I've been trying to figure out how you do a star test or collimate with a star as its supposedly the most accurate method. but how on earth do you see those airy discs/rings.
I'm primarily using a camera and not an eyepiece and have tried short exposures, defocused, and defocused slightly as well. as I defocus, the star gets bigger and I can see the shadow of the secondary in the middle and the spider, but that's about it. no disc.
anyone had any luck with this method?
The disc is the dot at the centre of the Airy Pattern - which may have a lot of light in the surrounding rings if the seeing is less than perfect . Why don't you post one of your pics of the in focus star image ?
As posted by Ken & Mark, it's tough to see it on the CCD. If you can use a EP then that would be ideal as you get instant feedback and dont need to worry with the CCD. I have used the method you linked to for my reflector and had no issues seing the disks using a 17mm or 13mm EP. However it isn't important that you see the rings; just the shape of the out of focus star and position of the secondary in the middle of the disk.
The whole point really is to see how the disk (Defocused star) is 'pinched' and how the shadow of the secondary (the dot in the middle) is centered. If the disk isnt circular then the optics are either pinched (Too tight on one side) and is being distorted or the optics itself isnt round. In your case if you see this then it would be the first since you have good optics. Furthermore the dot in the middle of the disk should be exactly in the center. If it isnt then it's a very good indicator that your optics arnt aligned and is a good way to adjust your optics to get them bang on. This is also a similar approach I use on my RC8, and gets me very close.
Dont worry too much about not seeing the actual rings in the disk, they would just be concentric with the outline of the defocused star and thus you can just use the shape of the disk. The rings wouldnt really get used nor are critical to get the collimation because of this fact
ok thanks
I should update the title to read airy rings rather than the disc.
so viewing it through a ccd at prime focus at f4 I guess is out of the question, even with a 2x barlow.
I do have a 15mm EP, I might try that.
so do you first focus, then defocus just a bit? or does it have to be focused to see the rings. do you use a fairly bright star?
getting it on the ccd would've been great as i can then see if there is any astigmatism or pinch in the optics.
My biggest problem is centering the secondary.
I won't go into length on the offset for the secondary, but the problem is that for my optics, 10inch F4 and an 88mm secondary, the offset is around 5.5mm, and that means when I defocus on a star to see if the secondary is centered, I have to account for that 5mm offset and get it off centre.
I might instead measure the distance from the top end of the secondary to ota surface and distance from the other end of the secondary to the other side of the OTA and make sure one side is 5mm less than the other.
I do have the hubble artificial star and might try viewing it with an EP at a distance.
I'm not entirely sure why you cant see it on a ccd tbh, but yes using a EP is pretty good and simple. It just has to be defocused a little, and the rings may be very fine or almost impossible to see depending on a number of factors (Seeing being one of them as Mark mentioned). I always rake outwards but it doesnt really matter. I use a fairly bright star, I find it easier to see if the star seems bigger through the EP (As compared to trying to defocus a tiny dot that is a star).
As for the secondary offset, I would agree on just using a ruler or vernier for it. I am not sure if in the airy disk the secondary should appear offset, I think it should be in the center regardless. Someone else may know better than me
I went through the thread, but no one mentioned how they actually got the image of the fresnel rings. does it work only for sct's and rc's due to their long focal length?
Mark, silly question, what should we be aiming for during collimation, fresnel or diffraction?
It should work on any scope. You just wind the star image out of focus. Use a bright star, so you can throw it well out of focus to get a good sized disc to work with that's still bright enough.
It shouldn't take you too long to work out what adjustments change what part of the disc.
It really comes down to seeing. Attached is a photo I took with my hand-held iphone that shows the airy disc and Fresnel rings.
Thanks Jason,
what EP and/or barlow did you use and what scope? just to get an idea of the magnification.
so if you defocus more, would the rings appear bigger, or dissappear.
tonight might be clear so I will give it a shot myself.
Need to get these darn rings!!
I seem to be having problems getting the rings as well. Racking the focuser back and forward I get a "bigger" star, but I don't get any rings on either a 12" or 16" dob. Doing the same on an 11" SCT and I do get the rings.
So does either magnification or telescope type have a bearing on the result here?
You will get diffraction rings by using a bright star, as Sheeny said, but
also high magnification. Also as previously said, the better the seeing the
better the rings. I imagine that you were working at higher mags with
your much longer focal length 11" SCT.
Diff. rings are a function of light, and have nothing to do with the type
of scope.
raymo
Last edited by raymo; 09-02-2014 at 05:08 PM.
Reason: more info
Yes you're exactly right ..... I would have been 300x or great in the SCT when I last checked collimation on a bright star, and definitely saw the rings, whereas I was looking for them last night in the dob, but probably only at about 100x.
Yes I finally saw the rings.
I used a 6mm ep in my 10inchf4 and defocused slightly on a bright star and I saw them.
It was small as the magnification wasnt high.
I'll try it with a barlow and the 6mm.
If you defocus too much then it becomes a white disc with the secondary shadow in the middle.
But as someone asked here, how do you know which to adjust, the primary or secondary if the rings arent concentric or centered.
It doesn't really matter which mirrors need adjusting. Do a careful
collimation, and everything will become concentric, assuming that
the scope has no inherent faults.
raymo
Last edited by raymo; 09-02-2014 at 08:16 PM.
Reason: more info
