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Old 16-04-2016, 01:15 PM
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PHD2 - Removing EQ5/6 high frequency gear mesh oscillation

I think it's possible to use a modified guiding algorithm in PHD2 to get some extra guiding accuracy out of factory standard EQ5/6 mounts. It's most useful when seeing is good.

My write up is here:
http://home.exetel.com.au/df/Phd2_Mod.pdf

Some images from the pdf for a flavour of what's going on ...
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Old 16-04-2016, 01:47 PM
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Atmos (Colin)
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Looks quite interesting Sam.
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Old 16-04-2016, 01:51 PM
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Interesting idea Sam. Do you think the short guide exposure (<2sec) might cause this approach to not work as well when the seeing is poor?
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Old 16-04-2016, 03:12 PM
glend (Glen)
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I have the Rowan Belt Mod on my NEQ6 and it does help, as does a regular grease service
, but that depends on how often you use it. In my observatory it gets used alot with narrowband imaging even when the Moon is about. Careful alignment of the worm shaft drive gear is critical, even with a belt drive. I had to grind off a couple of mm from the case to prevent belt drag on the metal edge.
I run Metaguide and have no problems with my graph but i am using a guide period of 2 seconds, any thing faster leads to "over guiding oscillation". Long narrowband subs are no issue. I dither every sub these days as well, it really cleans out the artifacts.
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Old 16-04-2016, 04:46 PM
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Refreshing post Sam. Clearly explained with data.

It looks to me like you have put together a smart additional PE correction to remove a known high frequency component that would otherwise be neglected (because it is not harmonic to the worm PE?) - would that be a fair summary? If so, this general approach may also have application to the Losmandy G11, which can suffer from worm bearing lumpiness, that also does not have a nice relationship to the worm frequency.

keep up the good work and thanks for posting.

Last edited by Shiraz; 16-04-2016 at 06:28 PM.
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Old 16-04-2016, 06:01 PM
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Quote:
Originally Posted by peter_4059 View Post
Interesting idea Sam. Do you think the short guide exposure (<2sec) might cause this approach to not work as well when the seeing is poor?
It's really a matter of the amplitude of the mount oscillation vs the seeing FWHM. In my experience, poor seeing simply hides these type of high-frequency mount oscillations. With poor seeing, when you plot an unguided log, you just see noise superimposed on a long time scale oscillation.

My idea with shorter exposures is to try and get as many points as possible for good curve fitting, and to feed in pre-emptive corrections frequently. When you're dealing with a 10 sec cycle, you ideally want to be sending corrections every 1-2 seconds.

Quote:
Originally Posted by glend View Post
Rowan Belt Mod on my NEQ6 and it does help
I agree that a physical solution is definitely the best way to go, it is a mechanical problem after all. Software can only mitigate the problem so far. With a good mount it would be preferable to have a longer guide period, 2 secs plus to average out seeing. Credit to the NEQ6 of course. For most practical purposes, the standard factory build does a pretty good job for astrophotography.

Quote:
Originally Posted by Shiraz View Post
application to the Losmandy G11
Thanks Ray, that is a good summary. The 10.2 second cycle is the dominant short term cycle on the EQ5/6 mounts. I would imagine the same approach would work on a Losmandy G11, just with a different period. A starting point would be to look at an unguided guide log with good seeing to see what kind of short-term oscillation is apparent.
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