I received the laser collimator and the Cheshire collimator a couple of days ago. Both were in excellent shape and well worth the money I paid for them.
Now - the bad news...
I used the laser collimator to check the initial alignment of the Primary and Secondary mirrors and found out the eye piece focuser assembly is not aligned properly with the optical path within the telescope tube. Seems time, heat and humidity have conspired to change the cardboard tube of the telescope to offset the focuser towards one side of the scope tube - now I have to figure out how to correct this offset with the current focuser (which is not the best in the world) or purchase a better focuser for the beast! I may opt for the later. It appears the focuser is not perpendicular to the secondary mirror any longer - about 2-3 degrees off which is more than enough to cause alignment issues! For you to get the best images from a telescope you MUST have the optical path aligned pretty much DNO (dead nuts on).
A second observation - it looks like the primary mirror is "shifted" to towards one side of the telescope tube. The interesting part is the mirror is mounted to a particleboard plate using RTV which does not normal do such things so I must have offset mounted the mirror when the mirror was re-coated. DUH - this will play havac with not only the optical alignment but also the visual observations through the telescope in terms of secondary mirror refraction (not centered will cause side-ghosts - duplicate images that can not be focused to a single image, which is what I am observing in the scope now).
Not only did the laser collimator "show" these problems the Cheshire collimator confirmed the problems as I can not get the alignment correct without being "offset" in the secondary mirror images to get the Cheshire images "close" to where they should be - and I observe one "side" of the telescope tube where I would normally not see the tube side (primary mirror offset is causing this).
Before I can align this beast I have to fix the above listed issues...
Now I have to decide if I want to continue using the current Dobsonian-style mount or construct a different one - decisions, decisions!
More to follow - stay tuned!
Thursday, November 13, 2008
Sunday, November 2, 2008
Telescope Mirror Alignment
Alignment of the telescope's optics...
All Newtonian telescopes require alignment of the Primary and Secondary mirrors to the optical path for the best resolution of the image. Wow - that is sure a "simplistic" sentence which contains all sorts of implications! There are over six (6) variables I can identify which affect the alignment of the telescopes optics. This means there are six different adjustments, some of which interplay with each other, that directly affect the alignment of the optical path within the telescope.
Most of the instructions I have found to perform the alignment mention one of two popular methods. Several of the instructions actually mention using both methods to insure a proper alignment of the optics within the telescope. These methods are Laser collimation and Cheshire collimation. As it's name implies the Laser collimation method utilizes a laser beam to align the optics within the telescope. You place the laser collimator device in the eyepiece holder on the telescope then adjust the secondary mirror so the laser beam is striking directly in the center of the primarey mirror - then you adjust the primary mirror so the laser beam is reflected back to the eyepiece holder by way of the secondary mirror with the reflected laser beam striking the laster beam source. Sounds easy enough - you just have to "have" a laser collimator.
Building a laser collimator - or - Efforts in Total Frustration!
One would think building a laser collimator would be a simple thing to do - get a laser pointer and mount it in something to hold it within the eyepiece mount then align the telescope mirror....
Well....
Not so simple after all!
I have three laser pointers, two are the type used for presentations and one is built into a LED flashlight. After playing with all three I have determined NONE of them are aligned with the housing containing the laser diode! Normally, you would not notice such a discrepancy since you are using the pointer to point to something and you adjust for the "error" visually while highlighting whatever it is you are highlighting. This is NOT the case when you are using the laser pointer as a reference itself.
Compounding the issue is the pointer casing is much smaller in diameter than the telescope's 1.25-inch optics mount tube so you have to come up with "something" to not only mount the laser pointer in, but also make it adjustable to "correct" for the mis-alignment of the laser diode's mounting within the pointer housing itself. After checking hardware stores I have determined there is really nothing available for use as a mounting tube of the correct diameter. The closest I came was a piece of PVC tubing for 1-inch coupling - it is about 2 or 3 millimeters larger than the optics tube on the telescope but I do not have anything to turn the diameter down and maintain the parallelism of the tube so that is not a choice either. I can hear others saying "use a drill press and sandpaper"! That may or may not work! You really NEED to maintain the critical diameter so the laser collimator tube fits correctly within the optics mounting tube or you will introduce an error and you will not get the optical path collimated correctly - this manifests itself as a "smeared" image that will not focus correctly! I learned that the hard way!
My final solution? I ordered a laser collimator from a website on the web (grin) so as to remove all the headaches associated with attempts to fabricate one! Money well spent since I will need to collimate the telescope each time I move it around, which is every time I use it, and I have a second six (6) inch Newtonian telescope that will need the same collimation as well.
Cheshire Collimation Tool...
Well - after a couple of days attempting to build a Telescope Optics Alignment Tool and using it to align the primary and secondary mirrors I have come to the conclusion I do not have the required tooling to build such a device. On the surface it appears to be a very easy-to-construct device, cross-hair in one end, a hole in the side with a 45-deg reflector piece (that also has a hole in the center) and an end-cap with a hole in the center. I am afraid a hand drill just is not stable enough to drill the required holes and I have not been able to find anything that is 1.25-inches in diameter to fit correctly within the telescopes optics tube (the same problem as described earlier)!
Again - my "solution" was simple - Throw some money at the problem! I went on E-Bay and found a Cheshire collimator there for a very reasonable price and have placed a bid on it. Hopefully I will Win the bid and get the collimator, and if not, I will order one from the same location I purchased the laser collimator (grin).
Now I wait for the collimator hardware to arrive!!!
More to follow...
gm...
All Newtonian telescopes require alignment of the Primary and Secondary mirrors to the optical path for the best resolution of the image. Wow - that is sure a "simplistic" sentence which contains all sorts of implications! There are over six (6) variables I can identify which affect the alignment of the telescopes optics. This means there are six different adjustments, some of which interplay with each other, that directly affect the alignment of the optical path within the telescope.
Most of the instructions I have found to perform the alignment mention one of two popular methods. Several of the instructions actually mention using both methods to insure a proper alignment of the optics within the telescope. These methods are Laser collimation and Cheshire collimation. As it's name implies the Laser collimation method utilizes a laser beam to align the optics within the telescope. You place the laser collimator device in the eyepiece holder on the telescope then adjust the secondary mirror so the laser beam is striking directly in the center of the primarey mirror - then you adjust the primary mirror so the laser beam is reflected back to the eyepiece holder by way of the secondary mirror with the reflected laser beam striking the laster beam source. Sounds easy enough - you just have to "have" a laser collimator.
Building a laser collimator - or - Efforts in Total Frustration!
One would think building a laser collimator would be a simple thing to do - get a laser pointer and mount it in something to hold it within the eyepiece mount then align the telescope mirror....
Well....
Not so simple after all!
I have three laser pointers, two are the type used for presentations and one is built into a LED flashlight. After playing with all three I have determined NONE of them are aligned with the housing containing the laser diode! Normally, you would not notice such a discrepancy since you are using the pointer to point to something and you adjust for the "error" visually while highlighting whatever it is you are highlighting. This is NOT the case when you are using the laser pointer as a reference itself.
Compounding the issue is the pointer casing is much smaller in diameter than the telescope's 1.25-inch optics mount tube so you have to come up with "something" to not only mount the laser pointer in, but also make it adjustable to "correct" for the mis-alignment of the laser diode's mounting within the pointer housing itself. After checking hardware stores I have determined there is really nothing available for use as a mounting tube of the correct diameter. The closest I came was a piece of PVC tubing for 1-inch coupling - it is about 2 or 3 millimeters larger than the optics tube on the telescope but I do not have anything to turn the diameter down and maintain the parallelism of the tube so that is not a choice either. I can hear others saying "use a drill press and sandpaper"! That may or may not work! You really NEED to maintain the critical diameter so the laser collimator tube fits correctly within the optics mounting tube or you will introduce an error and you will not get the optical path collimated correctly - this manifests itself as a "smeared" image that will not focus correctly! I learned that the hard way!
My final solution? I ordered a laser collimator from a website on the web (grin) so as to remove all the headaches associated with attempts to fabricate one! Money well spent since I will need to collimate the telescope each time I move it around, which is every time I use it, and I have a second six (6) inch Newtonian telescope that will need the same collimation as well.
Cheshire Collimation Tool...
Well - after a couple of days attempting to build a Telescope Optics Alignment Tool and using it to align the primary and secondary mirrors I have come to the conclusion I do not have the required tooling to build such a device. On the surface it appears to be a very easy-to-construct device, cross-hair in one end, a hole in the side with a 45-deg reflector piece (that also has a hole in the center) and an end-cap with a hole in the center. I am afraid a hand drill just is not stable enough to drill the required holes and I have not been able to find anything that is 1.25-inches in diameter to fit correctly within the telescopes optics tube (the same problem as described earlier)!
Again - my "solution" was simple - Throw some money at the problem! I went on E-Bay and found a Cheshire collimator there for a very reasonable price and have placed a bid on it. Hopefully I will Win the bid and get the collimator, and if not, I will order one from the same location I purchased the laser collimator (grin).
Now I wait for the collimator hardware to arrive!!!
More to follow...
gm...
Subscribe to:
Posts (Atom)
Posts
