QHY 174M GPS and The Transit of Mercury

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smithn00
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Joined: Fri Jan 04, 2019 2:29 pm

QHY 174M GPS and The Transit of Mercury

Post by smithn00 » Tue Nov 12, 2019 1:42 pm

I prepared for the transit by finding what I thought was the proper exposure and focus on prior sunny days. I used a white light filter with a restricted aperture. Since there were no sunspot available, I tried another way to get near focus.
I went out at night without the filter and got best focus on a bright star. I marked the position on the tube. I thought adding the filter would only change focus a small amount.
But on transit day none of that worked. I final setup was exp.1/500, gain 200, 1x1 bin, FITS. The resulting image was slightly dim.

The result was no Mercury. Running the focuser in and out from the set point brought no Mercury. Changing exposure time up and down got similar results.
So I think I misunderstand some basic point of the process.
Any suggestions?

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admin
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Re: QHY 174M GPS and The Transit of Mercury

Post by admin » Tue Nov 12, 2019 6:55 pm

Hi,

If the filter was covering the objective lens of the telescope then I wouldn't have thought it would have changed the focus position at all. If the filter was somewhere in the light path after the objective lens then it probably would have an effect of no more than one third of the thickness of the filter glass.

The setup steps you describe certainly seem reasonable to me, although maybe I would have been tempted to try with the filter on the moon using high gain and long exposures to bring through enough light. Another option would have been to focus on the limb of the Sun – even without sunspots it should be possible to get that in sharp focus I would have thought.

Cheers, Robin

smithn00
Posts: 23
Joined: Fri Jan 04, 2019 2:29 pm

Re: QHY 174M GPS and The Transit of Mercury

Post by smithn00 » Tue Nov 12, 2019 8:41 pm

I forgot to mention that the scope is a 10" DOB. The entire aperture was covered except for a 4" hole for a glass white light filter. The filter aperture was further reduced by a cardboard mask to about 1/2".
So you don't see a problem with my procedure either?

vichman
Posts: 9
Joined: Mon Jun 18, 2018 7:05 am

Re: QHY 174M GPS and The Transit of Mercury

Post by vichman » Thu Nov 14, 2019 6:01 am

Hi,

I used the very same camera and Sharpcap to shoot images of Mercury's transit on Monday quite successfully.
My scope was much smaller though, an h-Alpha scope with a focal length of 600mm.

I used 6ms exposure times with a gain of 0. With this setting the histogram extended up to about 70%.
For focussing I used Sharpcap's contrast detection focusing tool (https://docs.sharpcap.co.uk/3.2/#Focusing), aiming on the edge of the sun. That worked perfectly, even though the seeing was horrible.

Some questions on your settings:
How did your histogram look like? Do you have a screenshot?
Did you auto stretch the image? (lightning button)
Why did you use such high gain? Are you sure that you did not overexpose?
Stupid question, but this happened to a friend of mine on Monday: Are you sure that you had Mercury in your field of view? Mercury is super tiny (only about 10", compared to 1800" of the sun), so finding it especially at high focal length can be very tricky.

CS
Stefan

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