It is not unreasonable to expect that a low end mount will have some slop in
the DEC axis drive. Then you don't want it to apply DEC corrections in both
directions, unless you have a balance bias to keep the gear to one side. In
that situation the DEC corrections may still not be as consistent as if you use
only one direction for DEC corrections. Your electronics might have a setting
that allows DEC nudges to be applied on only one direction. To avoid the need
of correcting in both directions, you may want deliberately to misalign
slightly (without misaligning enough to affect your exposures). By east-biasing
the polar axis balancing, you are already dealing with the similar slop in the
polar axis. An aside: If the east bias is achieved with the counterweight, the
torque achieved will depand on the angle of the counterweight shaft. This can
be a problem, especially when the shaft is pointing almost straight down. An
old trick is to wind a string around the mount with a separate weight attached,
to apply a constant torque without depending on the counterweight for east
bias.
-----Original Message-----
From: az-observing-bounce@xxxxxxxxxxxxx <az-observing-bounce@xxxxxxxxxxxxx> On
Behalf Of Michael McDonald
Sent: Saturday, March 23, 2019 10:52 AM
To: az-observing@xxxxxxxxxxxxx
Subject: [AZ-Observing] Oh no! He's at it again!
I went out and tried the new configuration last night.
First the good news: I can reach focus on both the guide scope ;(with the 2X
Barlow) and the C6 (with the reducer)!
The Barlow does make the guide scope a lot less sensitive to light. My normal
1-1.5” exposure went up to 3” for Orion in the West to 6” for M3 in the East.
And the 6” wasn’t long enough. I was still getting a very low SNR. Living in
the East Valley, I had expected to have more light pollution in the western sky
than the East. But that’s not what I’m finding. Something East of me is
generating a lot of light pollution (the Moon added to it last night!) while
Phoenix’s glow is farther away. So I may have to start shooting more towards
the West.
And my Dec guiding is still a sawtooth:
http://www.mikemac.com/Astrophotos/2019-03-22-Guiding.png
And a fairly regular one at that! If anyone who has any idea of what is going
on with my guiding, I’d LOVE to hear your ideas.
As one of you expected, the image circle of the C6 does NOT cover the ASI1600
sensor when I have the reducer installed. That’s a bit disappointing. I’m still
trying to decide if I can live with that or not. Also, Astrometry.net
calculated my effective focal length as 1139mm, not the 980mm I was expecting.
That’s OK, just different.
And for last night’s experimenting, I used a 3x3 binning for al of my images.
That still gives me roughly a 1500x1200 image as a result, which is good enough
for the web and testing.
The really bad news with the reducer is I was getting all sorts of weird rings
and ovals my images of Orion, as shown here:
http://www.mikemac.com/Astrophotos/M42_L_2019-03-22_20_Bin3x3_60s_reducer.png
Any ideas what’s causing those artifacts? Internal reflections in the reducer?
Those artifacts did not show up (noticeably anyway) in the later images of the
dimmer M3. But if I’m going to get that kind of junk in any image that has a
bright star, then the reducer will have to go.
The good news from last night is that of the roughly 100 sixty second images I
took, maybe a half dozen show an excessive tracking issue! And that’s despite
the terrible sawtooth pattern for the Dec axis while guiding!!
So, my take aways from last night:
1) I need a longer focal length guide scope that has a decent aperture so I can
keep the guide exposure times down to something workable
2) the jury is still out on the focal reducer due to those image artifacts and
an image circle that doesn’t cover my sensor
3) I still think I have an issue with my Dec axis
4) Binning can help hide some of the guiding issues
I hope to have my M3 processed later today so we can see what the final results
really are.
Mike McDonald
mikemac@xxxxxxxxxxx
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