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Monday 14th occasionally windy, clear start but cloud promised for the rest of the day. I'm watching the dome for the first signs of sunshine reaching it. There is a low hill to the east which blocks sunrise until later, depending on the time of year.Make sure your telescopes are balanced on the mounting to avoid errors as sagging occurs.
First and vital stage: Setting the camera perfectly upright. The Declination drive is used to move the image of the solar disk up and down the screen. While the camera is gently twisted until the limb moves parallel to the upright cross hair. To put it another way: The limb should touch the line where both horizontal arrows are shown.
In normal use the ZWO USB3 cable of my ZWO ASI-174sticks out of the bottom of the camera body when the image is upright on the computer screen. While the ZWOASI-120MC the cable sticks out of the top. Inversion of the camera will cause problems later. You can check uprightness of the image by focusing on a tree or a roof. Don't forget to replace the solar filter afterwards!
To save making a solar filter to protect the ZWO camera I just popped the home made, 90mm Baader Solar foil filter cup over the modified finder. Then I had an hour of sunshine before it completely vanished behind the cloud.
With the instructions in front of me I repeatedly adjusted the mounting mechanically to keep the solar disk within the circular reticule in SharpCap. I used 200% zoom to expand the sun into the circles after the first rough trials with maximum field of view.
Image drift on the SharpCap screen: Physically adjust the mounting as follows:
Image drifts North or UP on the screen? Move the top of PA to the EAST. [Clockwise]
Image drifts South or DOWN? Move the top of PA to the WEST.[Anticlockwise]
Image drifts East or LEFT? Increase PA Altitude angle.
Image drifts West or RIGHT? Lower PA Altitude angle.
Carefully Recentre the sun's disk in the reticule each time using the slow motion drives after each mechanical adjustments is made. Allow time for drift to be established before making further adjustments. Beware of false drift caused by cloud softening the disk's edges. Try zooming in SharpCap to produce a large image which best matches the reticule rings.
Make sure you don't have the mounting adjusters locked before making adjustments. Either nothing will happen. Or only partial changes will be made. In cloudy weather you can keep adjusting the gain or exposure in SharpCap to produce a sharp edged, bright image of the disk. Above all be patient. Make small adjustments at intervals long enough to be sure of the direction of image drift. If you do overshoot you will just have to work backwards using the same instructions.
My PA altitude was slightly low and was pointing too far east. Just before the sun was lost I had hardly any movement between the disk and circles over several minutes. As usual I levered the mounting in azimuth with a 3' scaffolding pole pushed through the altitude adjusting port. PA altitude adjustment was via the turnbuckle. There was no need for an external reference because it was all there on the screen. Nothing more was required other than the sun's disk remaining central in the reticule. Naturally, the AWR drives were set to Solar first.
Second stage: Centring the sun's disk in the circular SharpCap reticule and hoping it stays there. If not, then slight, mechanical adjustments are made to the mounting. Only use the drives afterwards to bring the disk into the centre of the reticule. I kept playing with SharpCap's Exposure and Gain settings to keep the disk just visible behind the earlier and thinner cloud. Eventually the cloud won but not before I was very close to being correctly aligned. A very satisfying result.
I'll post some pictures after morning coffee. The sky is becoming darker and greyer now. Exactly as forecast. It's a good job I took a chance on the early sun appearing rather than going for my morning walk.
I should add that the physical adjustments required to the mounting were all very minor. It wasn't possible to see any real change by eye alone. A gratifying approximation using only a compass and a digital clinometer on the PA shaft
That said. the sun used to sink slowly on the screen as I captured videos. Though it should be remembered that this often involved using a 2x Barlow on a 1200mm focal length. The equivalent of 2.4m focal length. Plus the 5.6x crop factor produced by the tiny chip in the ZWO 120MC-S.
Later glimpses of the sun teased me into trying again but were too short lived to be certain of image drift. Later still, there was watery sunshine allowing me to check image drift at intervals between heavy cloud while I scribbled on my blog.
Click on any image for an enlargement.
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