RA clutch tightness?

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maceemiller
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RA clutch tightness?

#1

Post by maceemiller »

Hi all.

Now this seems to me to be a silly question but i will ask anyway.

Ive just been given an EQ2 mount and a celestron motor drive. Ive just been out taking images of the Orion Nebula with my DSLR and Im quite happy with the results but as Im sat here waiting for DSS to do its thing Im wondering this.....should the RA clutch be tight or loose? I had mine as tight as I possibly could and while the nebula stayed in the centre of my view, every 2nd or 3rd image seemed to trail a little (25 second exposures) but in 2 different directions (top left to right and top right to left)

If its PA then im ok with that as this was a rough test yet the clutch thought is bugging me!
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Re: RA clutch tightness?

#2

Post by OzEclipse »

The direction doesn't mean much unless you know the camera orientation. You can establish that by doing a short tracked image then power off the drive during the exposure. You'll get a bright star with star trailing to the west. In general, a tight clutch isn't a bad idea. You don't want clutch slip during an exposure.

The reversing drift, if in RA, could be periodic error. The EQ2 mounts have a record setting PE of about 140 arc sec. The EQ2 has a 100 tooth worm and hence a 14 minute period so you'd expect a reversal every 7 minutes not 75s which seems too quick for a reversal. In the illustration, I have attempted to illustrate how the star can reverse drift directions or appear stationary due to periodic error in subsequent images.
Periodic-error-lucky-imaging.jpg
Joe
"34 South: The Hilltops Observatory"
Central West NSW
Bortle 1-2 skies, 148 E, 34 S

Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops

web site : http://joe-cali.com/
SCOPES - ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80, M70
MOUNTS- EM-200, iEQ45, Push dobsonian with Nexus DSC, Various homemade EQ's
CAMERAS : Pentax K1, K5, K01 / VIDEO CAMS : TacosBD, Lihmsec
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maceemiller
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Re: RA clutch tightness?

#3

Post by maceemiller »

OzEclipse wrote: Wed Jan 13, 2021 3:25 am The direction doesn't mean much unless you know the camera orientation. You can establish that by doing a short tracked image then power off the drive during the exposure. You'll get a bright star with star trailing to the west. In general, a tight clutch isn't a bad idea. You don't want clutch slip during an exposure.

The reversing drift, if in RA, could be periodic error. The EQ2 mounts have a record setting PE of about 140 arc sec. The EQ2 has a 100 tooth worm and hence a 14 minute period so you'd expect a reversal every 7 minutes not 75s which seems too quick for a reversal. In the illustration, I have attempted to illustrate how the star can reverse drift directions or appear stationary due to periodic error in subsequent images.

Image

Joe
Hi Joe.

Thankyou for the explanation and chart.

Not sure if this makes a difference but the mount is a CG3.
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Re: RA clutch tightness?

#4

Post by OzEclipse »

Your post said EQ2. Doesn't make any difference to the explanation but the periodic error magnitude might be different. If the CG3 worm has more than 100 teeth, it might explain the 90 sec direction switch.
"34 South: The Hilltops Observatory"
Central West NSW
Bortle 1-2 skies, 148 E, 34 S

Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops

web site : http://joe-cali.com/
SCOPES - ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80, M70
MOUNTS- EM-200, iEQ45, Push dobsonian with Nexus DSC, Various homemade EQ's
CAMERAS : Pentax K1, K5, K01 / VIDEO CAMS : TacosBD, Lihmsec
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maceemiller
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Re: RA clutch tightness?

#5

Post by maceemiller »

OzEclipse wrote: Wed Jan 13, 2021 7:21 pm Your post said EQ2. Doesn't make any difference to the explanation but the periodic error magnitude might be different. If the CG3 worm has more than 100 teeth, it might explain the 90 sec direction switch.
Hi again.

I feel I'm going backwards here.....

My canon 100d is on the mount as shown in the picture. Its a really light camera so I thought i didn't need to balance it with a counter weight, but do I? Could this be a possible cause for some erratic (small) star trails?


I know this sounds stupid and probably obvious but I wonder if I've totally overlooked an obvious point?

I only set this up for the first time last night and it was very late so I was tired.....

The motor drive is quite noisy and it didn't sound like it was struggling at all. The camera was more upright on the mount than the picture shows when I was using it.
received_845596412959826.jpeg
received_880336419448972.jpeg
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Re: RA clutch tightness?

#6

Post by OzEclipse »

Running like that is not a good idea. If there is any play in the gear train, then as you cross balance points the mount will shift. I always balance my precision EM200 mount. On older less precise mounts, I balanced then very slightly off-balance so there was a little more weight to the east. So the mount is pushing up hill. THis involved changing the counterweighting slightly after the mount crossed the meridian.
"34 South: The Hilltops Observatory"
Central West NSW
Bortle 1-2 skies, 148 E, 34 S

Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops

web site : http://joe-cali.com/
SCOPES - ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80, M70
MOUNTS- EM-200, iEQ45, Push dobsonian with Nexus DSC, Various homemade EQ's
CAMERAS : Pentax K1, K5, K01 / VIDEO CAMS : TacosBD, Lihmsec
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maceemiller
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Re: RA clutch tightness?

#7

Post by maceemiller »

OzEclipse wrote: Thu Jan 14, 2021 8:06 pm Running like that is not a good idea. If there is any play in the gear train, then as you cross balance points the mount will shift. I always balance my precision EM200 mount. On older less precise mounts, I balanced then very slightly off-balance so there was a little more weight to the east. So the mount is pushing up hill. THis involved changing the counterweighting slightly after the mount crossed the meridian.
Thankyou for the reply.

For some reason I never thought about counter weights with a DSLR.

Last night I was playing around with the mount and found that even with the clutches unlocked it hardly moved.
Took a few bits apart and cleaned off the super excessively thick, gooey grease that had been put in and now its super free when the clutch is unlocked.

Certainly I believe the grease was causing unnecessary "friction"......for example I unlocked the DEC clutch with my camera and heavy lens purposely lens heavy and it hardly moved!

Proof is in the pudding and I will make sure I over balance a little to the East. Never done that before but I understand why :)
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