to try to find a richer star field.

cheers,

Robin

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Re: Helpful tip

Post by oopfan » Mon Jul 10, 2017 5:09 am

Great news, Robin, PA went superbly well tonight. I took your advice and boosted my Gain to 100%, Minimum Star Width to 4px, and set the Exposure to 2 sec. (Previously I would run Gain of 50%, the default Minimum Star Width of 1px, and Exposure of 4 sec. I think that the Plate Solver did not like all of the hot pixels.) Seeing conditions were excellent and the Sun was 12 degrees below the horizon. I am sure that helped.

Perfect! Thanks very much.

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Re: Helpful tip

Post by oopfan » Mon Jul 10, 2017 2:29 pm

After last night's success, I have reflected upon the lessons learned. As a consequence I think I may have stumbled upon the answer for the up-down-left-right problem that many of us have reported:

First a bit of background. For many years I worked in the railroad rail testing space. Sensitive instruments glide over the rail on a truck using various technologies to see inside the rail in the quest to find defects early before they lead to a derailment, loss of life, etc. The sensitivity is turned way up. This leads to a great number of suspects that turn out to be false positives.

It is with that background that I am beginning to see that SharpCap's Plate Solver may be adversely influenced by a great number of hot pixels resulting in false positives. Certainly, false positives will impede the Plate Solver from finding a solution; we have all witnessed that. But I am also beginning to think that there is a secondary effect that results in a solution but an inaccurate one. In all of my life I have never come across an algorithm that is impervious to garbage inputs.

For me, increasing the Minimum Star Width from 1 pixel to 4 pixels seems to have done the trick. I will certainly keep an eye on it and report any discrepancies.

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Re: Helpful tip

Post by SdA » Sun Aug 13, 2017 3:43 pm

Hi oopfan and all members,

My combination is:
Skywatcher Startravel Refractor 80mm/400mm scope (without focal reducer) and ASI120MC.
If i use like you 2sec exposure 100 gain (or maybe 85~95) with dark frame subtraction and minimum star width 4 pixels do you believe that i can do a good PA or i have to use a focal reducer ? Yesterday night i spent 1 hour with no result.

With a finderscope 50/175 it was very easy for me to do a very good PA with SharpCap, question of a few minutes.

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Re: Helpful tip

Post by oopfan » Sun Aug 13, 2017 4:56 pm

I would suggest getting the 0.5x focal reducer.

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Re: Helpful tip

Post by oopfan » Mon Aug 14, 2017 12:21 am

Let me take that back...

If using your finderscope at 175mm focal length works then use it! You will save the cost of the focal reducer. The focal reducer will make your 400mm scope into 200mm. There isn't much difference between 175 and 200, plus you will get a slightly wider field of view at 175mm which is a good thing. I say go with the finderscope and save money!

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Re: Helpful tip

Post by SdA » Mon Aug 14, 2017 6:39 am

Thank you very much oopfan. I'll change the 400mm with the 175mm.

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