pmrid
10-05-2014, 04:53 PM
I have a new (used) GSO RC8 that I'm trying to get right and have been approaching it with 3 different tools. I thought I might make a few notes here in the hope others may find them useful.
The 3 methods I'm using are:
1. A plain Cheshire;
2. A Takahashi Collimating scope;
3. A Howie Glatter laser with circular hologram attachment.
I'd like to be able to say one of these methods has produced the required fine collimation. But that isn't the case. I rapidly concede the problem could be operator-error. Like computer trouble, 99% of the problem lies between the seat and the keyboard. The same (suitably modified) is true of telescopes.
The Cheshire is a non-optical device so what you see is what you get. It presents a non-magnified view so its usefulness depends as much on good eyesight as anything else and since my eyes are not good, I felt I needed a bit more help. I think that once I get the scope collimated, a quick check with a Cheshire will be all I need. But getting it there seemed to require something more for me.
So I bought a Tak collimating scope. And I watched a few YouTubes and read the guide on http://www.dosborne-astroimages.co.uk/equip_image/collimation_article.htm which was clear and helpful. Starting with the secondary and finishing with the primary, the result seemed to be bang-on. But my images still showed distinct elongation NE-SW. I began to suspect tilt in the focuser/filterwheel/CCD and therefore ordered a collimating tilt-plate from Bintel. Getting the focuser pointing directly at the centre-spot on the secondary made sense and here was a way to do that.
At the same time as waiting for the tilt-plate to arrive, I happened on the blog on collimating an RC8 at http://steveastro.blogspot.com.au/2013/05/collimating-my-gs0-rc8-telescope.html and that led me directly to the third method. Steve referred to the Cheshire EP method as well but my main interest was in the use of the Howie Glatter laser. That method is a 3-step process:
1. Use your tilt-plate to centre the focuser on the secondary. To do this you have to stand on your head and peer around corners almost in order to see down the throat of the scope to the reflection of the secondary centre-spot. You can do it but it's a real contortionists delight. Using the adjustments on the tilt-plate you use the small single spot from the Glatter and get it in the middle of that refleced spot. OK. Not easy but do-able.
2. This one is difficult. You put the circular hologram adapter on the Glatter and fir it up. It throws out a series of concentric rings which hit the secondary, are reflected back onto the primary and finally out towards you standing at the front. The trick with this step is to be able to block out the bright centre-spot on the Glatter and concentrate JUST on the reflected circles on the surface of the primary. It helps if your mirror is dusty so there is something thjat will show up where the rings are. The object of the exercise is to get those concentric circles centric with the primary by using the adjustments on the secondary. I found it very difficult to complete this step with any confidence. The maze of reflections coming out towards me was a jumble and it took quite a lot of effort to position yourself well enough to get it done.
Step 3 was to then project the same holgram onto a plat wall or similar a few feet away. Keep the scope level and at right angles to that surface so there are no distortions. The concentric circles a clear enough but it's not clear to me what I ought to trying to achieve. The innermost ring (others of smaller diameter are blocked by the secondary shroud) was incomplete as was the outermost ring. No amount of adjusting the primary was able to expose both rings fully so I contented myself with getting the rings seemingly of good shape and properly centered. I measured them to see that they were the same on all sides. Not sure what else to do.
NOW the result of all that was that the clouds rolled in and have stayed for the past few days and promise to stay for longer. As usual. So I don't know if I have succeeded or not.
However, I suspect I have not because I put the Cheshire and the Tak both back in the scope to see whether they all agreed. They didn't. The result of the Glatter method was that the primary was well out of whack.
I'm sure others have tried these and other methods and my guess is there are quite a few who would like to get this right. So feel free to tell me what I'm doing wrong and lets see if we can find a workable answer.
EDIT: I've solved the problem of Step 2. Use a compass and felt pen (or similar) and scribe a circle equal to the diameter of the primary - in my case 204mm. Also scribe circles at about 20mm intervals inwards. Then, using a pair of calipers carefully measure the diameter of the baffle tube that comes out of the primary and scribe a circle about 1 mm larger in diameter on the same sheet of paper. The tube is wider at the base than the opening end. I made the diameter 67 mm and that seems about right. Cut out the centre hole and also cut out the outer line so you end up with a fat donut the same size as your primary with a hole in the middle the same size as the baffle tube. Use paper that is a little stiff so it will slip down over the baffle tube and sit on the primary lightly. That becomes a screen onto which the hologram can be projected from the secondary and without the distraction of the reflections off the primary and also giving you a clear guide for the orientation of the secondary mirror using the secondary collimation grub screws. When you're done, remove the paper and proceed to step 3. A HINT, attach a bit of sticky take firmly to the paper with some cotton thread or similar into it so it is not too hard to pull the paper ring back out when this step is finished.
I've now gone through the process again using the īmproved" Step 2 method above. As long as the centre circle for the baffle tube is cut right and the paper ring does not slump, it is fine. The rings project back clearly and it isn't at all hard to get them concentric with the baffle tube.
The third step remains a bit unclear. I projected onto a white board 3 feet away and the rings were clear, complete and of equal width - my best assumption was that the scope was now collimated. The problem is thjat the actual shadow of the secondary assembly does not show up distinctly and you have to intuit it somewhat.
BUT, the problem was that the Tak scope did not agree with the Glatter. It still placed the primary as being a half-turn off in 2 points. So we have quite a disconnect between these 2 methods.
Until I get some sky I won't be able to tell you which one is best.
peter
The 3 methods I'm using are:
1. A plain Cheshire;
2. A Takahashi Collimating scope;
3. A Howie Glatter laser with circular hologram attachment.
I'd like to be able to say one of these methods has produced the required fine collimation. But that isn't the case. I rapidly concede the problem could be operator-error. Like computer trouble, 99% of the problem lies between the seat and the keyboard. The same (suitably modified) is true of telescopes.
The Cheshire is a non-optical device so what you see is what you get. It presents a non-magnified view so its usefulness depends as much on good eyesight as anything else and since my eyes are not good, I felt I needed a bit more help. I think that once I get the scope collimated, a quick check with a Cheshire will be all I need. But getting it there seemed to require something more for me.
So I bought a Tak collimating scope. And I watched a few YouTubes and read the guide on http://www.dosborne-astroimages.co.uk/equip_image/collimation_article.htm which was clear and helpful. Starting with the secondary and finishing with the primary, the result seemed to be bang-on. But my images still showed distinct elongation NE-SW. I began to suspect tilt in the focuser/filterwheel/CCD and therefore ordered a collimating tilt-plate from Bintel. Getting the focuser pointing directly at the centre-spot on the secondary made sense and here was a way to do that.
At the same time as waiting for the tilt-plate to arrive, I happened on the blog on collimating an RC8 at http://steveastro.blogspot.com.au/2013/05/collimating-my-gs0-rc8-telescope.html and that led me directly to the third method. Steve referred to the Cheshire EP method as well but my main interest was in the use of the Howie Glatter laser. That method is a 3-step process:
1. Use your tilt-plate to centre the focuser on the secondary. To do this you have to stand on your head and peer around corners almost in order to see down the throat of the scope to the reflection of the secondary centre-spot. You can do it but it's a real contortionists delight. Using the adjustments on the tilt-plate you use the small single spot from the Glatter and get it in the middle of that refleced spot. OK. Not easy but do-able.
2. This one is difficult. You put the circular hologram adapter on the Glatter and fir it up. It throws out a series of concentric rings which hit the secondary, are reflected back onto the primary and finally out towards you standing at the front. The trick with this step is to be able to block out the bright centre-spot on the Glatter and concentrate JUST on the reflected circles on the surface of the primary. It helps if your mirror is dusty so there is something thjat will show up where the rings are. The object of the exercise is to get those concentric circles centric with the primary by using the adjustments on the secondary. I found it very difficult to complete this step with any confidence. The maze of reflections coming out towards me was a jumble and it took quite a lot of effort to position yourself well enough to get it done.
Step 3 was to then project the same holgram onto a plat wall or similar a few feet away. Keep the scope level and at right angles to that surface so there are no distortions. The concentric circles a clear enough but it's not clear to me what I ought to trying to achieve. The innermost ring (others of smaller diameter are blocked by the secondary shroud) was incomplete as was the outermost ring. No amount of adjusting the primary was able to expose both rings fully so I contented myself with getting the rings seemingly of good shape and properly centered. I measured them to see that they were the same on all sides. Not sure what else to do.
NOW the result of all that was that the clouds rolled in and have stayed for the past few days and promise to stay for longer. As usual. So I don't know if I have succeeded or not.
However, I suspect I have not because I put the Cheshire and the Tak both back in the scope to see whether they all agreed. They didn't. The result of the Glatter method was that the primary was well out of whack.
I'm sure others have tried these and other methods and my guess is there are quite a few who would like to get this right. So feel free to tell me what I'm doing wrong and lets see if we can find a workable answer.
EDIT: I've solved the problem of Step 2. Use a compass and felt pen (or similar) and scribe a circle equal to the diameter of the primary - in my case 204mm. Also scribe circles at about 20mm intervals inwards. Then, using a pair of calipers carefully measure the diameter of the baffle tube that comes out of the primary and scribe a circle about 1 mm larger in diameter on the same sheet of paper. The tube is wider at the base than the opening end. I made the diameter 67 mm and that seems about right. Cut out the centre hole and also cut out the outer line so you end up with a fat donut the same size as your primary with a hole in the middle the same size as the baffle tube. Use paper that is a little stiff so it will slip down over the baffle tube and sit on the primary lightly. That becomes a screen onto which the hologram can be projected from the secondary and without the distraction of the reflections off the primary and also giving you a clear guide for the orientation of the secondary mirror using the secondary collimation grub screws. When you're done, remove the paper and proceed to step 3. A HINT, attach a bit of sticky take firmly to the paper with some cotton thread or similar into it so it is not too hard to pull the paper ring back out when this step is finished.
I've now gone through the process again using the īmproved" Step 2 method above. As long as the centre circle for the baffle tube is cut right and the paper ring does not slump, it is fine. The rings project back clearly and it isn't at all hard to get them concentric with the baffle tube.
The third step remains a bit unclear. I projected onto a white board 3 feet away and the rings were clear, complete and of equal width - my best assumption was that the scope was now collimated. The problem is thjat the actual shadow of the secondary assembly does not show up distinctly and you have to intuit it somewhat.
BUT, the problem was that the Tak scope did not agree with the Glatter. It still placed the primary as being a half-turn off in 2 points. So we have quite a disconnect between these 2 methods.
Until I get some sky I won't be able to tell you which one is best.
peter