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GOTO accuracy for stars vs DSOs
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Adding a shim under either front or rear ring will not change your polar alignment. You say you have a good polar alignment. That's close enough for the following orthogonality check. Roland has explained this many times and I think he has come up with another newer procedure but I have not committed the new procedure to memory. I understand the older procedure on finding which ring needs the shim. I start with the scope on the West side and star on the East side, close to zenith. When the star is less than 1 hour from your meridian, center star ReCal Force a meridian flip to the East side. Star should be visible in FOV If not then you have an orthogonality error. Re-adjusting your polar alignment will not remove this orthogonality error. One way to find out which ring needs the shim. The counter-weight shaft should be about horizontal on the west side. Lightly press the top end of scope away from the counter-weights If the star moves closer to center of FOV then the front/top ring needs the shim. Lightly press the bottom end of scope away from the counter-weights If the star moves closer to center of FOV then the rear/bottom ring needs the shim. Aluminum cans are popular shim material. What is an orthogonality error? With the counter-weights down and scope pointing at NCP. An orthogonality error will cause the viewing point to be above or below NCP. To the left or right of NCP is not an error. Move Dec to remove any left or right error. Rotating RA will move the viewing point around NCP forming a circle. Radius of this circle is your orthogonality error. Diameter of the circle is 2 times your orthogonality error. You will have to shim the front or rear ring to bring your viewing point back to NCP. As always, I may be wrong again. Jimmy |
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Joe Zeglinski
Roddy,
For some reason I just assumed you used Ray’s
“PemPro’s PA Wizard” option to at least do the PA - even if you skip the
somewhat more complicated, PEC Curve Creation wizard.
PemPro is a really terrific tool. What did you use
otherwise, since you don’t seem confident that your PA is accurate enough?
Joe
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rodddryfoos@...
I checked--everything is tight. No rocking at all in Park 3. I really think its PA. If the mount has a defect, it would be pretty amazing if the only time it showed up was meridian flip accuracy. I spoke with George at AP and this was not even mentioned. I will call him an inquire.
Thanks, Rodd |
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Joe Zeglinski
Rodd,
Have you checked the attachment of the dovetail to your
mount’s head? Maybe one of the screws isn’t tightened squarely, or
tightly enough, so the entire scope, whichever one you have mounted at the
time, and the AP saddle rock just a bit, to the other extreme after a
meridian flip. That would result in a major shift and loss of your target from
the original centered FOV.
If the dovetail is in fact tight, solidly and
squarely bolted on, then maybe you may have a minor defect in this particular
production run of the mount – Again, as evidenced by a slight rocking of the RA
axis.
You can check this at PARK-3 by rocking the end of the end of the
counterweight bar up & down, to see if there is any play, or even a
minor separation crack between the DEC and the RA, as you rock the
axle.
Tony, (his AP-GTO Group handle is: “Harley
Davidson”). had that problem which he demonstrated in his video, last Labour
Day.
See his group post:
Even though his is a different model from your Mach-1,
perhaps yours has the same unique production/assembly problem. Tony had to
return the mount for an internal mount part replacement, not something an owner
can do himself. It works fine now.
Good luck. Hope this helps,
Joe
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rodddryfoos@...
That sounds like a Zebra. I use 4 scopes-each with there own rings and mount plates--and each has the same issue. I think its related to PA to tell the truth. I can routinely take 30 min subs and my stars are small and round--so I am not keen on starting to add shims and change things like that. I should do a gnats behind drift PA and that will answer the question--but that's beyond my ability and I don't want to lose the imaging time
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Joe Zeglinski
Hi Rodd,
I’m surprised no one has yet mentioned this potential
cause, but I had the same problem as you have, a couple of years ago. Kept
losing a centered target from my STL-11000 FOV ... after any meridian flip. As
per your comment, it took quite some time, and a lot of persistence, very slowly
jogging the scope, to get the target into the frame again – it was way off, even
after a perfect centering on the previous side of the pier.
That was until I realized, after a VERY long period of
“denial”, that the entire problem was scope “Parallax Error”. After all, I told
myself, I have an excellent top class AP mount, and an extremely expensive scope
RC-14.5”, so it couldn’t possibly be due to a “mechanical problem”. I spent a
large amount of time and effort using PemPro to get the polar alignment
very near perfect, thinking PA was to blame, and yet I lost the CdC target
immediately after any meridian flip, no matter which side of the pier I started
my tests from. It was frustrating, even with a target star right at Dec = 0 and
less than about 15 minutes east/west of the PM as the initial framing point for
the tests. It gets worse at higher DEC’s toward Zenith.
Once I calculated an approximate required shim thickness
and slipped it under the appropriate end of the OTA’s cradle, the problem
was solved quite to my satisfaction, and indeed ultimate relief.
Definitely, read the section on Parallax correction in
your AP mount’s User Guide. One of your saddle ring’s “riser” may need to be
shimmed to nullify the (possible) Parallax Error.
Good luck,
Joe Z.
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I have been in this hobby for less than a year and agree with David. I tried Astro Photography Tool (APT) first but quickly switched to SGP because it was easier to get support for. It did take me a couple nights to get everything configured but I have gained MUCH more data over the same time than without it. I am new and can be collecting data in less than 5 minutes after polar alignment. Before SGP it could easily take me an hour to do exactly what you are describing. If you ever need help with SGP there are many of us out there that would be happy to help.
Thanks, Charles |
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rodddryfoos@...
To change to SGP and auto focus would require at least a night for me to learn--and probably more. I do not have internet at the scope (except on my cell phone) and getting advise at 2:00 am is not likely. I get maybe 2 nights a month to image due to weather and Moon issues. To use it up learning something new, when the old way works, is a tough sell. If I had mnore clear nights I would be more willing to experiment.
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David
Rodd,
toggle quoted message
Show quoted text
If you use a computer, it should never take more than a couple minutes. You need to check out some program like Sequence Generator Pro. Its what I use when mobile imaging, and its so simple. I simply align with the RAPAS, which takes about 1minute, get the scope roughly in focus to start, and then I simply point the scope somewhere in the eastern sky and click “Solve and Sync”. This shoots a short length image, typically a couple seconds, and automatically plate solves the image, and syncs the position to the mount. Done. Thats it, its that simple. Now pointing works great all across the sky. Also, you can do your framing ahead of time in SGP with the framing and mosaic wizard that sets up the sequence with exact coordinates. When starting imaging, you simply set it up to plate solve the initial position and it moves the mount until the framing and positioning is perfect and away it goes. It should never take an hour to find a star to focus on. Not to mention, if you also have an electronic focuser, SGP (and many other programs) will do an automated focus run for you and put your setup in perfect focus automatically. I used to struggle a little as well, but plate solving and an automated focus routine is the best thing for speeding this up. David
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rodddryfoos@...
Thanks Rolando. No--I use a computer. Right now I use CdC as a planetarium platform--seems to work ok. I haven't learned how to plate solve. I frame manually--kind of a pain but it works. I don't mind that as much as hunting for a star that is out of the FOV--because you don't know in which direction to look--so it can be frustrating. Learning new things is hard due to the very limited clear sky time I get--losing imaging time trying to learn something drives me crazy--so I muddle through to get the data. Its really at the beginning and after a flip--fortunately with broadband I can generally focus in the FOV--I zoom way in and choose a bright star (most stars in field are not very bright--so its a relative term) and use a B-Mask. The mount is still guiding so framing is not lost.
Rodd |
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Roland Christen
A lot of people use a plate solve which gets you to the exact spot you want very quickly.
One other way is to do a quick model with APCC Pro and that will allow you to find and center objects very accurately on your first slew.
One question, are you using the keypad to go to a star or object? Right now the keypad RA/Dec does not send high resolution data to the mount. We are working on a new database with sub-arc sec data, which will be available for download soon.
Rolando
-----Original Message----- From: rodddryfoos@... [ap-gto] To: ap-gto Sent: Thu, May 2, 2019 2:34 pm Subject: [ap-gto] GOTO accuracy for stars vs DSOs Greetings, I use a Mach 1GTO with several scopes ranging from 4-5" refractors to an 11" Edge. regardless of the scope I use, I find my initial GOTO to be off. I use the RAPA scope and routinely take 30min subs with round stars and decent FWHM values (with all scopes), so PA seems pretty good. With the FSQ 106-especially at F3, GOTO accuracy is irrelevant as the FOV is so big, the initial star (focus) is somewhere in the FOV--not so the TOA 130 at F7.7 and especially not the C11Edge. I find myself having to hunt for the first focus star--sometimes for 45min to an hour. Its frustrating. Once I have it, and tell the planetarium program it's centered--subsequent moves are pretty good. But I invariably run into the same difficulty after a meridian flip--I have to hunt for the star (or target). A guidescope helps with the TOA 130, as I can use it as a photographic finder that has a wide FOV--so the star is typically in or very close to the guidescope FOV. But with the c11Edge--a guidescope is not preferable.
Any suggestions?
Thanks Rodd
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rodddryfoos@...
Greetings, I use a Mach 1GTO with several scopes ranging from 4-5" refractors to an 11" Edge. regardless of the scope I use, I find my initial GOTO to be off. I use the RAPA scope and routinely take 30min subs with round stars and decent FWHM values (with all scopes), so PA seems pretty good. With the FSQ 106-especially at F3, GOTO accuracy is irrelevant as the FOV is so big, the initial star (focus) is somewhere in the FOV--not so the TOA 130 at F7.7 and especially not the C11Edge. I find myself having to hunt for the first focus star--sometimes for 45min to an hour. Its frustrating. Once I have it, and tell the planetarium program it's centered--subsequent moves are pretty good. But I invariably run into the same difficulty after a meridian flip--I have to hunt for the star (or target). A guidescope helps with the TOA 130, as I can use it as a photographic finder that has a wide FOV--so the star is typically in or very close to the guidescope FOV. But with the c11Edge--a guidescope is not preferable. Any suggestions? Thanks Rodd |
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