Doug6952
sage
Reged: 07/24/08
Posts: 397
Loc: West Milford, NJ
|
|
I have a new Cg5- advanced GT mount. While doing some imaging the mount tracks well but there is movement of the object in the FOV. The object will stay in the FOV for quite some time, but it moves either up and down or left and right slightly. This is noticable in a 15 second exposure. After each exposure you can see that a star has moved a small distance since the last exposure. It might move in any direction for maybe 20 to 60 seconds then reverse and move back. At first I thought it might be the weight of my scope, but tonight I tried it with my ETX 90 OTA and it was doing the same thing. Could this be due to not doing a precise polar alignment?
I do a 2 star alignment then add at least 2 calibration stars. I then use the polar align function. I have not tried doing drift alignment yet. I have a limited view due to trees. I also have not used the mount calibration function yet. Don't know if this will make a differnce or not. I may try doing the mount calib. tonight if it's clear.
If any of you experienced CG5 users have any thoughts to resolve this situation please let me know.
Thanks,
Doug
--------------------
Doug
Sometimes I wonder......."Why is this frisbiee getting bigger" ...... Then it hits me!
Orion EON 72mm
Meade 8" starfinder
Meade ETX 90 OTA
CG-5 Advanced GT Mount
DSI Pro III
Orion SSAG
Stellarvue F50
Baader LRGB and Ha filters
|
gordianknot
sage
Reged: 09/04/05
Posts: 446
|
|
How much movement are you noticing? If you could tell us how many pixels it moves over what period of time, what focal length the scope is and what camera you're using, that would be great. The CG-5 isn't a great tracking mount and without autoguiding requires reasonably short exposures, though 15s sounds a little too short to me. Depending on focal length I think that 30s to 1 minute subs should be possible on it.
I would suggest:
1. Check the polar alignment. Make sure if you use the software polar align technique that you use a high power or reticle eyepiece when centering the stars. A drift align is even better. You can use the Utility menu of the hand controller to supposedly "check" the accuracy of an align.
2. Make sure mounting hardware is snug and quite rigid. Make sure all the bolts are tight, etc. Also, check on the dovetail plate as some people have reported that certain dovetail plates aren't study enough for large loads (I've never seen this myself however).
3. Check the camera/telescope connection and the focuser. Is there any play in this?
4. Consider an environmental effect like wind, people walking around the scope or you touching the camera to take exposures. It takes surprisingly little to muck up a long focal length exposure.
5. If using a battery make sure it's above 12 V, some Celestron mounts have been known to perform oddly at low power.
If you're still experiencing a little motion, it's probably just because of tracking error inherent in the mount. The only solutions to this are guiding or buying a fancier mount. If you have lots of error (many pixels), then you may have a defective mount or some issue that I haven't thought of.
|
Doug6952
sage
Reged: 07/24/08
Posts: 397
Loc: West Milford, NJ
|
|
Thanks for the info,
"How much movement?" I would have to take a guess and say maybe 1-5 pixels in a 15 sec time period. Not sure on how to accurately measure that. It will usually move 4 times then stop and reverse for 4 times(these are all 15s frames). I'm using a DSI Pro 1 on a 8" F/6 1220FL scope. The ETX is 90mm F/13.8 1242 FL.
Everything seems tight and rigid to me. There is no play in the focuser. When I first noticed this there was no wind, nobody else around and I was far away from the scope. I'm getting a steady 12.06V from my power supply. I'll try doing a more precise polar align and see what happens.
Doug
--------------------
Doug
Sometimes I wonder......."Why is this frisbiee getting bigger" ...... Then it hits me!
Orion EON 72mm
Meade 8" starfinder
Meade ETX 90 OTA
CG-5 Advanced GT Mount
DSI Pro III
Orion SSAG
Stellarvue F50
Baader LRGB and Ha filters
|
Telescopeman54
Vendor - Trapezium Telescopes & Services, LLC.
Reged: 08/17/07
Posts: 1152
Loc: New Hampshire
|
|
That sounds like it might be the motors "stepping". Check your anti-backlash settings. Try reducing them to zero and if see if that stops the motion. I've done this on a few mounts where the image was moving in such a manner and was visible such motion was visible at higher powers.
CS
Steve
--------------------
Steve Forbes
6.25" f/15 Mogey/NASA refractor on GOTO GEM
6" f/15 Jaegers refractor on GOTO GEM
5.1" f/8 Burgess on CG-5 w/GOTONOVA
4" f/15 Jaegers on ORION ATLAS GOTO
2x 6" f/6 Edmund Palomar reflectors on GEMs w/ RA and DEC drives
2x 4-1/4" Edmund Palomar Jr. reflectors on GEMS w/ RA drives, (all circa 1965 - 69)
3" Edmund reflector on "EQ" mount from @1963
4" Criterion reflector with "EQ" mount from @1956.
150mm f/12 MAK w/iOptron Mini Tower
Lots of other stuff, too!
|
Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 2059
Loc: Massachusetts
|
|
If the object drifts in a constant direction of declination it's polar alignment. If it moves back and forth in any direction, it's the mount.
--------------------
17.5" f/5 Discovery Truss
IM715 7" f/15 MCT, Eon-120ED
Lunt 60mm single etalon HA
CG5A coffee grinder, Orion Skyview Alt-AZ
35,19,15 Pans.9 Nag. Meade 24.5 4kSWA, 4.7 5kUWA.
BO-TMB 7mm planetary.
Zeiss Diascope 85
Zeiss, Leica, Canon IS, Fujinon, Nikon binos
One each generic rescue Greyhound (pictured)
|
rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1660
|
|
Quote:
That sounds like it might be the motors "stepping". Check your anti-backlash settings. Try reducing them to zero and if see if that stops the motion. I've done this on a few mounts where the image was moving in such a manner and was visible such motion was visible at higher powers.
CS
Steve
Nah...the CG5 does not use stepper motors; it uses servos. The movement he is noticing is likely due to:
declination drift caused by less than precise polar alignment.
back and forth RA movement caused by normal periodic error.
--------------------
Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
|
rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1660
|
|
Quote:
I have a new Cg5- advanced GT mount. While doing some imaging the mount tracks well but there is movement of the object in the FOV. The object will stay in the FOV for quite some time, but it moves either up and down or left and right slightly. This is noticable in a 15 second exposure.
Yes, the final resolution is to guide during exposures. Even if well balanced (slightly east heavy) and well polar aligned, the mount will not produce too many perfect unguided frames longer than about 30 seconds. The good part is that I've found that I can usually expose for as long as I want if I am autoguiding.
If I add four calibration stars, do the HC polar alignment routine, and balance east-heavy I find I can get a decent number of untrailed 30 second exposures. That's what I used to do when I was takin' pictures with my li'l DSI.
--------------------
Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
|
Doug6952
sage
Reged: 07/24/08
Posts: 397
Loc: West Milford, NJ
|
|
back and forth RA movement caused by normal periodic error.
This is what I was thinking it was. Like I said it stays in the FOV for a long time. I've had the object centered in the FOV for over an hour with only the back and fourth motion and maybe a very slight drift out of FOV.
--------------------
Doug
Sometimes I wonder......."Why is this frisbiee getting bigger" ...... Then it hits me!
Orion EON 72mm
Meade 8" starfinder
Meade ETX 90 OTA
CG-5 Advanced GT Mount
DSI Pro III
Orion SSAG
Stellarvue F50
Baader LRGB and Ha filters
|
Doug6952
sage
Reged: 07/24/08
Posts: 397
Loc: West Milford, NJ
|
|
I was able to obtain this image with my scope and mount combo. Out of 181 15sec exposures I had enough good ones to stack 138 subs for the final image.
--------------------
Doug
Sometimes I wonder......."Why is this frisbiee getting bigger" ...... Then it hits me!
Orion EON 72mm
Meade 8" starfinder
Meade ETX 90 OTA
CG-5 Advanced GT Mount
DSI Pro III
Orion SSAG
Stellarvue F50
Baader LRGB and Ha filters
|
Jeff in Austin
professor emeritus
Reged: 02/26/07
Posts: 587
Loc: Austin, TX
|
|
Doug, you have recieved good advice so far. The only comment I want to add is my experience with the same mount. After a few months of use the RA and Dec axes began to get loose. I took off the covers and found there was excessive play between the first two drive gears (one is mounted on the drive itself). After tightening everything up it works great, but 6 months later it's getting loose again. There are no "locking" features to the hardware that maintains the clearance....no lock washers, locking inserts, or loctite...so regular maintenance is in my future. I don't know if all of these mounts are the same way, but if you can wiggle the scope a few degrees in either direction when it's mounted you may have the same issue.
--------------------
Stellarvue SV102ED
Celestron C8-SGT XLT GoTo
Astro-Tech AT66ED
Canon 50D unmodded, TC80N3 timer, Stiletto Focuser.
DMK21AU04.AS
Coronado PST (Lunt LS60THa on order)
Supportive family, including First Grader w/ scope, curious toddler, and lovely wife.
Sponsor of Austin CSC, Austin CalSky.
|
Doug6952
sage
Reged: 07/24/08
Posts: 397
Loc: West Milford, NJ
|
|
Quote:
Doug, you have recieved good advice so far. The only comment I want to add is my experience with the same mount. After a few months of use the RA and Dec axes began to get loose. I took off the covers and found there was excessive play between the first two drive gears (one is mounted on the drive itself). After tightening everything up it works great, but 6 months later it's getting loose again. There are no "locking" features to the hardware that maintains the clearance....no lock washers, locking inserts, or loctite...so regular maintenance is in my future. I don't know if all of these mounts are the same way, but if you can wiggle the scope a few degrees in either direction when it's mounted you may have the same issue.
I took the plastic motor covers off this morning just to see what was under them. The gear mesh from the motor gear to the second gear seemed a bit loose on both axis. I tightened up the mesh to reduce any play. I'll test it out tonight if the sky stays clear.  
--------------------
Doug
Sometimes I wonder......."Why is this frisbiee getting bigger" ...... Then it hits me!
Orion EON 72mm
Meade 8" starfinder
Meade ETX 90 OTA
CG-5 Advanced GT Mount
DSI Pro III
Orion SSAG
Stellarvue F50
Baader LRGB and Ha filters
|
Jeff in Austin
professor emeritus
Reged: 02/26/07
Posts: 587
Loc: Austin, TX
|
|
I actually had mine apart today as well. I found another source of "looseness" hidden behind the drive motors. There are 2 screws and a set screw in the middle. I tightened up the 2 screws and it was tight as a drum, no wiggle what so ever. It sounded like the motors were working extra hard at first so I backed it off slightly. It seemed to work much better under the stars tonight.
--------------------
Stellarvue SV102ED
Celestron C8-SGT XLT GoTo
Astro-Tech AT66ED
Canon 50D unmodded, TC80N3 timer, Stiletto Focuser.
DMK21AU04.AS
Coronado PST (Lunt LS60THa on order)
Supportive family, including First Grader w/ scope, curious toddler, and lovely wife.
Sponsor of Austin CSC, Austin CalSky.
|
rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1660
|
|
Quote:
I took the plastic motor covers off this morning just to see what was under them. The gear mesh from the motor gear to the second gear seemed a bit loose on both axis. I tightened up the mesh to reduce any play. I'll test it out tonight if the sky stays clear.
Be careful about doing that. Often "good and tight" in one part of the gear's travel means "locked up and stalled" in another. Usually there must be some play.
--------------------
Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
|
Doug6952
sage
Reged: 07/24/08
Posts: 397
Loc: West Milford, NJ
|
|
Quote:
Be careful about doing that. Often "good and tight" in one part of the gear's travel means "locked up and stalled" in another. Usually there must be some play.
Rod,
I didn't make it overly tight. There was quite a bit of play before I adjusted it. I just closed the gap a bit.
Thanks for the warning though.
The mount worked well last night. It still had the back and forth motion, but I was using a .5 focal reducer so it wasn't as noticeable. I didn't do a drift alignment. I have a limited FOV from my site. I set up on my driveway and had trouble even finding calibration stars that weren't behind trees.
Doug
--------------------
Doug
Sometimes I wonder......."Why is this frisbiee getting bigger" ...... Then it hits me!
Orion EON 72mm
Meade 8" starfinder
Meade ETX 90 OTA
CG-5 Advanced GT Mount
DSI Pro III
Orion SSAG
Stellarvue F50
Baader LRGB and Ha filters
|
Doug6952
sage
Reged: 07/24/08
Posts: 397
Loc: West Milford, NJ
|
|
I think I may have found the problem.
Since Tropical Storm Hanna decided to show up, I figured it was a fine day to dissasemble my mount and check things out. After removing the worm I cleaned off the grease and it seemed rather tight and felt as though it was grinding. It was actually binding in a few spots. I loosened it and then tightened it just enough to remove the play.
After re-assembling the mount it doesn't seem to be straining to move the scope as it was doing. I won't be able to see if the movement while imaging is fixed until the skies clear, but I'm hoping this fixed it. I also adjusted the gearing so that everything moves nice and smooth with little to no backlash.
Doug
--------------------
Doug
Sometimes I wonder......."Why is this frisbiee getting bigger" ...... Then it hits me!
Orion EON 72mm
Meade 8" starfinder
Meade ETX 90 OTA
CG-5 Advanced GT Mount
DSI Pro III
Orion SSAG
Stellarvue F50
Baader LRGB and Ha filters
|
Doug6952
sage
Reged: 07/24/08
Posts: 397
Loc: West Milford, NJ
|
|
Well it seems to be a bit better. I was able to push my exposure time up. 30 sec was ok, but some frames had issues. I was able to get good subs at 22 sec.
I'm wondering if it makes a difference on how straight the worm is mounted. If the bolts that mount the worm shaft are not tightend evenly would this skew the worm causing it to track differently? It's kind of difficult to see if it is on perfectly straight. It would be nice if there was a viewing window to see the worm shaft.
Doug
--------------------
Doug
Sometimes I wonder......."Why is this frisbiee getting bigger" ...... Then it hits me!
Orion EON 72mm
Meade 8" starfinder
Meade ETX 90 OTA
CG-5 Advanced GT Mount
DSI Pro III
Orion SSAG
Stellarvue F50
Baader LRGB and Ha filters
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 
