Odd shaped stars, what could it be?

[Kaptain Klevtsov]

During last night's session I had an issue with the stars showing up as lines instead of points. I had guessed that it was due to wind wobbling the 'scope as the exposures were in the minutes, no big deal and not a lot to be done about it. After getting four subs off through the OIII filter I wondered if the collimation could be out. I checked it using the laser and the secondary was out enough to put the red dot 10mm off centre, just outside the circle on the primary. 2 minutes later that was sorted and the camera was back on. The plan was to use the CLS filter instead, to get more subs.

Slewed over to Vega to get the focus sorted, sub-framed around Vega and exposure time of 0.3 seconds. Vega came out as a line, which I thought was odd. A thin straight line with a ratio of about 4:1 length to width. I've seen coma, and the star turns into a shuttlecock shape, or goes egg shaped, but never a line. Any ideas what could be going on here guys? I don't think it was wind, as the short exposure time should have let me get a couple of good ones amongst the bad ones, but the images kept coming up the same. I was very zoomed in on the image and unbinned.

The mount is an HEQ5 BTW, and the guiding was connected but turned off in software (K3CCD Tools).

Kaptain klevtsov

[themos]

forgive the stupid question but did you actually look at the image with your eyes or are you describing only what the chip measured?

[Kaptain Klevtsov]

I was looking at the computer screen.

So that it makes sense, if you're not an imager, the easy way to focus is to have short exposures - so something bright, and only download a portion of the image from the camera to the computer so that the transfer time is as short as possible. This way you get to watch the focus change in (almost) real time and the plan is, obviously, to get the star into as few pixels as possible. Zooming in on the image to 4X or more makes the point of best focus more obvious.

So, while I was doing this, the camera was snapping away quite quickly, probably two frames per second. It was whilst watching the screen doing this that Vega became elongated The shape was similar to a liquorice torpedo. http://www.aquarterof.co.uk/liquorice-torpedoes-p-213.html?osCsid=c5446856a8c12139fa76b0858d1921e4

Kaptain Klevtsov

[Kaptain Klevtsov]

Just remembered another detail. When I had added the OIII filter, the stars were quite out of focus. The big donut thing that shows up had the hole off centre. I presume this means that there is a collimation issue, but which bit? The laser said it was all good, maybe I need to re-do the laser?

Also, could I fine tune the collimation by looking at the donut and adjusting the primary? I could get it to guide so that the image stays centred if that's going to be an issue.

Kaptain Klevtsov

[anthony]

Was the camera nose in fully so as to make sure the chip was properly aligned to the optical path? I wonder if the chip being at a slight angle might cause what you saw.

Anthony

[Kaptain Klevtsov]

T threads between camera and filter wheel, and filter wheel to 'scope adapter. The adapter might have been in skew whiff, its still in there so I'll go have a gander.

Kaptain klevtsov

[themos]

Which scope are you using?

From what I've read, KK, collimation is primarily a primary affair and the secondary is secondary(!). It's most unlikely for the secondary to go off. From what you say, I understand that you used a laser to move the secondary but did nothing to adjust the primary. That, I think, is wrong. It may well be that your laser is wrong. Many writers caution against relying on the laser for adjusting the secondary. Do the secondary visually (just a sight tube to make sure your iris is centred) and then do the primary with a Cheshire, aligning the primary ring-spot with your Cheshire slit.

There, I've just told my grandmother how to suck eggs!

[Kaptain Klevtsov]

No mate, what you're doing is helping me think through the problem, thanks for that. With the laser last night it shot out of the focuser and bounced off the secondary, but missed the centre circle on the primary by a bit. I tweaked the secondary to get it centred in the circle and that brought the beam bang on centre in the bullseye of the laser. Straight back up the hole as it were.

I'm beginning to think that the laser is off or something got askew while I was adding the filter in the dark.

Kaptain Klevtsov

[themos]

KK, you mean you never actually looked to see if stars had spaghettified in an eyepiece?

Ok, here's what a Sky and Telescope guide says (my emphasis)

Step 2: Here you adjust the tilt of the secondary mirror to aim the focuser's axis at the center of the primary. First, remove the cardboard from the spider. Now, while viewing through the sight tube, carefully adjust the screws that tilt and rotate the secondary until the primary mirror's reflection appears centered in your field of view. If your sight tube has cross hairs, align the primary's center spot with them; otherwise, center the outer edge of the primary within the sight tube. (Make sure that the sight tube is racked in far enough to let you see the whole primary mirror.) A laser collimator is even better for this step — just center the laser beam on the primary's center spot.

A small error in secondary alignment is usually not a problem. As long as the pointing error is no more than 1 or 2 percent of the main mirror's diameter, it makes no visible difference. However, if you plan to use a laser collimator in Step 3, you should be aware that even a tiny misadjustment here will throw off the final collimation.

If you have a truss-tube telescope, you will need to repeat Step 2 each time you reassemble the scope. With a solid-tube reflector, you need only check this once in a while.

http://www.skyandtelescope.com/howto/diy/3306876.html

[Kaptain Klevtsov]

KK, you mean you never actually looked to see if stars had spaghettified in an eyepiece?

Now there's a thought. Might see if I've got one of those lying around somewhere.

Kaptain klevtsov

[AndyH]

Hi.

From what i remember reading on the TALScopes yahoo group, you're not advised to touch the secondary unit on a Klevtsov.

Hope everythings works out fine for you.

Andy.

[Kaptain Klevtsov]

Sorry Andy, its the 200mm Newt. that I'm playing with. Just checked the collimation of the collimater, that looks fine, re-tweaked the 'scope (it was half a tiny bit off accoring to the laser). Now I just have to wait for stars to arrive and the clouds to leave.

Kaptain Klevtsov

[AndyH]

Hehehehe.......... dooohhhhh.

Andy.