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Help with tracking issues with Meade DS-16

Astrophotography Reflector Mount Meade
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#1 JoshJ

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Posted 25 October 2021 - 02:56 PM

Hey all,

 

I've been into astronomy for decades, but I've just recently gotten into astrophotography. I have a Meade DS-16 that I bought from a nearby CN member a few years ago. It's a monster of a scope and mount, but the views are magnificent.

 

The issue that I've been having is tracking error with the mount. I'm limited to ~4 second shots, and even 4 seconds without star trails is pushing it. Does anyone else have experience with this scope/mount who can give me some advice on what I should be able to expect?

 

A few notes:

- I poured a concrete pier and I have the mount permanently fixed to the pier. I've eye-balled the polar alignment, but it's (obviously) not perfect.

- The control box that came with the mount seemed to be finicky and it was not tracking correctly at all, so I've removed that from the equation - I have the mount directly plugged into power.

- It is somewhat difficult to manually rotate the mount on the RA axis (it's even worse along the declination, but that's another topic), but maybe that's just due to the clutch friction and the inertia of this monster? I don't see any way to release the clutch, so I've just been rotating it as-is.

 

I'm not sure if my issues are polar alignment, a worn mount needing a rebuild/oiling, or if this is as good as I should be able to expect from this setup with an 1800mm FL.


Edited by JoshJ, 25 October 2021 - 02:57 PM.


#2 the Elf

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Posted 25 October 2021 - 03:25 PM

At that focal length you need auto guiding. Does the mount support it? If not you probably have to put it onto a more modern mount that can deal with the weight.

You can do polar alignment using the star drift method aka Scheiner method. You can also use electronic helpers.

Without guiding and given the production year of the mount I'd expect 10 sec or so. Educated guess, I never owned such an old mount. My RC8 is 1600mm focal length, not that much shorter but I use it with a reducer to bring it down to 1100. That does not work with a newton.

 

If you want to know in what condition your mount is, point the scope towards the equator. Altair is a good star for the test. Offset your polar alignment about half a degree towards east or west and take a single 30 min shot with the lowest gain/ISO and if you have some dark filter. The star will drift due to the intended polar error but it will also walk along a sign wave that represents the periodic error of the worm. Post the result here and someone will comment on it. From the amplitude you can calculate the maximum exposure time.

 

Maybe you can pick up some information from this article:

https://www.elf-of-l...t_unguided.html



#3 John Rogers

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Posted 25 October 2021 - 03:57 PM

You should take the time to ensure that the mount is correctly balanced in the configuration that you are using for astrophotography.  An unbalanced telescope will appear to track properly in some parts of the sky and perform horribly in other parts.  Start by removing the cover from the RA drive system.  It is probably equipped with a slip clutch.  Detailed photos should help in identifying where to start. 

 

At a minimum you will need to completely disengage the motor from the drive.  This could be as simple as backing out the slip-clutch adjustment screws all the way, or removing the worm/motor drive assembly. 

 

 

Again, photos will help the group here provide recommendations.



#4 ckr

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Posted 25 October 2021 - 06:44 PM

I have the DS-16 on the original "Push-To" mount, with a 120 volt synchronous motor on the RA axis, no fine or hand or any controls but a switch for the RA drive.

 

Loosen the 3 large round head Phillips screws around the RA worm gear to relieve clutch pressure.

 

Mine wouldn't loosen until taken apart from lack of use, the "Push-To" mount screeched like a barn door due to plastic "bearings" (really bushings) being old and dry. I took it apart,cleaned it up, and added some nice flat roller bearings on both axes,. I bought a pair of Byers 7.1"  gears and worms w/ stepper motors, intending to computerize the mount, but have gotten sidetracked...

 

Good luck!!



#5 JoshJ

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Posted 25 October 2021 - 11:20 PM

Thanks for the input, everyone!

The next clear night I’ll get out and do the test with Altair, and also try out the drift method for polar alignment. I’ve skimmed over that article a couple times. - I need to spend some more time pouring over it to really understand it - thanks for passing that along!

I’ll see what I can do for getting the scope better balanced. I didn’t think about how that could be causing the problem.

It’s great to hear from someone else who has the same setup. How much of a job was it to take it apart and clean it up? I have a habit of letting projects linger on for too long, and I’d hate to be without the scope and mount for a long time.

#6 ckr

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Posted 10 November 2021 - 04:04 PM

I nibbled at it over a winter and have dragged it out a few times since, but even after all that I can never quite Push-To where I want to go, always a little too far or not quite far enough, need the goto (or tangent arm, some kind of fine control) but having trouble finding a controller/driver that's just right. Also, DON'T GET THE GSO 16" MIRROR CELL!!!  My 70's particle board cell fell apart so I got the fanned GSO cell, great cell but too small for my mirror. My DS-16 mirror is 16.23" and doesn't fit the cell, I started to grind away excess aluminum to open it up a bit, but that's also been waiting for attention (and beyond returning now...). I will say that even with an eighth inch of dust on the mirror straight out of the barn, in a Meade 5.5mm UWA I could see the the Cassini div in Saturn's rings.

If I had it to do over again, I'd take mine to a machinist and have him mill out for tapered bearings, I was going to do this but worried about play if not preloaded tightly enough among other things. I used flat roller bearings, but tapered would take up the load better along the length of the roller, flat ones have very small contact points not having the bevel. Anything beats the plastic shower rod bushings mine came with.


Edited by ckr, 10 November 2021 - 04:11 PM.



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