Well I have not really cracked Guiding after all!

[Astronome]

Hi all,

Seems obvious now I look at the geometry:evil6:

If you have the guide scope on the same mount but displaced by say 4 inches on the mounting bar, even though you are guiding fine and to within 1 pixel the stars on the camera frames will slowly (very slowly) drift across the image. So what is the answer mount the guide scope on RA rotation axis?? So what if you have 2 scopes on the same mount in a line?

I need to do some geometry...

Anyone know the answer. It will be an RA drift won't it?

Not a major issue but over 500 seconds exposure I can see the stars ever so slightly out of round.

Regards,

John

[themos]

You could be hitting field rotation due to imperfect Polar Alignment and an excessive angular displacement between your guiding star and the centre of your imaging frame.

[themos]

Also, dreaded differential flexure could put a limit on the length of your guided exposures.

[Astronome]

Interesting. I can sit there for an hour without guiding and only very small shift in the star. Field rotation is interesting. Maybe I should check opposite sides of the H18 images?

ATB,

John

[Astronome]

Just checked the images of M43 from two nights ago on 1 filter image 1 and 10 on blink and it moves linearly acros the image, no rotation. So I am sure it is something to do with guide scope offset. The fact that I have the guide scope to one side of the imaging scope?? In order to balance the mount weight the imaging scope is only very slightly of the rotational axis but the guide scope is about 5 inches to the left.

John

[SteveL]

Sounds more like differential flexure to me.

[themos]

Let DSS analyse the offsets and angles. If you don't see a progression in the calculated Angle, then it's not field rotation but differential flexure.

[Merlin66]

As long as the guide scope is securely fixed and pointing close to the area you're imaging it doesn't matter if its 4" or 4' away!

The stars are almost at infinity (no discussions please!) so the light that arrives here is parallel. So as long as both scopes track together you're in business.

[Whippy]

Have you got an image you can show us with these stars?

Tony..

[Astronome]

Well you would need two images and each is 48MB. I have measured the drift and it is 24 pixels per hour. FOV is about 1.4deg and H18 is about 2000 across in pixels.

don't forget the guide scope has not deviated from the guide star by more than 1 pixel (actually abour .4 pixel)

Distance between the two scopes is about 8 inches.

I think I can see some geometric issues. What I am doing is effectively fixing the mount around a point 8inches from the DEC rotation axis.

I will do some calculations. Alignment not perfect but close enough for EQMOD to fix any small errors.

Ah I will measure the star drift without guiding....

Watch this space.

ATB

John

[themos]

That sounds like differential flexure to me. I think MetaGuide has a procedure for identifying it.

[opticalpath]

What Merlin66 said ..... the drift cannot be due to offset of the guide scope because the light sources are so distant and the offset separation is so (relatively) tiny.

(Are all the stars elongated in the same direction? If not, it's an optical distortion.)

Assuming the elongation is consistent across the field, the most likely cause is differential flexure. That is: tiny changes in the relative pointing directions of the guide camera and imaging camera. The guide camera keeps the guide star dead centered in the guide scope and thinks it's doing just fine, but that position is slowly changing relative to the frame that the imaging camera is seeing. The result is elongated or trailed stars, usually in a diagonal direction that varies with the sky position.

Flexure can be very hard to track down (ask me how I know!) Common sources include: small movements of the primary mirror (flop in a SCT) as the telescope tracks, insufficiently rigid attachment of the guide scope to the main OTA, slight movement of the imaging camera or the guiding camera in their focusers, weight of connection cables pulling on cameras.

These effects result from gravity pulling at overhanging components. Any part not very rigidly mounted and firmly clamped is susceptible to differential movement. At typical imaging scales, the movement needs only to be tiny to cause trailing. Because it's gravity pulling, the magnitude and direction varies with position and orientation of the setup.

Good luck tracking down the cause. Post JPG versions of some images to show what you're getting.

Adrian

[opticalpath]

Here's a simple test you can perform to check for differential flexure, if your guide camera software allows you to monitor the guide camera image.

1. Centre a star on the guide camera image cross wires.

2. Switch autoguiding OFF

3. Take a short (few seconds?) exposure with the main imaging camera.

4. Leave everything alone and come back 30 minute later.

5. Centre the same star in a fresh Guide camera image again and take another short exposure with the main imaging camera. Compare the two main camera images.

In this test, whatever the PE or tracking error of the mount, both guide and main scopes have experienced identical tracking and errors. Since you re-centered the guide star before the second exposure, the two main-camera images should be virtually identical. If the field of view in the two images has shifted, one relative to the other, the only possible explanation is some differential movement beween guider and main scope during the interval.

Remember, if this was autoguiding, the guide camera would in effect be saying, I'm still dead centered on the guide star. 20 pixels per hour sounds a lot but that still represents only a tiny angle of perhaps 1 or 2 arcmin, depending on your image scale. You can probably imagine a camera shifting in the drawtube by that small angle and not noticing it.

Adrian

[Astronome]

Ok Opticalpath, sounds like a good test.

The only thing that is confusing me is that over many hours the drift is linear???

Let me do some more testing. Error on unguided is only about 3 minutes/hr x and 1 minute/hr y. Just done the calculations on some older images.

Regards,

John

[themos]

Any small change will first show up as a linear effect...

[SteveP]

John

Can you post an image showing the star elongation? You may have to resize it first if it's 48Mb.

Although your problem may be as a result of flexure in the system, I wouldn't rule out poor polar alignment at this stage. If your guidescope is so far removed from the imaging scope, any field rotation will be centred off of your image, ie on the guidestar itself

Steve

[opticalpath]

John,

If we're talking about the SX-H18, the FITS mono image files should only be about 16 MB at full 1x1 resolution. Not sure how you're getting 48 MB unless you're converting them to 16 bit colour format. If you're posting an image, converting it to 8 bit JPG (grey scale) format will dramatically reduce the file size.

The H18 sensor is actually 3326 pixels and 18mm wide, so if your FOV is about 1.4 degrees, you must be imaging at about 740mm focal length and 1 pixel would represent about 1.5 arcsec at 1x1 or 3 arcsec at 2x2 resolution. So your drift of 20 arcsec per hour represents an angular movement between the two imaging trains of only 30 (or 60) arcsec. That looks a lot on the image but requires only a tiny mechanical flexure between the guide and main scopes.

The amount of drift you mentioned that you are getting UNguided suggests that you do still have a polar alignment error that is worth working on. That alone would not explain the *guided* drift amount but it may complicate the diagnosis a bit by contributing some field rotation. (FWIW, I try to get my unguided drift in Dec down to 0.1 arcmin per hour or less on the meridian.)

The fact that the drift is linear over several hours is a little strange; drift caused by differential flexure usually changes direction for different orientations of the OTA. It would be worth doing some tests taking successive images of the same field like I suggested, but over several hours, with the OTA first east and then west of the meridian. That usually causes a change in direction of flex and therefore of drift. The benefit of testing with the auto-guiding OFF, but manually recentering the star in the guider field each time, is that you take the auto-guider program out of the equation and remove the possibility that some odd setting or behaviour of the program is contributing to the problem.

Keep us posted on how you get on.

Adrian