10 min unguided exposure

[cygnusx1]

Hi folks

I'm trying to determine where the error(s) in my HEQ5 Pro mount / setup are affecting my autoguiding accuracy.

I can't seem to reduce my errors to under 2-3 arcsecs suing either PHD or Guidemaster.

Here is a cropped image of a 600s unguided exposure

Any idea what the kinks at the beginning and end are?

Could that be backlash or flexure?

The rest of it looks ok to me.

[barkis]

Kevin, are you imaging through a scope here? The two tick like streaks at the extremes of the drift, (which by the way are pretty uniform in brightness as you can see), look like something has nudged the camera, at the beginning and at the end of the exposure.

The uniformly bright drift period, looks simply like it may simply be an alignment discrepancy.

Someone may have an alternative diagnosis. I used to get a similar effect when using an SLR Camera on my 12" Newt caused by the simple act of operating the cameras shutter with the remote cable. It was the mirror in the camera as it opened, and as it closed.

Beginning and end of exposure. More likely my hand as I got hold of the relese cable both times and shook the scope slightly.

Badly explained, but I think you understand what I am saying.

Ron.

[Kaptain Klevtsov]

Could be you need to do the PEC thing. The shape you have might turn into a sine wave looking wiggly worm if left longer ,which would indicate that the polar alignment is off by a hair or less, on top of the PEC, or it could be the flex in the mount caused by wind just does that. There's no easy way of telling if the mount was oscillating due to wind to give that shape or if the star started at one end of the curve and ended at the other end.

From what I've read recently, you should line up the camera so that dec is up and down and RA is left right. That way you know which direction its wonky in, which gives you big clues as to whats going wrong. I might start doing that, as it seems eminently sensible. Any idea which way is up in this image?

Kaptain Klevtsov

[Beyond_Vision]

I dont know why you are getting these problems but my set up is very similar to yours and I get perfectly round stars with 20 minute exposures guiding with PHD. I have not carried out any backlash training or PEC or done any drift alignment yet it works fine. What guiding hardware are you using?

Regards

Kevin

[cygnusx1]

KK, the orientation is RA : N/S DEC : E/W if that helps at all.

BV, i can get good 3-4 mins unguided exposures on my 66mm F/6 APO, but remember i'm imaging with a C9.25 @ F/6.3 f/l 1400.

Much less margin for error at that f/l.

Additional tests last night resulted in similiar size errors. I even removed the side by side plate setup and just tested with the 66mm APO, htinking that the weight of the dual setup may be causing overload on the mount. No difference.

My conclusion is either I have a mount with above average inherent error, or i've hit the limit on accuracy with my setup at the large f/l i'm imaging at.

If that's the case i'll be v disappointed.

[steppenwolf]

Hi Kevin,

Where are you getting the 2-3 arcseconds error figure from? I am guessing that you are getting this from PHD's readout error though your guide 'scope? Assuming that is the case, last night, using MAXIMDL and my EQ6 mount, my absolute worst case errors were .48 of a pixel which equates to 2.4 arcseconds using the SXV Guider so I suspect that your mount is probably OK BUT that focal length of 1400mm will, of course show this error somewhat amplified. I get slightly elliptical stars with my 1200mm Reflector (in fact you and I have spoken about this on the 'phone as I suspected that I had image creep) but as the error is being corrected by the system, you should not actually get much in the way of trails unless you also have some 'creep'.

[Kaptain Klevtsov]

KK, the orientation is RA : N/S DEC : E/W if that helps at all.

If that means that error in RA would be up/down, then you have an alignment issue, as well as some PE. The up and down movement is caused by the tracking speed being slightly wrong and the side to side error is alignment error.

Kaptain Klevtsov

[barkis]

Kevin, why dont you put a guiding eyeiece in the Celestron whilst you are guiding. If you align the crosshairs parallel to the RA direction, try to find a star near the meridian. then the RA will be E to W, although these directions will be reversed in the eyepiece.

You will see any drift, both in RA and Dec. Then compare what your eye sees, with what the data tells you.

I have no Idea if this suggestion is valid, it is exactly the same as for drift aligning, so the results are meaningful.

Ron.

[blinky]

Am I missing something here the image is a 600s UNGUIDED one. I thought the resulting star trails were normal even for a perfectly aligned mount as they are due to imperfections in the gears/cogs/motors

[MartinB]

If RA is N/S then the sharp horizontal movement is nothing to do with the mount. The mount will not move in dec at all so the movement is down to wind, vibration, flexure, or movement of the mirror (if you were using your SCT). Wind would be the most likely but I guess mirror movement could do something similar. Impossible to say how good or bad your PE is without knowing the scale but you should be able to work out your total peak to trough PE if you know your sampling rate. A movement of 20-30 arc secs would be reasonable, I would have thought, for an HEQ5. Just as important as PE are random errors which this image doesn't show.

You could try offsetting your polar alignment by 5-10 degrees and running a long unguided expsoure, this will clearly show the state of play of your RA tracking. In Maxim you can set up guiding and keep a log of tracking errors. By not allowing guiding corrections you can record the PE. Transfer into excel and you can produce a PE curve. Don't know if you can do this in PHD.

The jist is that this image doesn't show any problems with the mount itself.

[cygnusx1]

Hi Martin are you impying that I may just have hit a limit of accuracy with the setup/weight I have on the mount?

I ran a test using Guidemaster and my PE maxed out at about 20-25 arcsecs.

I doubt it was flexure (or wind ) - im using a sturdy aluminium side by side plate at least 1/2 - 1" thick.

[MartinB]

Hi Martin are you impying that I may just have hit a limit of accuracy with the setup/weight I have on the mount?

I ran a test using Guidemaster and my PE maxed out at about 20-25 arcsecs.

I doubt it was flexure (or wind ) - im using a sturdy aluminium side by side plate at least 1/2 - 1" thick.

No, I'm saying that the squiggles at the top of your image are nothing to do with the mount since they are not in the RA direction and that is the only axis being driven by the mount. A peak to trough of 20-25 arc secs obviously equates to +/-10-12 arc seconds which is perfectly reasonable for this class of mount. This is perfectly amenable to guiding out. You need to get an RA curve running over a couple of cycles to see whether you have an problems with random errors and to assess how smooth your PE is. If this is all looking good you could try getting a bomb proof polar alignment and doing some runs using RA guide corrections only. Then start adding in dec corrections to see what impact that has.

If the dec corrections mess things up offset the polar alignment a little so that the dec corrections are all in the same direction to establish whether the problem could be due to backlash.

[iancandler]

Are you locking the mirror on your DSLR up prior to starting the exposure? as those "kinks" could be vibration from the mirror flipping up and dropping down as it does at the start and end of each exposure.

A good way to get past it is if your camera has a 3 second delay facility for when you use a remote, enable it. It will then flip the mirror up when you click the shutter, but not start the exposure for another 3 seconds, long enough for any vibrations to die down.

At the end of the exposure just place a black card in front of the scope and then stop the exposure.

[Kaptain Klevtsov]

Good spot Ian, that would explain why they are in opposite directions. One way for mirror up, the other way for mirror down. I think.

The level of brightness in the wiggles bothers me though with this explanation, as the mirror event would be very fast. The closing of the shutter would at least be over in less than the blink of an eye. Thoughts anyone?

Still needs more data to get it sorted, this has me puzzled just because I've no idea why it looks like that.

Kaptain Klevtsov

[Beyond_Vision]

Yes KK I think the wiggles would be less bright than the middle line if it were the mirror movement in the camera.

Regards

Kevin