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100/800 fuzzy image

Started by senordan, 11/26/2005 05:59PM
Posted 11/26/2005 05:59PM Opening Post
I recently purchased a used 100/800 as well as William Optic's ZS 10th year anniversary fluorite doublet. I'm impressed with both. However recently when looking at the moon at 250X I noticed that the image is fuzzy in my 100/800 while the ZSFD is sharp. What can be causing this fuzziness? When I take the scope to 400X the image becomes extremely fuzzy and dull. While the ZSFD still remains sharp.
Posted 11/26/2005 06:42PM #1
Daniel,
Just thought to pass along a little tip that I learned from Barry Simon some time ago. When ever I am browsing for a refractor the first thing I do is pop in my Takahashi Collimator (A good quality Cheshire eyepiece will work well also) and check the collimation. Although not quit as good as a star test this little procedure will generally tell you a lot about optical alignment and or collimation. Now I realize that this is not something that you can do when buying mail order it is something that you can do as soon as you receive the optic. Many high quality APO's have adjustable lens cells so often the fix is relatively simple should you wish to do it your self. If the lens cell is fixed then you need to send the scope back to the manufacturer for repair. There can also be miss aligned focuser issues which can also be determined using several methods. I use a laser myself and measure where it hits the objective. There is allot of information on the how to's of all this on the internet as I have not the time to give you a course on this via this medium. A care full search of some of the forums right here will show you numerous examples of collimation problems and the fix for them. As has already been suggested start out with a good star test and report back. Also consider buying a good quality Cheshire like the one Tektronix makes. The Takahashi is very expensive and really just makes the process a little more convenient. Some people like the Ronchi device for diag. problems as well but that is for another day.
Good luck,
Dan

Danny Halstead
Moderator APM Forum
Posted 11/29/2005 09:11AM #2

Good ideas from everyone here, check out all their suggestions.

You definitely need to look at star images at high power, like 250x. Focus back and forth on a bright star, what does the focused and defocused star look like? Circular concentric rings?

I ask because I've owned 8 TMB lenses over time. Every single one has been a superb performer under the skies, and every TMB100/8 in particular have been perfect beyond any errors my eyes or seeing conditions could ever reveal. So a dramatic problem with your lens would surprise me.

One TMB lens I owned did give me a problem. Something must have happened to this one particular lens in the hands of the "Dreaded UPS Guy" 8O because like you said, images were just kind of fuzzy. Sure, it could magnify an image, but they were never very impressive. Fuzzy.

Under high magnifications, the stars looked like the enclosed picture. Thats about 2 waves of coma error. One of the three glass elements had become decentered. The lens had to go back to be serviced and then it turned into a typical TMB performer.

Good luck with your lens. I'm going to pick up a TMB100/8 or a TMB105/6.2 as soon as the budget permits. I've got to have a little TMB again.

Mike


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