Polar alignment with Sharpcap tool

[Roysten]

Over the last few months I've been trying my hand at imaging DSOs, in learning to do this I've found that you really can't get away with exposure times longer than a few seconds without motors and a solid polar alignment. Initially I tried this by eye using Polaris, the best I achieved was around 8 seconds of exposure before the inevitable drift of whatever is in my view. I thought about buying a polar scope or learning the drift technique, then I happened upon the polar alignment facility in Sharpcap 2.9 on this forum and gave it ago. Jut thought I'd write a few things about how that's gone for me.

Firstly I tried using the software with my 150P with a field of view of <1/2º, unsurprisingly this didn't work; I just couldn't see enough in the field of view for the software to lock onto anything. I calculated the field of view of my 70mm Travelscope with my cam and still found it to be shy of the 1º requirement. So I bought myself a cheap 0.5× reducer to achieve ~1.8º across field of view.

As most people have found the weather has been shocking with weeks of cloud at a time. Since I got the reducer at the beginning of Feb I've only been able to get out three times. It's only on the third time late last week I was able to finally achieve a polar alignment using the software! It took me over an hour; the light pollution in my subburban part of Hull is pretty bad and the software was rarely able to consistently detect the same dozen stars with every refresh of the image. I had to have my gain quite high with an exposure time of 5 seconds to get much out of my images. I was getting quite a bit of noise in the images but increasing the minimum star-size in Sharpcap from 1 to 2 pixels seemed to help. Quite often I'd get a lock for the first RA angle but not on the second, which was very frustrating. The times I got a lock from both angles were then hampered by the lack of the yellow line, which dictates which way to move the Alt-az and altitude on my mount, obviously in those instances I was very far away from the NCP. It isn't an overstatement when I say I was over the moon once it finally locked onto the correct point!

I achieved a PA less than 1" from the NCP, I have seen people who have achieved much better than that but this has been an order of magnitude better than anything I've done previously, where I was lucky to get within half a degree.

For those who struggle with PA I can definitely recommend the Sharpcap feature, it is fairly intuitive, easy to use and quick.

To show the difference it can make I've attached an image of everyone's favourite - M42. Multiple other problems with the image aside I'm pleased with the improvement of the image with good PA compared to the one by eye. Both images were taken as 30 second exposures on a motorised mount.

I hope this is helpful for those who have struggled like me and I'd be interested in hearing other people's experiences with PA.

[hornedreaper33]

Hey Roysten awesome picture of M42 well done

I am days away from having my first t-Ring and adapter and will be setting up for taking my first images. Is there no way of getting an accurate PA without using software? It seems every time I log into here to read the latest, there is always a new thing to learn.

[The-MathMog]

2 hours ago, hornedreaper33 said:

Hey Roysten awesome picture of M42 well done

I am days away from having my first t-Ring and adapter and will be setting up for taking my first images. Is there no way of getting an accurate PA without using software? It seems every time I log into here to read the latest, there is always a new thing to learn.

Drift alignment as mentioned, is a possibility, as all it really needs is a camera capable of doing exposures of a few minutes. And it can be really accurate depending on how much effort you wanna put into it, and it takes software calculations (All Star Polar Allignment for example) out of the equation.

And to the original post, it sounds like an interesting feature. Are you just using your main imaging camera, or how do you go about it?

[astrosathya]

Hi Roysten,

I tried using sharpcap to polar align from my city apartment, but the Light Pollution is simply too much and hardly any stars are visible in the region. What are the settings you use for PA with Sharpcap?

[Roysten]

Hey @hornedreaper33, that a Dungeon Keeper reference there? I think a lot of people generally use a polar scope, which you can pick up for ~£30 depending on your mount, FLO and 365Astronomy both sell one for my EQ3 mount. I've heard good things about Polar-Master's but they look very expensive! Before using Sharpcap I was just using my finderscope and aiming about 1º away from Polaris in the direction of Dubhe and Merak in the Plough, admittedly very crude!

@The-MathMog, I used my ASI120MC with a 0.5× reducer attached to my Travelscope 70 to get the alignment then switched to my 150P and DSLR to capture the main images. Probably be better if I could mount my smaller scope ontop of my 150P but I don't really have all the necessary attachments.

Hi @astrosathya, the light pollution is made near me too! To get many stars at all I was having to use an exposure time of 5 seconds and gain at near 100% to get a lock. Seeing was pretty fair, better than the other time I tried it. Will be interesting to get a little into the country and see if it's any easier.

[hornedreaper33]

3 hours ago, Roysten said:

Hey @hornedreaper33, that a Dungeon Keeper reference there? I think a lot of people generally use a polar scope, which you can pick up for ~£30 depending on your mount, FLO and 365Astronomy both sell one for my EQ3 mount. I've heard good things about Polar-Master's but they look very expensive! Before using Sharpcap I was just using my finderscope and aiming about 1º away from Polaris in the direction of Dubhe and Merak in the Plough, admittedly very crude!

@Roysten you got it. I loved that game and when I first got the internet in the late 90's it was the first thing that came to mind.

I was reading the follwing link:

https://www.cloudynights.com/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760

Once I finally got my head around the process I think I should be able to align somewhere near. Good luck with your imaging.