EQ6 Tracking problem

[OzRob]

Last night I had my first real try at capturing a DSO. An easy target was the way to go so M42 was picked. While the resulting image is OK (other than some trailing) I think I have a bit of a problem with my mount. It doesn’t appear to be tracking smoothly.

The first attached image shows what the trailing looks like in one image (I have zoomed in on one are and all images are from the same area) and then the next attached image is the very next frame. The trailing is in different directions! I stacked all the images in startrails to see how it looked over the whole run and a section is shown in the third attached image. The following image shows what I would call rabbit’s ears and then the next image shows a frame with much better tracking. The last image shows a very quick stack of all the frames. It is the result of 19 two minute exposures.

Can anyone tell me what is going on? I am using a EQ6 Pro mount.

[Dennis]

Hi Rob

Was the mount just tracking at the sidereal rate, or did you have an auto guider connected and operating, so that a separate chip was sending guide star corrections back to the mount motors?

Cheers

Dennis

[OzRob]

ooops I forgot to state that there was no guiding, only tracking at sideral rate.

[Dennis]

Okay. Assuming the mount is accurately polar aligned and that you were imaging through the WO 110 (F7?) with the Pentax K200D, then what you are looking at appears to be the periodic error of the mount.

The WO 110 at F7 will have a focal length of 770mm giving an image scale of around 2 arcsec/pixel.

I don’t know the tech specs for your mount but I suspect the periodic error would be around 15 to 20 arcsecs. So, if the mount is just tracking, you would expect a star to make excursions of around 15 to 20 arcsecs over the period of a single worm revolution (say 7 to 10 minutes?) so what you appear to have recorded is the PE of your mount?

To “negate” this order of PE, I would have to use a 70mm focal length lens on my Canon 40D which gives an image scale of around 16 arcsecs/pixel.

Autoguiding will allow you to expose for longer periods, at longer focal lengths, but this will require a guidescope and guide camera to actively control the mount via corrections made every say, 3 to 5 secs from the autoguider.

Cheers

Dennis

[bojan]

This is not only the PE..
Take note, you have errors in both RA and DEC. Was the mount properly aligned?
If it was, the pictures show the whole thing is wobbling.. or the tapered bearings (on the the RA shaft) are dirty and/or not properly adjusted/tightened.
BTW, PE on my specimen of EQ6 PE was around 23 arcsec pp. about the same as yours (minus wobbling, if there is one)

[OzRob]

The alignment was fairly close but not spot on. I did a quick drift alignment and all seemed OK. The thing that gets me is that the drift varies from one frame to another with a few being fairly good. I can't think of anywhere where a wobble could be generated and everthing was tight.

One thing that I did forget to do was re-balance the scope between a quick observation period to show some people the moon and imaging. The camera is a little heavier than the diagonal and eyepiece. Could this be the cause?

How tight should the RA axis be? Mine spins evenly but I wouldn't say freely. The Dec is similar.

[bojan]

Hmm
Balance does not have to be absolutely perfect, just a bit unbalance is allowed, but the mount should work AGAINST this unbanlace (or, tendency towards unbalance).
Did you have some wind while imaging?

Important thing is, you should not see (or feel) any free movement of the axes, apart from obvious freedom of movement that is expected.. not too tight, however you should feel viscosity of lubricant when moving the mount around axes..

[bojan]

For example, the maximum exposure I can obtain without guiding is 30 sec with my 1000mm lens.
From this I have 75% of frames usable, the rest are not as a result of jitter (caused by transfer gears, 47 teeth, from gear box to worm).

[Bassnut]

Is the mount on a permanent, isolated pier?, or tripod not on the ground, on a veranda or such?. Were you walking around the scope at the time?.

[OzRob]

OK, well maybe 2 minutes unguided at 770mm is just too long then!! Looks like I have to buy a guidescope.

There wasn't even a slight breeze last night. It will be last last dark sky night tonight. I will try to get a series of shots of the whole worm gear period with the polar alignment out and maybe that will give up some clues.

I can't feel any unwanted movement either.

[OzRob]

It was mounted on a tripod sitting on a clay rich soil. I had specators who where walking around as I did at times. Is that likely to have such an effect?

[Bassnut]

Well, I woundnt have thought a tripod on clay ground would be a problem (but possible), the distubance seems random, only thing I can think of.

Yes, IMO 770mm FL and 2 mins is too long unguided, but that doesnt explain the random result.

[33South]

[How tight should the RA axis be? Mine spins evenly but I wouldn't say freely. The Dec is similar.[/QUOTE]

Based on an old EQ6 (mine) that does not sound right. Theres no slipping clutch mechanism on the mount when the locks are locked hard you shouldn't be able to move either axis other than with the hand controller.

[OzRob]

I was describing when they are not locked.

[bojan]

Yes of course, no movement at all when locked.. but Rob probably tested movement when locks were open.

[OzRob]

I managed to get this done. I am no expert but they look OK to me, perhaps someone else can see something. The first image shows a stacked image of ten 10 second shots taken every minute. The second image is two 5 minute exposures stacked. The period of the worm gear on the mount is eight minutes.

[bojan]

Those were taken with mount deliberately mis-aligned, right?
If yes, they look OK to me.
Your M42 session must have been affected by something unusual..

[OzRob]

Yes, absolutely otherwise it would be difficult to see what was going on...;)

I just hope whatever happened the other night is a once off. Thanks for everyone's input.

[bmitchell82]

ide just bite the bullet and get your mount guiding... eq6's arn't the work horse's of unguided photography they just arnt good enough for that. with my mount perfectly polar aligned i can drag upto 3 min out of a image. but that was when i was first starting and though that a image out of my telescope was hevenly :).

for guiding you can track quite happily with a modified finder I did this to mine and have stars that are within 10% round which you cant see that the stars arnt slightly out of whack. all i did is turn up a brass puck, put thread at one end and used a 1 1/4 drill bit, put a few lock nuts in to hold the dsi and away i went.! all you need is a cheap CCD qhy,dsi are two very good candidates. I have yet to try out the finder guider on my 10" at 1200 FL but it shouldn't have a problem when setting minimum movement to .1 or less. i guess thats the joy of sub pixel guiding.!

[OzRob]

I am already looking at a guiding scope. I will first try the webcam that I have. I am thinking about a WO Megrez 72. This will look good with the FLT110 and the EQ6 (shallow I know) and provide a shorter FL for imaging.

[bmitchell82]

when im guiding i can see a regular peak occur in RA that is guided out, not to worry its all happy days.! as long as the resultant has round stars i know im caputring as much detail as i can get.

[Moon]

I've got one of these and it does look great, but it really needs a field flattener if you are going to use it for imaging. It works ok as a guide scope, but you need lots of extender to bring the guide camera to focus.
It's also quite heavy - there might be other lighter weight options out there.

[OzRob]

mmm...food for thought. Any suggestions for better options?

[bartman]

Hi Rob,
just to let you know I had the same as in pic 3 of your starting post.
No wind etc....I did notice a bit of play in the dec bearing and tighted the grub screws a little and now havnt seen those strange shapes anymore ( not that I have done much imaging lately).
i'm also yet to start auto guiding but need to sort out the communication between my software and cam ( PHD and Gstar - via a usb to rca cable -)
Bartman