tracking during deep sky photography using the Mead LX200

DRL333 · #1 05-01-2009, 04:30 PM

Even though I have had a telecope for year I am a newcommer to deep sky photography. My first project was to photograph M42 using the Mead DSI pro mounted on a Mead LX200 with the driven alt-azimth mount and tripod that comes standard with those models.

My outcome of my first attemps have made me respect , all the more, the wonderfull photographs I have seen on iceinspace.

My problem is that the image on my camera jitters as my scope tracks the object. The movement is about 10" (about the size of the trapezium in M42) and the jitter occillates at about 2-4 sec. Thus I can take good images with exposures upto 2 sec but beyond that I get substantial image streaking. The telescope allignment seems to be fine because it follows the object for an hour quite well and I don't think that it is atmospheric turbulence either.

So then, is tracking jitter of 10" typical for such a scope or have I got something wrong that I need to fix?

Where might this jitter come from?

Any help with this problem would be greatly appreciated. I would be especially interested in hearing from others who have used the Mead LX200.

Thanks

Derek

Ian Robinson · #2 05-01-2009, 06:06 PM

Periodic errors in the RA drive ....

Do you have capability to tune the drive for this - called PEC ?

I'm pretty vague on the LX200 , but I thought PEC was offered with this .

(Checked by Google - it is a standard feature on the LX200 see http://www.astronomyforum.net/meade-...-question.html).

Also found this http://www.mapug-astronomy.net/ragreiner/PECOper.html

Not a Meade LX200 user so that's all I can offer , hope it helps.

Omaroo (Chris Malikoff) · #3 05-01-2009, 06:16 PM

Derek - Don't bother about PEC training until you've verified the mechanics first.

What model of the LX200 do you have - Classic or GPS? Is it in polar mode (on a wedge) or are you forcing it to track in both RA & Dec simultaneously in AltAz mode? Tracking objects in AltAz is always a compromise, but you can do the following to at least soften the ill effects:

If it's relatively old, then you may benefit by replacing the cork clutch liners on both Alt and Az axes with Petersen Engineering EZ Clutch Kit upgrade from Bintel. Slipping clutches can lead to all sorts of irregularities.

If it already has brass gears then it is a relatively modern scope. If it's not, the old nylon gearing is most likely wearing and you may benefit from a set of Bucks Precision stainless steel gears. Firstly though, try removing the cover plates on both axes and see if the original red grease is still present and well applied - if not it's probably time to clean and re-grease with a good synthetic.

Once these have been at least checked THEN go ahead and try PEC training. Your problem sounds almost like a tooth is missing or you're getting some slippage - not merely that it isn't PEC trained.

DRL333 · #4 05-01-2009, 07:07 PM

Thanks for your insights into the problem. Just to answer Tak's questions. It is a relatively new scope (GPS) which I purchased second hand. The scope is 18 months old and not heavily used. I did notice some Jitter when I first used it at home but the camera makes the jitter more obvious.

Judging by your answers, 10" of arc jitter is not normal and I need to do something about it.

The gears were tuned for whiplash just after I purchased it by bintel but I don't think they did the PEC

Thanks for your help

Derek

Omaroo (Chris Malikoff) · #5 05-01-2009, 07:29 PM

Don't forget the clutches Derek.

Periodic Error Correction can't necessarily compensate for wild jumps like you're experiencing. Still sounds mechanical to me, but see what happens. I'm keen to hear the result! Remember that in AltAz mode you're asking the mount to track in two dimensions at the same time. Remember - PEC training is performed separately on both the RA & Dec axes when in AltAz mode. There is also a "DEC PEC" training mode. Errors are going to come in to play. Apart from PEC training there is a "Train Drives" menu item in your Autostar II controller that takes you through basic gear slop and backlash adjustment.

Another thing - how's your balance? Are you using the appropriate SCT counterweights to offset the camera's weight? The DSI doesn't weight much, but balance is still important.

bmitchell82 (Brendan) · #6 06-01-2009, 10:58 AM

Okay, i can speak in confidence on this subject with this mount.

I run with a LX200R 12" meade... and i don't think its much chop at all. This "Jitter" your talking about i know all to well about it.

How to get rid of it and sort out your imaging?

1 of 2 ways

Get rid of the meade forks and drop the ota onto a GEM (good optics crapola mountings/drive)

Run the Alt/Az drives on a Wedge. this way it elimiates the stepper motors up down left right movement. As this "drift" your seeing is the tracking keeping up. Think of it like your drawing in paint. no smooth lines just blocks, and to get around the corner its up, across, up, across.

Another problem that i observe is that if you have any hint of wind it accentuates the problem 10x and in that case forget imaging, and go back to visual as you will not notice the movements.

Something that will help you substantially is putting a Off Axis guide scope on the tube (off axis = a small scope on the side that can be pointed in the general direction allowing a guide star to be found) with a CCD or Web cam (mono and sensitive is what you want/need). What this will do is lock onto a point source and send commands to the mount telling it that the mount must move to x, y position to keep it locked on.

On the lx200r that i use im flat out getting a decent image that has minimal trails on anything less than 30 seconds.

DRL333 · #7 06-01-2009, 03:22 PM

Thanks again for your feedback.

Brenden.

I am glad to come across another "ring bearer". It must have been a shock to you to invest in such an upmarket scope that still has tracking issues. Unfortunatetly, I had not picked up any rumors of that problem on the astro forum prior to me getting one.

According to your interpretation of the problem I should get less problems when the object has reached its zenith where only one drive controls the tracking. If tonight is clear I will check it out.

I did notice that a small puff of wind play havoc with tracking

Tak.
I take your point about about the likely futility of PEC training.
I had not realised that balance would be important for tracking jitter. I have not used any compensation for the camera. the balance is not exact but compared to the weight of the scope the balance is not far off.

thanks again
Derek

turbo_pascale (Rob) · #8 06-01-2009, 04:20 PM

I can only echo the others here but check out the mechanicals first.

PEC is useful way down the road when you start imaging (and in the current generation of firmware, it's a bit broken for us in the southern hemi - it plays it back inverted - you can get around it but it's painful).
Plus, the whole procedure requires a great deal of patience if you don't have some kind of guider to do the training with anyway - 20 minutes of sitting there tracking with the handpad is something I couldn't be stuffed doing!

I would check one thing first - given the scope is second hand, see if PEC is turned ON. It might be a bad training data. Ensure it is off.

Given you're AltAz, you're always going to struggle a bit because the scope is tracking on both axes.

A wobbling every 2-4 seconds (I assume in RA?) is very unlikely to be PEC. It sounds like a mechanical issue - some minor adjustments will probably fix it up.

Turbo

Virgs · #9 06-01-2009, 05:14 PM

One important point is the telescope needs to be on a wedge if you are going to do long exposure photography. Without a wedge you will experience field rotation

bmitchell82 (Brendan) · #10 06-01-2009, 05:49 PM

Like i said, i have found that anything over 30 seconds is not even worth keeping. Lucky i guess i have been imaging with a SBIG that is so sensitive that Orion neb anything over 3 seconds the trapizium is blooming and more to the fact 1 second is about heaps

Great guide CCD bad imager.

Run with what Tak said, then go a guide scope to help these things along. Also to this you can get balancing weights that attach like a dovetail setup. from memory they are about 150 or so

Omaroo (Chris Malikoff) · #11 06-01-2009, 05:57 PM

Who's Tak???

bmitchell82 (Brendan) · #12 06-01-2009, 06:04 PM

you :d TAK SHARP

turbo_pascale (Rob) · #13 06-01-2009, 06:28 PM

It's a bit harsh to call the SBIG 402-ME a bad imager! It has the same chip as my ST7.
You have a very high sensitivity/QE (85%) chip, which means you can image for shorter timeframes. I would say that given you're imaging one of the brightest objects in the sky, it is more than expected! Plus, these chips do not have anti-blooming, but it means the response is linear and you can do "science". Try imaging one of the really faint galaxies and you start to love the sensitivity!

As for 30 seconds, if you're alt-az, 15 seconds is about the limit - at 30 secs you will start to show star trails. Go polar!!!!

But I agree, they are awesome for guiding.

Turbo

bmitchell82 (Brendan) · #14 06-01-2009, 11:23 PM

yep i agree with you there, lucky that it is sensitive, i need it on the meade. though it will be tasked to guiding because it doesn't have the MP or clarity needed to produce really nice pictures. and the fact is that you can get a nice little cannon dslr for under 400 now that will produce stunning pictures. maby not award winning pictures but still something to brag about. so far i have captured the bug nebula, taranchula, triffid, 47 tuc, omega cent, swan/omega, eagle nebula, sculptor galaxy, southern pin wheel and a heap of others with it, just because it has low capture times made it possible. I wouldn't say these images are anything to talk about, but the average capture time is 10 seconds!