Polar Align with Atmospheric Refraction calc

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cat
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Joined: Thu Oct 26, 2017 4:55 am

Polar Align with Atmospheric Refraction calc

Post by cat » Thu Oct 26, 2017 5:15 am

Hi Robin
I'm planning to purchase a pro 3.x version esp for polar align feature. Have followed the online description demo of sharpcap PA and found it intuitive and easier than other methods. I live in Mumbai and places we go imaging are almost always 18-19deg latitude. That amounts to 2.7 arc minute atmospheric refraction as i checked in calculators. Does it mean i will always get that much error in final result?

If yes then any plan to account for those calculations(as an additional precision option) in future versions?

Lat Lon details can be precisely manually input by user since our phones have gps. We can also check and input manually approx temperature at site from online weather sites/local sensor. Can sharpcap then update the adjustment of alignment stars by that much extra correction?

Regards
Anu

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admin
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Re: Polar Align with Atmospheric Refraction calc

Post by admin » Thu Oct 26, 2017 10:23 pm

Hi Anu,

There is currently no correction for Atmospheric refraction in SharpCap's polar alignment, so you would be in error by the 2.7 arc minutes if you adjusted following the normal instructions. However, it should be possible for you to work around this issue very easily -

Polar align as normal, ignoring refraction to get 'correct' alignment. At this point your Azimuth will be correct but your Alt will be 2.7 minutes too high.
Now adjust the altitude, dropping it until the error readout on SharpCap is 2.7 minutes of arc.

This should manually correct for atmospheric refraction (since you already know how much to correct).

An automatic correction would (as you note) require the exact Latitude and Longitude. SharpCap now can read this info from your ASCOM mount if you connect one (or even an ASCOM simulator mount if you set one of those up), but for non-ASCOM users it would require me to add UI for the Lat/Long as well as the refraction calculations. I will consider this for a future version of SharpCap.

Hope this helps,

Robin

cat
Posts: 2
Joined: Thu Oct 26, 2017 4:55 am

Re: Polar Align with Atmospheric Refraction calc

Post by cat » Fri Oct 27, 2017 7:36 pm

Dear Robin,
Thank you so much for reply and solution.
Now using following tool, pre emptively checking refraction for observing sites i know have no connectivity.
http://wise-obs.tau.ac.il/~eran/Wise/Ut ... ction.html

It looks like mere half an arc minute play in most common T & P variations. 8-)

Wishing there could be something like dual speed add on similar to focuser knobs or atleast finer pitched polar alignment knobs to our mounts :lol:

Anu

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Re: Polar Align with Atmospheric Refraction calc

Post by admin » Fri Oct 27, 2017 8:17 pm

I can certainly agree with you about how poor the Alt/Az controls on many mounts are - unfortunately there are no cheap and good solutions to that :(

Half a minute of arc isn't going to be a problem - polar alignment is usually considered good enough for long exposure astrophotography if you get within about 2 minutes of arc of being correct.

cheers,

Robin

ken
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Joined: Thu Nov 01, 2018 10:10 pm

Re: Polar Align with Atmospheric Refraction calc

Post by ken » Fri Nov 02, 2018 12:42 am

admin wrote:
Thu Oct 26, 2017 10:23 pm
Hi Anu,

There is currently no correction for Atmospheric refraction in SharpCap's polar alignment, so you would be in error by the 2.7 arc minutes if you adjusted following the normal instructions. However, it should be possible for you to work around this issue very easily -

Polar align as normal, ignoring refraction to get 'correct' alignment. At this point your Azimuth will be correct but your Alt will be 2.7 minutes too high.
Now adjust the altitude, dropping it until the error readout on SharpCap is 2.7 minutes of arc.

This should manually correct for atmospheric refraction (since you already know how much to correct).

An automatic correction would (as you note) require the exact Latitude and Longitude. SharpCap now can read this info from your ASCOM mount if you connect one (or even an ASCOM simulator mount if you set one of those up), but for non-ASCOM users it would require me to add UI for the Lat/Long as well as the refraction calculations. I will consider this for a future version of SharpCap.

Hope this helps,

Robin
Hi Robin,

May I ask 2 questions.

1. In case my mount is connected via ASCOM and selected the mount type accordingly on hardware setting screen of SharpCap(2.9.3032) ,
is there still no correction made for Atmospheric refraction using the latitude information given by ASCOM ?

2. Mentioned that " There is currently no correction for Atmospheric refraction in SharpCap's polar alignment," Does this mean SharpCap does not make any correction, or correction is made based on estimating user's latitude to be 45 degrees North or South as mentioned in older version of user manual ?

Regards,

Ken

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Re: Polar Align with Atmospheric Refraction calc

Post by admin » Fri Nov 02, 2018 12:12 pm

Hi,

you need to update to a newer version of SharpCap (3.1 or 3.2) to take advantage of built in refraction correction - see https://docs.sharpcap.co.uk/3.2/#Polar% ... ment%20Tab

SharpCap 2.9 and 3.0 do not correct for refraction under any circumstances.

cheers,

Robin

ken
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Joined: Thu Nov 01, 2018 10:10 pm

Re: Polar Align with Atmospheric Refraction calc

Post by ken » Fri Nov 02, 2018 11:10 pm

Hi,

Thank you for your exlpanation.
It's all clear now.

ken

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