Odd shaped stars. Any suggestions what could be causing it.

[1ponders] (Paul) · #1 14-09-2009, 11:24 AM

First light for my Vixen VC200L last night and I'm not sure what's happening with the stars. Details:

Vixen with FR (vixen FR) = approx 1200mm FL
Canon 20D
Losmandy G11 (DD and PHD Guiding)
7/ISO800 @ 300sec

Frankly I'm absolutely chuffed at how well the mount autoguided at that length, but I'd be interested to know what is happening with the stars. The bright ones have a distinctive bulge at the top left, the medium stars are egg shaped as are the fainter ones

btw this is a full size crop of M7 from near the center

gbeal · #2 14-09-2009, 11:47 AM

Cheap guide camera at a guess, LOL.
Unless I am mistaken I think it stems from the rather thick spider vanes, and is a problem which most VC200L owners suffer. I could be wrong on both of course.
Gary

[1ponders] (Paul) · #3 14-09-2009, 12:19 PM

another sample - ngc104

renormalised (Carl) · #4 14-09-2009, 12:24 PM

How's the collimation?? It might need a slight tweek.

Dennis · #5 14-09-2009, 12:29 PM

Hi Paul

Firstly, congratulations on your 1st light images, the VC200L is an awesome imaging ‘scope.

In terms of your star shapes, the only time I have seen similar examples in my general reading, all suggested that pinched optics were the main cause of producing 3 lobed stars?

Cheers

Dennis

[1ponders] (Paul) · #6 14-09-2009, 12:38 PM

Carl the collimation looked ok, but I will check it again. I'm going to have to clean the mirror anyway so I'll recollimate anyway.

Thanks Dennis, I'm looking forward to getting the most I can out of it. That's what I thought initally Dennis, but I didn't think clips were used to retain the mirror cell. Again I'll be able to check that out when I clean the mirror. You don't happen to know anyone that has experience with cleaning VC mirrors do you

renormalised (Carl) · #7 14-09-2009, 12:44 PM

You wouldn't think on a scope that costs as much as a VC200 that they'd use clips to hold the mirror in. But that was my second choice for causing that wonky star look. Easily fixed, though, just a bit fiddly.

Dennis · #8 14-09-2009, 12:50 PM

Quote:

Originally Posted by [1ponders]

Carl the collimation looked ok, but I will check it again. I'm going to have to clean the mirror anyway so I'll recollimate anyway.

Thanks Dennis, I'm looking forward to getting the most I can out of it. That's what I thought initally Dennis, but I didn't think clips were used to retain the mirror cell. Again I'll be able to check that out when I clean the mirror. You don't happen to know anyone that has experience with cleaning VC mirrors do you

Mine (12 years old) did have 3 mirror clips, although you would have to really fiddle with them to produce mechanical strain on the mirror. There was a thin foam strip between the clip and the edge of the mirror.

Cheers

Dennis

[1ponders] (Paul) · #9 14-09-2009, 12:53 PM

Ok by the looks of it they use 'hooks' to hold the mirror in.

More digging needed if I'm going to clean this mirror then.

[1ponders] (Paul) · #10 14-09-2009, 12:55 PM

Ok, maybe not. I've just read the catalog and it says they have a ring and not hooks.

time to go look see.

[1ponders] (Paul) · #11 14-09-2009, 01:11 PM

Ok there is a ring on top of the mirror with three pads under the ring holding the mirror in place. It may be that these are too tight.

Dennis · #12 14-09-2009, 02:06 PM

Hi Paul

Also, try the Yahoo VC200L Group – they may have some instructions/photos?

http://tech.groups.yahoo.com/group/vc200l/

Cheers

Dennis

[1ponders] (Paul) · #13 14-09-2009, 02:17 PM

Damn, another group to join.

Thanks Dennis.

[1ponders] (Paul) · #14 14-09-2009, 02:49 PM

Ok found the scary stuff, Dennis

http://tech.groups.yahoo.com/group/vc200l/message/3496

Dennis · #15 14-09-2009, 02:56 PM

Hi Paul

I used to be a member but it seems my membership has lapsed, so I can’t take a peek in the Files section to see if there is any useful stuff for you.

Cheers

Dennis

[1ponders] (Paul) · #16 14-09-2009, 03:15 PM

I might join to see if there is anything there. But at least it looks like there is a solution

gbeal · #17 14-09-2009, 03:16 PM

I don't think it is collimation, nor pinching to be honest, I think it is simply a combination of the FAT vanes, and the large secondary.
Dennis is on the right track, there was a posting on the Vixen VC forum a while back, and I recall a Google search found much the same.
But........... I have been wrong before, once.
Gary

[1ponders] (Paul) · #18 14-09-2009, 04:07 PM

Quote:

I have been wrong before, once.

Actually that's twice now.

Check out the link I posted Gary and let me know what you think. I'm not a big fan of pulling things apart that i know little about, and then trying to get them back together. I always seem to have a few nuts and bolts left over.

avandonk · #19 14-09-2009, 04:08 PM

Paul the odd shape can be explained by tiny amounts of backlash or overcorrecting while guiding. It is no coincidence that the elongation is a combination in RA and DEC directions. Try some shortish exposures of bright stars without guiding and at least you can eliminate this as a cause. If you are using the 80ED to guide at native FL (600mm) you will find it is marginal for the Vixen at 1200mm.

Bert

[1ponders] (Paul) · #20 14-09-2009, 05:33 PM

Thanks Bert I'll look into that. I didn't use the ED80 to guide with, I was using my WO 72 FD f/6 so that is probably even worse.