I collimated my scope recently after purchasing a cheshire eyepiece and after the initial issues and learning, thought I had it pretty good after a quick view at some familiar subjects.
Seems I have jumped the gun. The other night I was looking at Saturn and Regulus and couldnt understand why everything looked like I was drunk (I wasnt) , even to the points right up to focus. I realised that my collimation had some issues and decided to fix it.
Well, that was 3 days ago and I have just about decided to get some fuel, some kindling and a few firestarters to make a nice, warm Newtonian Bonfire out the backyard to keep me warm while I use my binoculars instead.
My problem is this: I cannot seem to get the secondary mirror to line up with the focuser AND be round in shape AT THE SAME TIME.
I can adjust the secondary to look like it is lined up and circular, but then I put the Cheshire back in and adjust the tilt and it goes oval and I cant seem to fix it.
I have read everyone's and I mean everyone's instructions on collimation and have absorbed all I can on the subject. However, this doesnt help when you get an instruction that says: " Line up the secondary mirror so that it is in line with the focuser and then adjust the mirror's tilt so that you can see all three clips" and then goes to show you a pretty picture of what its supposed to look like! What happens if you CANT get it to look like that?
I have almost stripped the central phillips head metal off the screw head, I have turned it in and out so many times and have just about had enough of adjusting it this way and that, trying to get it closer to centre and then tilt and I have officially now lost the plot.
Can anyone please help me? I need a tip or some inside knowledge of how to do this easily. As I said, I thought I got it right the first time, I dont think I have busted anything, but now, I am ready to take a sledgehammer to it if I struggle over it any longer. Add to this the last two nights have been clear all night and well, you get my drift.
I would even be prepared to jump in my car with the POS (piece of s#$%) and take it to someone who obviously knows more about this than me if only I could get it back to some semblance of working order again. I live in Kellyville NSW so if anyone nearby can whack me upside the head with a piece of 4x2 and help me sort this out, then I would be forever in their debt.
I suspect they don't burn very well, so better give that thought a miss.
Last approach seems most productive. Get yourself and it to someone with a bit more experience and see what they discover. Try and do it on a clear evening so it can be tested immediately in the field.
Do you have the cheshire in the focuser 'just enough' so that the secondary is just smaller than the entire field of view of the cheshire? Did you also have some white paper between the secondary and the primary to reduce reflections? Moving the mirror up and down the tube shouldn't affect the circular appearance, AFAIK. Also adjusting the tilt shouldn't affect it that much?
When you decided that the collimation was out, when viewing Saturn, did you do a star collimation test?
Thanks, i think this could be the best (and safest for my scope) option.
Hi Programmer,
Yeah, I have tried all that, the cheshire is pretty much OK anywhere in the tube, I dont have any problems seeing the difference between the reflections of the primary and the secondary / outside of the cheshire reflection.
The focuser has a bit of slop in it as it is R&P but I did test the collimation on Regulus, Betelgeuse, Sirius and a few others and found that I didnt have concentric round circles either side of focus but a squashed looking halo of the star that reminded me of a smoke ring in a very slight breeze. It was crooked and bent like a froot loop that has gone soggy on one side. (sorry, all I could think of)
I certainly didnt get a nice round image off focus like all the guides show you.
Thanks, i think this could be the best (and safest for my scope) option.
Hi Programmer,
Yeah, I have tried all that, the cheshire is pretty much OK anywhere in the tube, I dont have any problems seeing the difference between the reflections of the primary and the secondary / outside of the cheshire reflection.
The focuser has a bit of slop in it as it is R&P but I did test the collimation on Regulus, Betelgeuse, Sirius and a few others and found that I didnt have concentric round circles either side of focus but a squashed looking halo of the star that reminded me of a smoke ring in a very slight breeze. It was crooked and bent like a froot loop that has gone soggy on one side. (sorry, all I could think of)
I certainly didnt get a nice round image off focus like all the guides show you.
Chris
Erick's is sound advice. I guess I'm just asking to help with my own feeble collimation attempts (which I *think* I have got down pat).
Just to clarify.. the star collimation test you did is the one to check collimation, not the optics (i.e. the 'airy disc') test?
But, I guess you'd know if your collimation was out or not. As I said, I'm just curious.
Hi Chris - that central bolt/screw moves the secondary up and down the tube: to get that (the sec) in the right position re the focusser tube is the first thing.
(1) slacken right off the 3 screws around that central one on the sec.
(2) look down the focusser tube with either "nothing*" - "the chesire sight tube" or "the ol' film canister with pin hole."
*using "nothing" but your eye requires getting your eyeball really centralized over the focusser tube!
(3) by turning the central sec mirror screw clockwise or anti, the sec mirror should be able to be centalized when you look down the focusser, thru whatever above method you've chosen. This means that when you then carefully grab the body of the sec with one hand and rotate it till it appears as if it's circular, it will appear concentric within the bottom of the focusser tube (or actually the sight tube/chesire, if you use one of these) - ie it will have an even gap all around its' perimeter in relation to the bottom walls/edge of the focusser tube (or sight/chesire tube.)
(4) holding it as steadily as you can in this position, twitch up the 3 surrounding screws of the sec till they all impinge slightly onto the back of the sec holder ,and this will hold the sec in aforesaid position: having someone to help, and/or "bob's knobs" helps immensely here.
(5) the above should have the sec centred and looking circular in the focusser tube (or chesire/sight tube) - the piece of white paper (A4) lightly taped onto the scope tube directly opposite the focusser tube (and thus behind the sec mirror, will assist the above process.)
(6) it's now time to start fiddling with the primary's screws to get everything aligned within each other!
Note: you do not touch/alter the central sec screw again after (3)
Thanks for the instructions, I have tried all this, but I have a slight discrepancy with what you have suggested as I have a secondary 2x barlow built in between the Secondary mirror and the primary mirror and this is held in by a sheath that shrouds the secondary mirror and basically passes behind the secondary and stops me from putting my hand in and holding the secondary while I tighten it all up.
The reason its there is because its a catadioptric short tube newt (500mm tube with 1000mm FL) so this is probably another reason for me to take to it with a baseball bat?
I have managed to get the secondary centred and round once when I first did the collimation but now try to line it up just back of centre so that when I turn the allen key screws the mirror tilts up towards the focuser tube and lines up nice and central. Unfortunately this is a hit and miss affair - hence the burred screw head as I have done it over and over and over and over.
I have even taken the secondary vane housing off from the end of the tube and adjusted it in my lap so that I can line up the secondary as close as I "think" is central to to opening of the shroud where the focuser should be once reassembled. After this, I put the secondary vane housing back on and what do you know, its NOWHERE near centred when looking down the cheshire.
I think I might take up tiddlywinks, its less stressful.
I did a quick google for "catadioptric short tube newt collimation" and got some interesting results ... its seems you're not alone. Looks like some potential "mounting/squareness" issues with the internal barlow mechanim according to the first 2 Google results.
Struth Screwie : I have never fiddled with one of those animals in my life!
So it it some sort of cowling/shroud - that is, a tube that the secondary is surrounded by, fixed on the vanes; with a cutaway/opening to allow the light to be reflected off the secondary and up into the focusser tube - with the part of this tube/cowling that extends towards the primary mirror holding the barlow?
And this cowling /cover/tube/shroud has an open end at the open end side of the scope tube to allow you to insert screwdrivers to adjust the central sec mirror screw and the 3 surrounding ones - but won't allow you to get your fingers/hand in to physically rotate the sec mirror?
Does this mean some sort of customized implement needs to be inserted from the open end of the scope down inside this shroud/cowling/sheath to grasp the sec mirror body so you can turn it as needed to get it circular looking? Could you make something up or use/modify some long tweezers or those kitchen tongs/snag holders etc?
Would like to see a piccy of the guts mate! Don't despair, where there's a will there's a way!!!
Just to clarify.. the star collimation test you did is the one to check collimation, not the optics (i.e. the 'airy disc') test?
But, I guess you'd know if your collimation was out or not. As I said, I'm just curious.
Hi Programmer,
Yes I focused in and out of focus on a bright star (or any star for that matter) and no, before you ask, it wasnt a binary or double star. Coming from far out of focus (focuser high in the tube) the image was a round donut with the four vanes visible which got smaller and smaller until I got closer to the focus point. At focus, or as close as I could see to it, the star appeared streaky or comet like with a "tail" off to the right and appeared like a piece of bird poo. Once I focused too far in, the star's image started to look like a thin smoke ring that kept getting bigger and bigger around the central part of where the star actually was. The problem is that smoke ring looks like it is dented on the right hand half as if it was being affected by a breeze and being pulled to the right as the focus goes further in the tube.
Additionally when focused, (only just remembered) the star or planet looks like the 3 colour pixels on an OLD colour TV where the red, blue and green images are slightly offset from each other, like when you move the individual colour channels around in Registax. It doesnt change with EP changes so I would assume this is a collimation issue with the streaky images and offset colours?
A final note, the scope was MUCH better than this before I collimated it. It may all come back to bite me on the ass.
reading up on this now. It seems from my reading that I have chromatic abberations which describe the shape and colour issues to a T. Check out this page to see what I mean about the shape and colour problems I am seeing. Maybe the adjustment of collimation has caused this. http://www.opticalres.com/gentsupp_f.html
Darryl,
Here are some pics down the focuser so you can see how bad things are at the moment, and in the front showing the secondary holder shroud and 2x barlow at the end towards the primary mirror.
Make sense now?
Chris
Last edited by Screwdriverone; 13-04-2008 at 11:41 PM.
Reason: added chromatic abberations comment
Yes Chris - rather like I described, but such that you can't even use some sort of forceps directly to get hold of the sec mirror, cos of the vane assembly that the central screw and the 3 surrounding one are on - this blocks your access!
But necessity is the mother of invention: when I speak of forceps, in the jewellery trade (which I used to ply) we had long, bent spring loaded tweezers that I reckon you could insert from the open end of the scope tube and get around the obstacle to grasp whatever the sec mirror is mounted onto, hopefully to turn/rotate it.
This may sound silly and it may well be, just throwing ideas/possibilities for you to consider or work/bounce off!
Yes, its EXACTLY as you described, now that i reread your post. I will give your idea a go, I think I have some long plastic (not as scratchy as metal ones) bent head tweezers my son had for a science kit on chemicals somewhere around here.
......I have a secondary 2x barlow built in between the Secondary mirror and the primary mirror and this is held in by a sheath that shrouds the secondary mirror and basically passes behind the secondary and stops me from putting my hand in and holding the secondary while I tighten it all up.
The reason its there is because its a catadioptric short tube newt (500mm tube with 1000mm FL) so this is probably another reason for me to take to it with a baseball bat?
OK, with that information, I change my advice. Smash it up with the sledgehammer and throw it on the fire. When you feel better, go and buy a nice 6" GSO tube unit - full length without that embedded x2 barlow!
Edit: OK, better not smash it up, but while I haven't tried to collimate one myself, there have been a number of people in here saying it was difficult if not impossible to successfully collimate a reflector with that intermediate barlow. I think that a reasonable quality tube without this barlow is readily available, quite cheaply, and that problem would then be gone. Not sure how well your EQ2 would cope with the extra weight and length?
Hoping you can get a purchase on that secondary mirror to rotate it Chris - if not, hoping you can get a purchase on another scope!!!!!
Just remember the old "where there's a will, there's a way" mantra as you struggle with it; we allways concede defeat too readily - which doesn't mean that there comes a time to do so sometimes!
And don't forget that if you can rectify it, it is a procedure that should not need to be done again. The other thought that comes to mind is with it being such a short tube, would removing the primary mirror and cell allow some manipulation from that end?
The primary is not necessary for the process in hand, and I know the shroud/cowling/tube/sheath has the barlow down the primary end, but....?
The other (quite possibly even more insane) suggestion is could you very accurately mark and cut small slits with a Dremel et al in the scope tube to allow radial adjustment/movement of the whole assembly (including the vanes) to achieve your objective?
This is most probably just before the sledgehammer process - but keep on "thinking outside the box" Screwie and something may evolve!
Is the barlow removable??. If it is, try that. It may help.
Better than a sledgehammer, goto a hobbies store, buy a couple of H-500 model rocket engines, fit a nose cone and some fins to the scope, fit the engines to the inside of the scope and try to make it the first "orbital" amateur telescope. Park it up alongside the Hubble
OK I have officially had it with this piece of crap.
I cannot for the life of me get the secondary mirror lined up under the focuser properly.
I adjust its position in the tube so that I can see a ROUND circle in the image, then tilt the secondary to get all three clips of the primary visible in the secondary mirror and the image then becomes OVAL shaped.
I back off the secondary allen key screws, adjust the position of the mirror to make it close to centre of the focuser and then when I try to tilt it again, it goes OVAL again.
I have tried Darryl's suggestion of snagging the secondary mirror and holding it in place while I adjust it up, but this doesnt work either.
For the life of me I cannot resolve it and am becoming extremely p'd off with this. Is anyone close to me available for me to bring it over and see if they can kick the tires and cast a more experienced eye over it?
What happens when you line up the secondary best you can
and by eye tighten it all up ?.. while I don't know these scopes is it possible your trying to line up your secondary to a primary whos tilt is way off ?. ..primary to secondary .. not secondary to primary.
Yup I agree with Graham Primary to secondary in the final process. Using
your Cheshire tool (sloted all the way in your focuer) just consentrate on
making the secondary mirror fit nice and evenly in the FOV of the Cheshire.
Dont worry about the clips on the primary at this stage, because your
primary might not well aligned. Aligning the secondary with the Cheshire will
automatical setup your offset. Once you have that completed (could take a
lil fiddling) then move to your primary and adjust that.
