14.03.2020 Sharpcap mounting alignment using sun's H-a image..

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Saturday 14th March 2020. 41F and windy.

Poor seeing and increasing wind were spoiling my imaging.

So I checked if SharpCap could be used to align the mounting using the huge image via the H-a 150/10 telescope. Last time I used a 9x50 finder with the eyepiece removed and a Baader, solar foil filter up front. The relatively low magnification made it easy to set the sun's image inside a graticule circle using SharpCap's zoom function. Any image drift, in any direction, was then easily seen.

In fact it proved to be quite easy using the much larger, magnified solar image, from the 1500mm focal length telescope using my ZWO ASI174 camera.

The first and vitally important step is aligning the camera's image perfectly upright. So that the edge of the sun's image exactly followed the graticule's vertical line from top to bottom.

This should occur as the up and down buttons on the paddle are pressed in turn. Ensure you have, normal, "upright" image orientation. An inverted image will certainly confuse the issue. Odd angles of only near uprightness will make alignment a complete lottery.

Once I was satisfied the camera image was upright on the graticule I set the lower edge of the sun's image on the horizontal line using the control paddle. The drive was left running at Solar rate. [Image above right.]

I strongly recommend you set the PA north pointing correctly before you try to set the polar axis, altitude angle. Any RISE or FALL of the solar image relative to the graticule's horizontal line requires the TOP of the PA axis to be moved EAST or WEST respectively.

Once there was no longer any up or down drift over several minutes, I moved on to check the polar axis altitude. I set the right [west] edge of the sun's image against the vertical line of the SharpCap graticule using the mounting control paddle. [See image right.]

If the sun's image drifted east or left I INCREASED the PA altitude angle. If it drifted right or west I LOWERED the PA altitude angle. Once it looked close to being correct on the screen I allowed at least five minutes for drift to occur before making any further changes.

Testing this way uses the actual telescope optics to test the mounting's alignment. The view is highly magnified which aids accuracy. Any error of misalignment, of the telescope relative to the mounting, is also [hopefully] reduced.

Trying to set the mounting, even using a good compass, relies on too many external factors. I had preset the north angle of the mounting using a 2m long, precision straight edge. Even allowing for local magnetic variation of north [deviation] and carefully avoiding all local iron or steel, the result was wrong. At least it was according to SharpCap's reticule test. My mounting's azimuth "slow motion" is a 4' length of scaffolding pole! Sad but true. 🙄

The same argument holds for using a digital inclinometer. They claim 1/10° accuracy but don't rely on it except for rough checking. Make sure you are measuring the PA altitude angle and not 90°-PA angle. I've done that a few times when the inclinometer body is rotated. I made no mounting adjustments but could clearly see that the reading was completely wrong.

You must Physically adjust the equatorial mounting as follows: Not using the drives! Which would achieve nothing at all! Hopefully your mounting will have slow/fine motions for adjusting both azimuth and polar altitude angle.


Rules for mounting adjustment using sun's image drift on the SharpCap screen.
Camera previously set accurately upright and mounting's Solar rate drive running:

Image drifts NORTH or UPWARDS?     Move the TOP of PA slightly to the EAST.
Image drifts SOUTH or DOWN?             Move the TOP of PA slightly to the WEST.

Image drifts EAST or LEFT?                   INCREASE PA altitude angle slightly. 
Image drifts WEST or RIGHT?               LOWER PA altitude angle slightly.

Only after any physical adjustments to the mounting can you return the sun's image to the appropriate reticule line using the paddle. Keep adjustments small and you won't lose the image from the video capture screen. You can use the movement on the screen to judge how small a change you have made. The slower the drift on the screen the less you need to adjust the mounting.

I found setting azimuth to true north much easier than adjusting the polar angle. Which tended to overshoot each time. I use a stainless steel, boating turnbuckle hidden inside the PA supporting fork to adjust the PA altitude. Being so coarse threaded, it seems not to be ideal for really fine, PA altitude adjustment.

A long, finely threaded screw may well be in my stars. As should screw operated azimuth adjustment. It only needs a couple of push screws on top of the pier to push the whole base by a few degrees. The mounting in an observatory should already be set to north close enough. It should not need major adjustments to point it north. A worm and wormwheel adjustment for mounting azimuth is total overkill! IMHO.
  

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