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Spherical aberration

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  • steve963
    Calling all experts. Hi Group, I m pretty much at my wits end and would like some help. I bought a TMB92SS, understanding that I was on my own when it came to
    Message 1 of 4 , Jun 12, 2012
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      Calling all experts. Hi Group, I'm pretty much at my wits end and would like some help. I bought a TMB92SS, understanding that I was on my own when it came to finding a Field Flattener. I gave the Hotech field flattener a try, and it works great for my ES 127 at F7.5, but falls woefully short for the TMB at 5.5. Rather than reinventing the wheel, has anyone here found a FF that works for this scope? I'm a little frustrated because I spent about 2K on this scope and it cant even properly illuminate an 8300 chip. I called Borg but they are sold out of anything of use and don't seem to plan on making anything in the future, and that was just for a reducer that may help. Here's a link to an image and the data. If anyone here has successfully illuminated an 8300 chip or better, I would love to hear about it. The TMB forum is a ghost town. Maybe for good reason.
      Steve
      http://www.flickr.com/photos/stephen63/7364245056/sizes/k/in/photostream/
    • Ron Wodaski
      Field flatteners are like eyeglasses: you need a prescription that fits your telescope. if the manufacturer can t give you that information ( Such and such a
      Message 2 of 4 , Jun 13, 2012
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        Field flatteners are like eyeglasses: you need a 'prescription' that fits your telescope. if the manufacturer can't give you that information ("Such and such a field flattener will work with your telescope"), then you are stuck with either trial and error or finding someone who got something to work with this telescope.

        My advice would always be to never purchase a telescope without knowing, at the very least, which field flattener and/or reducer you would use with it. Those kinds of accessories are by their nature very specific. I know that's not very encouraging news, but I need to caution you that it may turn out that either there is no flattener that will do what you want, or there may be one but it may not meet all of your requirements.

        Sorry to be the bearer of (potentially) bad news. My suggestion would be to either look into a return since the scope won't cover your camera, or spend some time trying to find a good match and then decide whether to return or keep the scope.

        Ron Wodaski



        On Jun 12, 2012, at 11:48 PM, steve963 wrote:

        > Calling all experts. Hi Group, I'm pretty much at my wits end and would like some help. I bought a TMB92SS, understanding that I was on my own when it came to finding a Field Flattener. I gave the Hotech field flattener a try, and it works great for my ES 127 at F7.5, but falls woefully short for the TMB at 5.5. Rather than reinventing the wheel, has anyone here found a FF that works for this scope? I'm a little frustrated because I spent about 2K on this scope and it cant even properly illuminate an 8300 chip. I called Borg but they are sold out of anything of use and don't seem to plan on making anything in the future, and that was just for a reducer that may help. Here's a link to an image and the data. If anyone here has successfully illuminated an 8300 chip or better, I would love to hear about it. The TMB forum is a ghost town. Maybe for good reason.
        > Steve
        > http://www.flickr.com/photos/stephen63/7364245056/sizes/k/in/photostream/
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      • Stan
        ... First - spherical aberration has nothing to do with field curvature and does not cause that problem. Second - the primary defect in that image is
        Message 3 of 4 , Jun 13, 2012
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          --- "steve963" <steve963@...> wrote:
          > ... bought a TMB92SS...
          > http://www.flickr.com/photos/stephen63/7364245056/sizes/k/in/photostream/

          First - spherical aberration has nothing to do with field curvature and does not cause that problem.

          Second - the primary defect in that image is asymmetrical coma caused by misalignment (collimation error). The scope might also exhibit field curvature with that large camera but that's not the primary defect though the effect is probably acerbated by curvature. Attaching a field flattener could make things worse because of the miss-collimation. You can prove this to yourself by trying to focus on stars in the comatic side – you will not be able to get a good star image; if the problem was only curvature then you would be able to focus the center or the periphery but not both simultaneously.

          Most people are at a complete loss when confronted by refractor collimation error because very few refractors have user accessible adjustments. Your best bet might be to send it to the manufacturer for repair (include that photo to prove miss-alignment). But if you are adventurous and like a challenge then examine the lens cell for possible ways to adjust it.

          Once you have achieved proper collimation then try your flattener again, if needed.

          Stan
        • steve963
          Thanks, Ron and Stan. Sorry for getting the terminology wrong. I looked it up and indeed I should have said field curvature. That said, there is definitely
          Message 4 of 4 , Jun 13, 2012
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            Thanks, Ron and Stan. Sorry for getting the terminology wrong. I looked it up and indeed I should have said field curvature. That said, there is definitely more elongation at one corner than the others. I also noticed a strange fan type pattern in that corner when taking flats. At first I thought it was the shutter but it shows up even in flats as long as 15 secs. The scope has collimation adjustment screws, but as you stated, Im among those people at a total loss as to how to make those adjustments. I'll try to find some references on how to go about doing that. But, I still believe that my flattener will still show a lot of curvature after collimating the scope and would like to hear what others have used for this scope.
            Thanks for the quick responses.

            --- In ccd-newastro@yahoogroups.com, "steve963" <steve963@...> wrote:
            >
            > Calling all experts. Hi Group, I'm pretty much at my wits end and would like some help. I bought a TMB92SS, understanding that I was on my own when it came to finding a Field Flattener. I gave the Hotech field flattener a try, and it works great for my ES 127 at F7.5, but falls woefully short for the TMB at 5.5. Rather than reinventing the wheel, has anyone here found a FF that works for this scope? I'm a little frustrated because I spent about 2K on this scope and it cant even properly illuminate an 8300 chip. I called Borg but they are sold out of anything of use and don't seem to plan on making anything in the future, and that was just for a reducer that may help. Here's a link to an image and the data. If anyone here has successfully illuminated an 8300 chip or better, I would love to hear about it. The TMB forum is a ghost town. Maybe for good reason.
            > Steve
            > http://www.flickr.com/photos/stephen63/7364245056/sizes/k/in/photostream/
            >
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