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

[[1ponders]]

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]

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]]

another sample - ngc104

[renormalised]

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

[Dennis]

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]]

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 :P

[renormalised]

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]

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]]

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]]

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

time to go look see.

[[1ponders]]

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]

Hi Paul

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

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


Cheers

Dennis

[[1ponders]]

Damn, another group to join. :lol: Thanks Dennis. :thumbsup:

[[1ponders]]

Ok found the scary stuff, Dennis

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

[Dennis]

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]]

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

[gbeal]

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]]

Actually that's twice now. :lol:

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. :shrug:

[avandonk]

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]]

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.

[Terry B]

Paul
The triangular stars was discussed at length on the VC200L group inthe past. It seems to be a bit idiosyncratic in it's appearance. I have never had them.
The consensus was that it was from slightly off collumation as wel as guiding.
Note you need to collumate without the FR in place.
Mine introduces a slight error across the field. The collumation looks different at the edge of the field to the middle with the FR but is perfect without it.

[[1ponders]]

Thanks Terry. Its going to be scary enough washing the mirror without pulling it apart to try to round up the stars. I'll check the collimation and the guiding and see how I go.

After Berts suggestion about the guiding I went back and checked M7, NGC104 and NGC2070. I did all these last night without changing the camera angle at all, and only one slight change of focus (using the "B" Mask). M7 was by far the worse, then Tuc 47, then the Tarantula. The Tarantula was fairly good, not great, but passable. So looks like it maybe some variable like guiding.

[avandonk]

When the corrections to guiding are nice and random even if over corrected you will still get nice round stars. Then you will say the seeing was 'poor'.

If on the other hand the corrections are skewed by the DEC axis only correcting 'poorly' and getting it right some of the time the RA axis will also correct and you inevitably end up with a triangle of luminance. This happens when the DEC axis spends more of it's time on the left or right.

This is why you get triangular stars.

Do not blame your optics until you have eliminated them by simply taking a short exposure without guiding!

Bert

[leinad]

This is happening to me to Paul. Havn't been able to get the scope out again yet to tune the scope further.

This was my latest pic from June after attempting to improve the collimation(the scope when bought was whacked out of collimation).

http://i611.photobucket.com/albums/tt197/Leinads_pics/Series1_165_011504.jpg

You'll see here the dreaded tri. shaped stars which round out more to the right of the image, but still aren't round.

I've searched the VC200L yahoo group and unfortunately one side of the fence says the mirror is pinched, other says they havnt had the problem.. yet.

At first I thought the focuser tube was the culprit then decided to collimate the scope as best as I could with a laser and cheshire, then collimating with the webcam on an unfocused star etc, but still couldnt achieve round stars. I ensured focuser was centered to spider vane, sec. and pri. were collimated the best I could.
Later testing at 2min and over 6mins I had the same strange shapes.

I used an ED80 with 2xbarlow and guiding graphs appeared Ok, but Im thinking that as avandonk mentioned mount corrections and guiding might be playing a part here also.

Let me know if you reach success as I'd be very interested how you beat this.
If you have questions on collimating send me a PM and I'll explain further the steps I went through.

[Tandum]

That's colimation paul, when it's really bad, stars look triangular. See what ccd inspector says.

[[1ponders]]

I will check the collimation Robin, as well as looking at the guiding. At this I'm open to suggestions.

On going back through the three areas I imaged I noticed there was no change in the orientation of the "triangles/eggs" (see attached of crops from similar areas on the images). I didn't change the position of my camera, so if it was guiding, to me it seems that the orientation would be different because NGC104 & 2070 are in different parts of the sky (including meridian flip) than M7 so the RA and DEC autoguiding corrections would have been very different.

[allan gould]

Paul
Ive seen this on another site where these star shaps are caused by the very thick spider of the scope. They machined down the thickness of the spider vanes and the squarish shape disappeared. Its not due to mirroe deformation or guiding

[bert]

Just to throw another spanner in the works to everybodies theorys. I am a VC200 owner and it is currenty getting a 'Doomsayer' conversion. While that is being done I have Guys personal vc200l. It has the vanes milled down thin and guess what? I am getting triangular/irregular stars in it the same as my regular tubed vc200l.

My hypothesis? The unique diffraction spikes are amplifing guiding errors that in other scopes would go unnoticed. I regularly change my scopes around and it seems that the vc being a longer focal length also makes the guiding look a lot worse. I am still working on my guiding and have not got it perfected.

To throw a spanner in the guiding focal length myth. I used to guide with an ed80 (fl around 600mm) with a stashoot autoguider, I sold it to a friend and now use a William optics 66 with a reducer (fl around 364mm) and the ssag. The guiding is much smoother. A long focal length is not needed with the centroid calculations (1/30th of a pixel)that are used in todays guiding programs. I did a calculator on the web somewhere with my image scale and focal length and It said my minimum focal length to guide at 1800mm is 100mm. I also have a few friends that use camera lenses for guiding very successfully.

Note: Good guiding is good guiding at any focal length, a shorter focal length just hides it somewhat.

Brett

[bert]

Note that Paul is getting triangular stars, Milling the vanes will reduce the SQUARE look of stars.

[Satchmo]

Paul, How do the stars look with a high powered eyepiece. When you defocus a moderatley bright star to half a dozen Fresnel rings, can you see a triangular shape rather than circle. It does look like the primary or secondary mirror are pinched by the retaining clips.

[[1ponders]]

Mark, I hadn't looked at the stars through an eyepiece except to center the webcam for polar aligning and then the 20D went straight on. I'm not going to get a chance to do anything with if for a few days so early next week I'll try the eyepiece test.

[Terry B]

This page explains the triangles well
http://www.mikesastro.com/analysis/vixen/stars.html

[[1ponders]]

Thanks Terry, that's great. Its helped me with both issues, triangular stars and how to disassemble for mirror cleaning.

[leinad]

Heres a few images from tonights collimation testing with the
FR. It still needs a few more tweaks, plus the guiding settings need to be fine tuned, and drift alignment done properly.

I didn't suffer from triangular stars as badly this time with the FR, although the tests didn't have a very populated star field and my guiding, collimation was much better.

More testing to be done!

[[1ponders]]

Looks like I'll need to fork out for CCD Inspector if collimation is going to be this critical.

[h0ughy]

but does it work with dslr's? is it stand alone?:shrug:

[[1ponders]]

It will work through CCDSoft and with the SBIG.

[allan gould]

Try Metaguide for getting your collimation spot on. Works very well with an SCT.

[[1ponders]]

I have metaguide somewhere. I'll give it a try Allan, thanks

[leinad]

I'm using a 40D. I would take a 30sec image of an evenly lit star field, then check in CCDOps. I don't think I did this properly though as at F/9 the results were slightly different.

Go over the Vixen instructions a couple of times Paul that's what I did although it confused me at first.

Center the focuser by removing the secondary(mark position) and use the cheshire to align. Check with laser to see that the laser shines at center of sec. holder. Then replace secondary and collimate adjusting secondary and primary as you would similarly to a newtonian.
Fine tune afterwards with star test, then and add salt, pepper to desired taste.

[[1ponders]]

Well never having owned a Newt, this should be fun. :P

First things first though is to check it how it is now. Then remove and clean the mirror, check again and adjust as needed.