#1  
Old 04-03-2015, 06:38 PM
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graham.hobart (Graham stevens)
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fix my stars?

Hi folks...
I have noticed the oblong stars on my pics-all pointing towards the top left. See the Horsey attached. I am trying to work out whether this is 1/ polar alignment 2/ collimation 3/ coma 4/ flexure or a bit of everything!!
A description of my system first-
Vixen RS 200 f4 Newt with after market Moonlite focuser, Modded canon 60Da (quite heavy) with coma corrector attached to T adaptor (I think correct spacing -MPCC) , PMX mount and guided by 80mm guide scope on ADM plate atop newt with lodestar.
Now I have included pics of stars inside and outside of focus to see if you think it it collimation -I thought it looked OK on the screen (a secondary offset is normal in f4).
I haven't rechecked my PA in a while, and have changed scopes so I could have bumped that- I will do another calibration run when the clouds go, and I have an OAG but haven't got around to setting it up yet.
So in essence I am asking you to see if you think there are obvious places to start to hunt down the issue/s!!
Thanks for any advice
Graham
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  #2  
Old 05-03-2015, 01:27 AM
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OK,
Polar alignment can be ruled out by doing a short exposure of a bright open cluster or the like. You are not going to get field rotation in a short time. It doesn't look like field rotation anyway.

I have had no experience with coma, but I don't think this looks like coma which radiates from the center outwards. your image doesn't look like this.

Flexure of the optical train is quite possible, resulting in defocused stars. point your scope to different parts of the sky to see of the elongation changes depending on where the scope points and which way the camera wants to sag.

My bet would be miscollimation. try tweeking your collimation while in focus and then viewing what happens to the stars - trial and error until you know what affects what.

2 other things, how is your guide graph? can you rule out Differential flexure and camera tilt (not orthogonal to the optical axis)? Although, as far as guiding goes, the star elongation isn't exactly all in the same direction.

Josh
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Old 05-03-2015, 10:14 AM
clive milne
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Quote:
Originally Posted by graham.hobart View Post
1/ polar alignment
No, otherwise all your stars would rotate around your guide star.

Quote:
Originally Posted by graham.hobart View Post
2/ collimation
It maybe a contributor. If you made a circular (cardboard) disk larger than the secondary holder and centred it in the ota, you should be able to tell.

Quote:
Originally Posted by graham.hobart View Post
3/ coma
No, nor does it look like a spacing issue, otherwise the stars would be distorted symmetrically around the field centre.

Quote:
Originally Posted by graham.hobart View Post
4/ flexure
It is not flexure between the guide scope and imaging ota, otherwise all the stars would be equally distorted.

The one thing it does look like is focal plane tilt (with maybe a little collimation error) The easy way to test this is to note your best focus position for each corner of the frame. It should be the same for all 4 corners. This is best done after you have eliminated the possibility of collimation error.


best
c
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Old 05-03-2015, 10:35 AM
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graham.hobart (Graham stevens)
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stars need fixing

Thanks Clive/Joshua.
Clive, do you mean make a cardboard cut out larger than the secondary holder and centre it on the primary mirror?
I have suspicions about the image plane tilt as the camera is really quite heavy and the moonlite is a compression ring adaptor.
There does seem to be some movement if one screws one screw in before the others, tilting the camera slightly.
Plenty to think about.
Thanks all
Graz
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Old 05-03-2015, 12:20 PM
clive milne
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Hi Graham,
Place the cut out on the secondary bolt
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Old 05-03-2015, 02:38 PM
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graham.hobart (Graham stevens)
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fix my stars

clive- can you clarify what the aim is here-is it to take the secondary out of the picture reflection wise so you can concentrate on getting the primary shadows concentric?
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Old 05-03-2015, 04:17 PM
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Looks like 2 things to me:

1. A bit of tilt, its worse in the top right corner.

2. It looks like an image you get when its windy and the autoguider gets pushed off or has a patch of large errors then settles back down.

Do you get these stars every time or is this a one off?

RS200 is sensitive to collimation. I had one once.

Greg.
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Old 06-03-2015, 10:56 AM
clive milne
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Quote:
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clive- can you clarify what the aim is here-is it to take the secondary out of the picture reflection wise so you can concentrate on getting the primary shadows concentric?
Correct.

Keep in mind though that there are more accurate ways to collimate a Newtonian

best
c
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Old 06-03-2015, 12:14 PM
clive milne
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An experiment you might like to try to determine the ota's sensitivity to focal plane tilt as a function of mechanical deformation:

This assumes you have access to a laser collimator?

1) Orient the ota such that the ota is pointing at the horizon and the focuser draw tube is horizontal (parallel to the ground)

2) Insert the mpcc into the draw tube of the focuser.

3) Tie a piece of string around the end of the mpcc.

4) Hang a weight off the piece of string that approximates that of your camera (plastic juice container filled with the appropriate amount of water for example)

5) Insert the laser into the focuser and turn it on. (leave it turned on until the end of the experiment)

6) Use a whiteboard marker to place a spot on your primary mirror that coincides with the laser spot. (don't be too concerned at this stage if the laser doesn't point at the optical axis of the primary - this is not a collimation exercise)

7) Cut the string with a pair of scissors and allow the container to fall to the ground being careful not to apply a force moment perpendicular to the string.

8) Inspect the primary mirror... is the laser still pointing at the whiteboard marker spot? If not, measure the displacement (in mm)

9) Repeat the experiment with the ota now pointing at the zenith and the focuser horizontal.

10) You can turn the laser off now.

Here's how to derive your focal plane tilt from the above steps:

Take your focal length (800mm) and divide by 57.3 (or 360/2xPi if you want to be precise) this gives you your image scale. ie)14mm per degree.

If the laser dot shifted 7mm in the above experiment, you know that you will experience +/- 0.5 degrees of focal plane tilt as a worst case example.

So... assuming the worst case example; +0, -1 degree of focal plane tilt and focus derived in one corner of the ccd (with a diagonal of 40mm) the opposite corner will be out of focus by an amount equal to:
The chip diagonal (or half diagonal if you focused on the field centre) divided by the primary focal length, multiplied by the laser deviation measured in steps 1 through 9 above.... Or, sine of 0.5 degrees x ccd diagonal if you prefer trig.

ie) 40 / 800 x (+/-) 7 = +/- 0.35mm

To find the extent to which your star images will increase in size for this focus error, simply divide +/- 0.35mm by your focal ratio (f/4) and multiply by 1000 to get the answer in microns

ie) +/- 87.5 microns. From this you can see that for every mm of deviation in the laser spot, you will increase the blur circles of your star images at the field corners by 12.5 microns.

best
c
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