I can hear that primary mirror crying in excrutiating pain from here, as I look at those mirror clips about to snap off under the strain.
Yes x 100.
Quote:
Originally Posted by ausastronomer
The clips are only there to stop the mirror falling out, not to hold it in place. You should tighten them so that you can slide a thin sheet of paper between the clip and the mirror.
Very good advice. Listen to the man, Eric. Use a bit of Loctite on the threads if you worry about screws coming loose.
I'd be happy to look at your scope. f/4 is very touchy with collimation. I can show you the barlowed laser method of collimating the primary. It's Ezy and precise!
I took your pic into PaintShop Pro and place orange circles where everything should be after collimation. All centred.
Then I placed a Green circle where you have currently set your secondry.
Then I placed a Blue circle where your primary reflection is.
As you can see nothing is collimated.
The only thing centred is the reflection of the focuser looking at itself.
My judgement is pinched mirror and collimation waaayyy out.
It looks like coma, and at f/4 is almost certainly that. Even a small chip will have a field of view large enough to show it. If the star images in the center appear focused, and the stars at/near the edge appear spread out along radial lines......it's coma.
Collimation: if you do not know how to collimate your laser, have someone help you who does. Collimating a scope with a miscollimated laser results in......miscollimation! I frankly don't like inexpensive point-source lasers. I prefer the "passive" and "repeatable" tools, like the 3 tools from www.catseyecollimation.com These are very accurate and will show you the inaccuracy in your laser (they did my Kendrick Laser. I had to spend an hour collimating the laser before it even came close to agreeing with these tools). If your laser agrees with the passive tools, consider yourself lucky--your laser probably came collimated (as Howie Glatter's always do).
Coma: easily (but not cheaply) fixed with a coma corrector. 3 good ones for photography that I know of--the Baader MPCC, the Lumicon Coma Corrector, and the TeleVue Photographic Paracorr. I'd look for a used one first if your budget is limited. Sometimes people sell them when they convert to a longer f/ratio scope.
Best of luck, and it looks like your image processing is pretty good. You'll be a pro in no time.
ok... i guess to solve my problem is to get a new scope or a coma corrector. myastroshop sells the Baader one's for $239. thanks ken for your pic showing the inaccuracy and i need to fix my collimation with a cheshire of course . thanks guys . it really helped me out.
oh forgot to ask, with a MPCC, that will extend the camera out? i was having problems with infocus and wouldn't the MPCC be an issue?
Don't spend money on a coma corrector just yet - Coma effects are massively exagerated by poor collimation. Have another long hard look once everything is lined up properly.
Fix one thing at a time, and it will all come good!
Quote:
Originally Posted by EzyStyles
ok... i guess to solve my problem is to get a new scope or a coma corrector. myastroshop sells the Baader one's for $239. thanks ken for your pic showing the inaccuracy and i need to fix my collimation with a cheshire of course . thanks guys . it really helped me out.
oh forgot to ask, with a MPCC, that will extend the camera out? i was having problems with infocus and wouldn't the MPCC be an issue?
thanks Lee. oh in regards to COMA correctors, i have found this:
Im not sure how meaningful it is in this context, but the site states that it was designed for the vixen r200 scope. I believe its an 8" f4 just like yours
Perhaps our resident optical experts fresh from active discussion on such a topic may wish to comment on the differences between visual and photographic coma correctors.
No diff in functionality. One has an eyepiece in front of it to deliver light from the (corrected) focal plane to the observer's eye, the other has a sensor/film in the focal plane.
Im not sure how meaningful it is in this context, but the site states that it was designed for the vixen r200 scope. I believe its an 8" f4 just like yours
I hope it will work ok. but would anyone know if i will be able to reach in-focus with a coma corrector?
Typical coma correctors do require in-focusing--at least this is true of the Lumicon and the TeleVue.
The lens you've linked seems to be threaded to attach to something (the bottom of the focuser drawtube?), which is interesting. It wouldn't work attached to the bottom of an eyepiece (too close to the eyepiece focal plane), though, even if it could be attached there.
Since the active part of a coma corrector is the lens, there's no reason to have a tube if it isn't necessary.
I believe this is a Ross corrector, so it's intended for photographic use. But, like the Baader MPCC, so long as you pay attention to maintaining the correct distance between the eyepiece's focal plane and the lens, it should be usable visually as well.
The eyepieces most likely to require more in-focusing would be your short focal length ones, which are probably 1-1/4". There are 2"to1-1/4" adapters that drop the eyepiece down 12+mm, so you might be able to achieve extra in-focusing without resorting to a shorter focuser. More the issue is whether you can achieve the correct distance between the camera's focal plane and the lens. It might require experimentation.
Hi Eric, your last pic is much better maybe a little tweek will get it spot on if you dare that is..
With the baader coma corrector will give you a extra 1cm outwards focus so should be ok. Can`t remember if the others change your focal plane postion much but some have to be much father from the focal plane too which would be a problem..
To your picture of the secondary having a dark crescent with your eye,focuser refle being shifted away from the primary in the very centre of your reflections. This is how it should look! That means your secondary is offset correctly to the focuser and mine is the same and you can see pics of the same on this site.... http://www.fpi-protostar.com/collim.htm..
So you are almost there...
Look forwards to your new post pics...
Gary
thanks for all your informed posts guys. really helped me out. if the coma corrector gives me more outer focus that is exactly what i need. Just ordered the Vixen R200SS from AStro Optical . Should be getting it tomorrow i'll post a pic with it on for comparsion. another question how would i mount the coma corrector and will it take filters?
Hi Eric,
The collimation looks a whole lot better than when you had it in the shop. I didn't think to look any closer at the primary retaining clips, glad you've fixed that. But looking at your image the coma is magnificently apparent on only 2/3 of your field. That was proven by the fact that you could only see 2/3 of the primay mirror when you brought it in. You need to drop that secondary towards the primary.
Offset = minor axis of secondary*(Diameter of primary - minor axis of 2ndary)/4*(focal length-[distance of secondary mirror to focal plane)
Should be a couple of millimetres. However, having said that, I think you will find that your secondary mirror is slightly oversized anyway and offset will not be necessary if you have the secondary centred.
At f/4 coma is more than objectionable at 10mm off axis where the image of a star will be smeared over 5 or 6 times the size of the Airy disc. Your sweet spot is only about 5mm or so across. If you wish to take a photo of a larger field then you need to use a coma corrector.
sweet spot only 5mm? nooo! i think i need a new scope. thanks Roger. The Vixen R200SS coma corrector arrived today. But it doesnt fit I will also need some Vixen adapter for the thread as well. I thought coma correctors were universal, though wrong. I wait for Merlin's one in the mail next week. Hopefully this will solve the issue.
sweet spot only 5mm? nooo! i think i need a new scope. thanks Roger. The Vixen R200SS coma corrector arrived today. But it doesnt fit I will also need some Vixen adapter for the thread as well. I thought coma correctors were universal, though wrong. I wait for Merlin's one in the mail next week. Hopefully this will solve the issue.
I already mentioned that at the earlier part of this thread. This is the first time i saw what a 'sweet spot' looks like after seeing your DSLR widefield image. :-) Anyway, the sweet spot can be fixed in place at the center of your FOV by using the barlowed laser collimator method. I have one and i can make the collimation adjustments in less than 30 seconds. That is the duration of placing the mirror's center spot sticker shadow on the collimator's center marker using the collimation screws.
Use a bit of Loctite on the threads if you worry about screws coming loose.
and precise! 
. thanks guys . it really helped me out. 
I will also need some Vixen adapter for the thread as well. I thought coma correctors were universal, though wrong. 
