Focal length for sharpcap polar alignment

[Wonderweb]

Hi. Sorry if this topic is not in the right forum. I wasn't sure where it belongs.

I have been discussing with a friend about polar alignment using sharpcap. I use my guidescope at 240mm focal length and he uses his main scope at 750mm. 

My question is, does he get a more accurate polar alignment than I do as he is looking at  smaller area of sky and would there be any benefit from doing both (starting with the guidescope and using the main scope to finish)? 

 

Thanks 

[MCinAZ]

  I also use SharpCap Pro as a polar alignment tool, and have been quite pleased with the results. For my mounts, I fabricated adapters which thread into the opening for the polar alignment telescope cap. The adapters provide a standard 1/4-20 screw which is used to mount a ZWO ASI120 camera. Since I have a small collection of Canon lenses, I use the ZWO Canon EF adapter and a 100 mm lens, as pictured below. I've found that this to be a good compromise between field of view and on-sky resolution, and can generally align to within a few arcmin of the pole (as reported by the software) in a matter of a few minutes.

  Because SharpCap reports polar alignment values to the arc-second, it's easy to chase perfection that will likely never be achieved. There are a number of error terms which are often of no real consequence but will affect the numbers displayed. Seeing, image resolution, accuracy of the algorithm used to extract centroids, flexure, etc., can all contribute discrepancies which are apparent at the arcsec level. In most cases, however, polar alignment to with one or two arc-minutes is more than good enough. What I've found works as well as I ever need is to perform the initial alignment per the image/rotate/image procedure, then rotate back through 90 degress, then an additional 90 degrees, noting the errors at the three points. If they look bad -- say, three arcmin or worse -- I'll touch things up if I need high accuracy. I sometimes image with a camera that provides a 12 x 9 arcmin field of view, so in order to ensure that my targets will fall on the sensor following a slew, I like to be polar aligned to with a couple of arc-minutes. If I'll only be viewing with an eyepiece, alignment within 10 arc-minutes is all that's needed.

  Given the focal lengths you describe, I would think that coarse alignment near enough to the pole to allow successful plate solutions and unintended movement in declination during the 90 degree rotations could be challenging. Even at 270 mm, I can't imagine that image scale is your limiting factor. The QHY Polemaster uses only a 30 mm f.l. lens, and lots of people are very happy with them.

[StuartT]

I also use SharpCap to polar align (though the new PA tool in NINA looks much nicer, so I'll prob switch to that).

But I am not using a guide scope for PA, I actually use my main scope only (808mm FL). Even though it can only see about 1 degree x 1 degree of sky, the SharpCap routine work just fine.

I agree with @MCinAZ that chasing perfection is rather pointless. If I get down to 1 arcmin of PA error, I think that's fine and I stop there. Prob even 2 arcmin is fine.

[tooth_dr]

I agree with the above, I use a 185mm focal length to PA using SharpCap, and it seems to be adequate for my imaging needs.

[Wonderweb]

Thanks for the info. 

It sounds like I'm spending far too much time and effort on polar alignment Although  I thought this was important so your guiding wasn't doing unnecessary work. I try to get down to a few arcseconds in both axis but I'll do a bit of experimentation in future to see what effect less accurate polar alignment has on my guiding. I'd rather spend my time imaging than messing around with p/a. 

I've also heard really good things about the nina p/a. I use nina for everything else so I may give it a try. 

[Neil27]

Hi @Wonderweb

I’m just perfecting my PA routine and am planning on using my ‘smaller’ scope the Altair 80mm, focal length of 560mm I think and this seems adequate.

For sharp cap as long as you can see 1 degree x 1 degree of sky and are no greater than 5 degrees away from pole you should be fine, well thats the theory anyway 😉👍

Not tried it in anger yet, just bought the sharp cap pro licence.

 

[StuartT]

15 hours ago, Wonderweb said:

Thanks for the info. 

It sounds like I'm spending far too much time and effort on polar alignment Although  I thought this was important so your guiding wasn't doing unnecessary work. I try to get down to a few arcseconds in both axis but I'll do a bit of experimentation in future to see what effect less accurate polar alignment has on my guiding. I'd rather spend my time imaging than messing around with p/a. 

I've also heard really good things about the nina p/a. I use nina for everything else so I may give it a try. 

you definitely don't need to get down to a few arcsec of polar alignment. 1 arcmin would be pretty good and 30 sec would be pretty excellent. Unless you have a long focal length main scope you really don't need to fuss too much. What is your main scope's focal length?

[The Lazy Astronomer]

17 hours ago, Wonderweb said:

Thanks for the info. 

It sounds like I'm spending far too much time and effort on polar alignment Although  I thought this was important so your guiding wasn't doing unnecessary work. I try to get down to a few arcseconds in both axis but I'll do a bit of experimentation in future to see what effect less accurate polar alignment has on my guiding. I'd rather spend my time imaging than messing around with p/a. 

I've also heard really good things about the nina p/a. I use nina for everything else so I may give it a try. 

I'm going to buck the trend a little here, because I do polar align more accurately, usually to within 10 - 30" total error as reported by Sharpcap. I always set up in the same place in my garden, and find it only takes a couple of minutes to get the polar alignment down to that level. 

I've run the guiding assistant a couple of times and that reports a PA error of about half an arc min, so pretty good agreement with Sharpcap. My guide graph also shows barely any dec corrections throughout imaging runs.

The way l see it, anything l can do to make the mount's job easier is for the better. 

[Wonderweb]

On 04/10/2021 at 11:05, StuartT said:

you definitely don't need to get down to a few arcsec of polar alignment. 1 arcmin would be pretty good and 30 sec would be pretty excellent. Unless you have a long focal length main scope you really don't need to fuss too much. What is your main scope's focal length?

My main scope is an rc6 at 1370mm which I often reduce down to 1027mm.

[Wonderweb]

On 04/10/2021 at 13:00, The Lazy Astronomer said:

I'm going to buck the trend a little here, because I do polar align more accurately, usually to within 10 - 30" total error as reported by Sharpcap. I always set up in the same place in my garden, and find it only takes a couple of minutes to get the polar alignment down to that level. 

I've run the guiding assistant a couple of times and that reports a PA error of about half an arc min, so pretty good agreement with Sharpcap. My guide graph also shows barely any dec corrections throughout imaging runs.

The way l see it, anything l can do to make the mount's job easier is for the better. 

That was how I saw it but I will try different levels of polar alignment and see what effect it has. 

[rwillett]

I use a cheap 130mm F4 lens with a altair Astro guide cam. Its accurate enough for me.

Rob

[Wonderweb]

I feel like I'm happy with the accuracy of my polar alignment using my guidescope but it does take a lot of messing about trying to get down to the level I was trying to achieve. I was really wondering if it would make it easier or quicker to get to a decent level of p/a with a longer f/l? 

[StuartT]

11 hours ago, Wonderweb said:

I feel like I'm happy with the accuracy of my polar alignment using my guidescope but it does take a lot of messing about trying to get down to the level I was trying to achieve. I was really wondering if it would make it easier or quicker to get to a decent level of p/a with a longer f/l? 

Well.. I think the effect of focal length kind of cancels out. Shorter FL may get you a less accurate PA, but then again they are less fussy about the tracking accuracy.

I guess the question is how accurate do you really think you need to be? As I said I really don't see the point in trying to get down to a few arcsec for the sake of it, unless you are seeing mis-shapen stars