I recently purchased a used 4" 500mm Celestron spotting scope. I wanted a second, smaller, quick peek/travel scope and had seen several people say this worked fine as a astro scope. There is nothing wrong with it, but I have one problem. I can't reach focus with the 2" eyepiece I tried on it, a 26mm GSO kellner. I do not yet have a 2" diagonal, so I put it straight in. The focuser does not go far enough in to reach focus, so it is not an issue of an extension tube or anything. I also have a Meade 32mm series 5000 2", but have been hesitant to put that in the focuser until I get a diagonal, because the focuser is sans compression ring. So, my questions are- 1. Will the 32mm have the same problem? 2. Will a 2" diagonal correct the problem, or would it be a waste of money? Thanks- Matt
Celestron 102mm focus problem
Started by rednavy5, 04/08/2009 03:20AM
Posted 04/08/2009 03:20AM
Opening Post
Posted 04/08/2009 03:34AM
#1
Matt: I am quite sure that the problem is that you do need to use a diagonal or an extension tube to reach focus. It is quite normal that a refractor will not reach focus unless there is either an extension tube or diagonal.
The easiest way to see if this is the case is to do just as you have done but remove the eyepiece from the focuser and move away while still centered aligned with the objective, you should reach focus.
Jon
The easiest way to see if this is the case is to do just as you have done but remove the eyepiece from the focuser and move away while still centered aligned with the objective, you should reach focus.
Jon
Posted 04/08/2009 04:49AM
#2
Hi Matthew
Can you get focus with a 1.25" diagonal and 1.25" eyepieces?
The focal plane of the 2" EP's I have (not many) appear to be near the field stop, which is near the shoulder of the EP's. If you are truly lacking in-travel straight-thru without any kind of diagonal, then it seems unlikely that 1.25" eyepieces with a diagonal could ever reach focus, as that would move the EP focal plane back another couple of inches at least?
Am not competent to be a mr-know-it-all on the matter. It is just an ignorant observation. Please take my comments with many grains of salt.
1. Is there any chance the previous owner upgraded with a nicer replacement focuser which accidentally happens to be a lot longer than the factory focuser?
From viewing pictures of your guy, I think the factory focuser on your C102 500mm typically looks about the same as the factory focuser on my C102 650mm-- A cast-aluminum cone shape with a rack'n'pinion focus mechanism.
If you happen to have some other shape focuser on your scope, a Crayford or whatever, then perhaps the focuser has been swapped out with something a little too long? That would not necessarily be a bad thing, except for adjusting for the length change.
2. I've been very happy with my C102 650mm FL for the money. Mine was straight out of the new box with no mods. It came with the 2" rack'n'pinion focuser and a 1.25" not-too-bad mirror diagonal and a couple of basic eyepieces.
My C102 would bring all my 1.25" eyepieces to focus with the 1.25" diagonal, but had ordered a 2" dielectric diagonal along with the scope. The 2" diagonal added another inch or two to the light path, and there were several 1.25" and 2" EP's that had insufficient in-focus with the 2" diagonal. I griped quite awhile that for whatever reason they put a 2" focuser on that thing, but cut the tube about an inch too long to play well with a 2" diagonal.
A lot of folks, with various brands of refractors, will decide to shorten the tube for one reason or t'other, but I wasn't wild about the idea of hacksawing a brand-new scope. Am too inclined to tinker, and wanted to put a crayford focuser on the C102 anyway.
Rather than buy the factory crayford adapter for that model scope (which would only duplicate the too-long factory focuser position and require cutting the tube anyway), I made a little aluminum adapter dimensioned to move the new crayford about 1" closer to the objective. I did some careful measuring of what in-focus would be required on all the EP's before deciding the ideal length of the new adapter.
====
Anyway, my experience proves that it isn't impossible for Celestron to make the tubes a slightly-wrong length. But in my case, all of my EP's would have easy-focused without any diagonal at all, unless for some EP's there may have not been sufficient OUT TRAVEL without a diagonal (though the factory focuser has a long throw, and there probably would have been sufficient out-travel). I was only an inch or two long in the special case of a 2" diagonal.
You would want to make CERTAIN of your problem, and how much to remove, before taking a hacksaw to your scope, but it isn't that onerous a job.
Can you get focus with a 1.25" diagonal and 1.25" eyepieces?
The focal plane of the 2" EP's I have (not many) appear to be near the field stop, which is near the shoulder of the EP's. If you are truly lacking in-travel straight-thru without any kind of diagonal, then it seems unlikely that 1.25" eyepieces with a diagonal could ever reach focus, as that would move the EP focal plane back another couple of inches at least?
Am not competent to be a mr-know-it-all on the matter. It is just an ignorant observation. Please take my comments with many grains of salt.
1. Is there any chance the previous owner upgraded with a nicer replacement focuser which accidentally happens to be a lot longer than the factory focuser?
From viewing pictures of your guy, I think the factory focuser on your C102 500mm typically looks about the same as the factory focuser on my C102 650mm-- A cast-aluminum cone shape with a rack'n'pinion focus mechanism.
If you happen to have some other shape focuser on your scope, a Crayford or whatever, then perhaps the focuser has been swapped out with something a little too long? That would not necessarily be a bad thing, except for adjusting for the length change.
2. I've been very happy with my C102 650mm FL for the money. Mine was straight out of the new box with no mods. It came with the 2" rack'n'pinion focuser and a 1.25" not-too-bad mirror diagonal and a couple of basic eyepieces.
My C102 would bring all my 1.25" eyepieces to focus with the 1.25" diagonal, but had ordered a 2" dielectric diagonal along with the scope. The 2" diagonal added another inch or two to the light path, and there were several 1.25" and 2" EP's that had insufficient in-focus with the 2" diagonal. I griped quite awhile that for whatever reason they put a 2" focuser on that thing, but cut the tube about an inch too long to play well with a 2" diagonal.
A lot of folks, with various brands of refractors, will decide to shorten the tube for one reason or t'other, but I wasn't wild about the idea of hacksawing a brand-new scope. Am too inclined to tinker, and wanted to put a crayford focuser on the C102 anyway.
Rather than buy the factory crayford adapter for that model scope (which would only duplicate the too-long factory focuser position and require cutting the tube anyway), I made a little aluminum adapter dimensioned to move the new crayford about 1" closer to the objective. I did some careful measuring of what in-focus would be required on all the EP's before deciding the ideal length of the new adapter.
====
Anyway, my experience proves that it isn't impossible for Celestron to make the tubes a slightly-wrong length. But in my case, all of my EP's would have easy-focused without any diagonal at all, unless for some EP's there may have not been sufficient OUT TRAVEL without a diagonal (though the factory focuser has a long throw, and there probably would have been sufficient out-travel). I was only an inch or two long in the special case of a 2" diagonal.
You would want to make CERTAIN of your problem, and how much to remove, before taking a hacksaw to your scope, but it isn't that onerous a job.
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