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.
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.
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!
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.
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.
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
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.
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.
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.
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.
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.
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