# Finding Solar North?

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I would like to gather a set of solar images to allow my GCSE pupils to calculate the differing rotation speeds of the sun’s surface. The problem is establishing which way is up! Is there an easy way to work out where north is on the Sun and place that at the top of the camera chip? I look forward to hearing what you all recommend. My current method is to compare my live view with the most recent image on GONG and rotate the camera to get as close as possible to that orientation. I sometimes tweak rotation in post processing too.

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I have my camera aligned with RA/DEC. The rotational axis of the Sun can then be found using a Stonyhurst disc, see for an animated one:

Nicolàs

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1 hour ago, inFINNity Deck said:

I have my camera aligned with RA/DEC. The rotational axis of the Sun can then be found using a Stonyhurst disc, see for an animated one:

Nicolàs

Thank you very much Nicolàs, that is great. Everyday I discover a little more how little I know about astronomy!!! I now need to ask a multiple questions regarding how to do all of that...!

Am I right in saying that aligning the camera with RA and Dec is as simple as rotating the camera until movements in RA cause the image to move in a level line across the middle of the sensor from side to side and in DEC until level up and down across the middle?

With regards to the Stonyhurst disc - are you able to elaborate a little more on how to actually use that with images on the computer, please?

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Posted (edited)
47 minutes ago, PhotoGav said:

Am I right in saying that aligning the camera with RA and Dec is as simple as rotating the camera until movements in RA cause the image to move in a level line across the middle of the sensor from side to side and in DEC until level up and down across the middle?

Hi Gav,

yes, that is as simple as it gets. Of course it is difficult to get it perfectly aligned, but with some effort you should be able to get the orientation better than 1 degree. I always use a digital cross-hair (from ZWO's ASICAP) and then run the scope indeed in RA-direction with a star at the horizontal cross-hair. Then turn the camera slightly and repeat that procedure until no significant deviation can be noticed. If you have an automatic camera rotator (e.g. Moonlite Nightcrawler) you should be able to get it better than that. You can test the orientation at night by plate-solving an image of the sky.

47 minutes ago, PhotoGav said:

With regards to the Stonyhurst disc - are you able to elaborate a little more on how to actually use that with images on the computer, please?

Once the camera is aligned, the Stonyhurst disc can be used as an overlay on top of the image. Here are solar images of yesterday and today with the discs superimposed on them:

Now it is easy to see how the spots follow the parallels of the Sun.

What I did was to explode the animated gif (the one I linked to in my first post) using https://ezgif.com/split and then downloaded the separate frames using the "Split to Frames!"-button (below the animation). Finally it is just a matter of rescaling and dragging into position.

Nicolàs

Edited by inFINNity Deck
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Nicolàs, you total superstar, thank you for your detailed explanation of the techniques required. That is just so perfect... I love this forum!!!

Your help will enable the next generation of astronomers to be inspired.

Many thanks,

Gav.

• 1
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I knew there would be more questions...

What do you use to work out which 'day' from the Stonyhurst discs to use?

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Hi Gav,

a quick calculation with a calculator: 31 (days in January) + 28 (February) + 31 (March) + 21 (today's date) = 111, so I used "day 110" from the animation (they are provided at 5 days intervals). If you want more precision, you either need to rotate the overlay slightly, do the calculations yourself (creating your own overlay), or use software like HelioViewer2.

Nicolàs

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Ah ok, as I was typing I did wonder if the day number was literally the day of the year. I reckon it's always better to ask a silly question to get an easy answer though! Thank you for your help. All I need now is a run of sunny days and plenty of surface activity at varying latitudes to follow!

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Posted (edited)

I use Les Cowley's Tilting sun ( www.atoptics.co.uk/tiltsun.htm) to show the solar directions and add graphics to images.

As mentioned, the solar image should be orientated E-W, N-S on the chip - easily checked by moving the mount slightly in N-S and watching the image - should tracking vertically in the frame.

Easy, straight forward and impressive.

Edited by Merlin66
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Thank you @Merlin66 - that looks good. Shame it’s not for Mac too!

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The wonderful clear sky this morning gave me the chance to try all of this out and here are the results:

Very happy with that!

All I need now is a clear sky in a few days time to get another shot...

Thank you for your help folks.

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Hi Gav,

which of the methods did you use for the overlay?

Nicolàs

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8 hours ago, inFINNity Deck said:

Hi Gav,

which of the methods did you use for the overlay?

Nicolàs

I used the day 115 frame from the animated gif that you posted towards the start of this thread.

Quick question - why the two latitude scales on opposite sides that are reversed??

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Used Upwards is for Bo=-5 (April) and turned upside down is for use at Bo=5 (around Sept)

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1 hour ago, Merlin66 said:

Used Upwards is for Bo=-5 (April) and turned upside down is for use at Bo=5 (around Sept)

Ah ha! Thank you.

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