What is causing my stars to look like this?

[White Rabbit]

Hi guys.

After upgrading my focuser to a Moonlight Focuser my stars are coming out rather strangle. I also upgraded my camera at the same time so it could be the camera. I suspect it to be reflections in the draw tube but thats pure speculation, could it be pinched optics somewhere. My image train is as follows.
STF 3800m/WO Flattner reducer .86/Astonomik 2" CLS filter/ orion premium refractor 102 714mm F7.
I never had this issue when imaging with my 5D MKII and GSO focuser so it has to be the focuser or the camera.

Here is in image crop so you can see the star close up.

Thanks

[DavidU (Dave)]

Did you collimate the scope after the focuser upgrade?

[White Rabbit]

No, it's a refractor, i wouldn't be game enough to try.

Thanks

[DavidU (Dave)]

You need to collimate the new focuser.
I put my laser into the focuser tube and made a white paper circle the same diameter as the dew shield. The paper circle would have a pin hole dead center if you use a compass. Most good focusers have collimation adjustments. Just adjust it so the laser red dot is dead center of the paper circle and your good to go.
I suspect your lens is in collimation but the new focuser is not.
cheers

Originally Posted by White Rabbit

No, it's a refractor, i wouldn't be game enough to try.

Thanks

[White Rabbit]

Ah, thanks. I'll give that a go.

I have a laser collimator for my Newts I suppose I could use that one, correct?

Thanks again.

[DavidU (Dave)]

Yep.
http://www.cloudynights.com/ubbarchi...6/Main/1083000

Originally Posted by White Rabbit

Ah, thanks. I'll give that a go.

I have a laser collimator for my Newts I suppose I could use that one, correct?

Thanks again.

[pmrid (Peter)]

While you're at it, it would be worth double-checking that you have your distance between the reducer and sensor correct. Not sure what the distance is for the WO .86 but I'm sure you can get it from their website or similar.

Having done that, have a look at how you achieve focus. Do you use a Bahtinov or the FWHM or HFD numbers in Maxim? Make sure you have achieved the best focus you can and have another look. I know these sound basic things and I'm probably teaching granny to suck eggs.

Peter

[LewisM]

Peter is onto it. But I am also thinking, could this perhaps be well-bloating in the camera too?

My focusing is purely by hand, using FHWM method in Maxim or APT. I will generally take about 30 minutes to focus, as I allow each minimum value position to settle for 5 minutes to see if it changes much, and remember that when I lock off focus, that it MAY change (since the screw applies pressure and will shift the drawtube fractionally in or out)

It does look like a slight focus issue, (perhaps focuser as mentioned, hence why I RARELY will change to an aftermarket focuser), but perhaps a bloat in the camera???

[White Rabbit]

I'll need to do a bit more research on the subject but from inserting my 2" laser colimator that I use to climate my newts, the laser appears pretty much dead centre. I'll need to measure it properly (as much as you can with a ruler ) but it looks pretty good to me.

The image was pretty well focused with a mask, I would imagine that an out of focus image would mute the effects of poor colimation.

Also I'm pretty sure I have the distance between the camera chip and the flatter correct as the images have a pretty flat field.

I'll post some results tonight from ccd inspector. I just purchased it last week and haven't delved into it to deeply as it looks like a steep learning curve.


Thanks guys.

[Terry B]

I'm not sure what the problem is. The image is of a saturated image of a bright star that is slightly octagonal. It seems to have even diffraction rings all around it. If it was a collimation problem then the image would not be even.
It is a refractor so no diffraction spikes should be present.
Are there any non circular baffles in the scope or focuser?
This is a similar effect to what the aperture ring makes with a camera lens.
The only other think I can think of is that the light from the saturated star is bleeding along the lines of pixels in the camera. I don't know what camera you used but does it have microlenses on the CCD?
It may be an effect from the antiblooming in the camera.

[jjjnettie (Jeanette)]

Clutching at straws here, but have you checked for spider webs inside your focuser?

[Shiraz (Ray)]

it's probably diffraction from the microlenses on the KAF8300 CCD. http://www.cloudynights.com/ubbthrea...5/Main/5391241

Seems to be a minor effect on the brightest stars and FWIW I think it looks quite attractive.

[White Rabbit]

Thanks for all the replies. Having done a bit of reading on it I think Shiraz is correct about the micro lense on the ccd.
A bit disapointing as there doesn't seem to be a work around.

Thanks all.

Sandy