Page 1 of 2 12 LastLast
Results 1 to 10 of 15
  1. #1
    Fred's Avatar
    Fred Guest

    Default New to refractors



    I just received my third scope. An Apogee widestar 90. I also have a
    15" dob I built myself and a Meade 4.5" Newt with a goto that never
    quite gets there.
    I bought the Apogee because it was cheap, I did not care about
    chromatic aberration which I expect form it, and I wanted something in
    a EQ mount for a change, so that I could be sure how the sky looks
    like around hard to find DSOs. Trusting myself to get the coordinates
    right rather than a faulty goto or estimating star hopping distances.
    In fact the mount is very impressive. But the scope, it has something
    much worse than chromatic aberration, which I have not notice yet.
    Stars are not stars, they are comets!
    I know how to fix this on my newts. But I have never seen coma so bad.
    There is no center marked mirror. There are no collimation knobs. How
    do I go about getting rid of this coma????

    Please help!

  2. #2
    Frank Bov's Avatar
    Frank Bov Guest

    Default New to refractors

    Fred,
    You're trying to do the same thing as a Newtonian, but you have different
    parts. First, make sure both elements of the doublet are aligned properly.
    Cells get loose, and worse yet, they can be over tightened, pinching the
    optics. If the cell is snug and the element spacers are properly seated, you
    need to adjust the cell in the tube.

    Put the lens cap on, and in daylight, look in the empty focuser. You should
    see concentric annular reflections, like a star test only much larger. Each
    surface has a defocused reflection of the focuser and they should all be
    concentric. Loosen the cell and tweak it so the rings are concentric. If
    some are but others aren't, go back to the first step and make sure the
    elements are seated properly and spaced uniformly. If you get close, but not
    quite there, try the same process with the focuser.

    If none of this works, talk with Apogee about a replacement.

    HAve fun,
    Frank, who's been very luck with his refractors to date.

    "Fred" <tackium@yahoo.com> wrote in message
    news:ae44bc9f.0312121848.441c5bf2@posting.google.c om...



  3. #3
    David Randell's Avatar
    David Randell Guest

    Default New to refractors

    "Fred" <tackium@yahoo.com> wrote in message
    news:ae44bc9f.0312121848.441c5bf2@posting.google.c om...

    [snip]...



    Hi Fred,

    If the Widestar comes with an adjustable objective glass (OG) cell (which I
    doubt) then you may be able to reduce the "comets" if its the result of
    miscollimation. Other than that, some adjustment may be available on the
    focusser end - but this is only a cure-all if the focusser itself is not
    square on. Having taken apart several Synta-clone f/5 'fast' refractors
    trying to eliminate mis-collimation, decentred lens elements, and focussers
    not squared on to the tube, I know correcting these aberrations is not
    necessarily easy. But saying this its important to realise that unless you
    are going to invest in a TMB or other high-end 'fast' refractor, you should
    expect not get pristine images from what is basically a RFT scope.

    For diagnosing the problem, I would first check out the diagonal for a
    source of astigmatism. The easiest way is to remove this and place it in the
    path of a small scope having an excellent star-test on an artificial star.
    Astigmatism from a less than flat mirror will soon become apparent. If the
    Widestar has a 45-degree erect image diagonal - replace it with a 90-degree
    diagonal, which tend to be much better quality. Then for general gross
    miscollimation errors, try using a Cheshire eyepiece - expect here you cap
    the OG, and shine a light into the Cheshire to see a set of reflected
    annular rings bouncing of the lens elements surfaces. If all the rings are
    concentric the collimation is pretty good. Decentred lens elements in these
    scopes are relatively easy to spot, but a pig to cure. A reference book on
    this stuff is Sidgwick's: Amateur Astronomers Handbook.

    Having said that, many of these scopes are intended for low to medium power
    observing only. Also its worth remembering that achieving perfect
    collimation with a 'fast' refractor will tend to be much more difficult than
    with its long-focus counterpart.

    Hope this helps,
    Dave Randell



  4. #4
    Jon Isaacs's Avatar
    Jon Isaacs Guest

    Default New to refractors

    >There are no collimation knobs. How

    Others have suggested some good techniques to check the collimation.

    First some questions:

    1. Does this scope have a plastic focuser??

    2. Are you seeing coma across the entire field, the center included or just at
    the edges?

    3. Have you tried the scope with no diagonal? if you still have the 45 degree
    erect image diagonal in the optical path, remove it and test the scope again.

    And regardless of whether the Coma disappears, get yourself a 90 degree star
    diagonal, those 45 degree erect image diagonals are pretty poor and definitely
    degrade the image at higher powers. And some of them also can cause
    vignetting at lower powers.

    Stick you laser collimator in the focuser and see how the alignment looks, you
    may find the focuser is not square with the OTA.

    THe scopes are pretty basic, a 90mm scope with an EQ mount for $150 is quite
    inexpensive. But with a bit of work, you should be able to get some decent
    results.

    I purchased the previous incarnation from Apogee Inc, the GanyMeade 185, a 80mm
    F5 scope on a photo type tripod for $99.

    I reworked the focuser, loosened up the objective retaining ring a bit and I
    have been able to do quite well with it. It will split the double-double on a
    regular basis and I was able to see a shadow transit as well as the cassini
    division. Those were one time only near perfect seeing but it does work. Not a
    scope for a perfectionist by any means but an nice scope for a tinkerer with
    realistic expectations.

    So hang in there and do give Apogee a call, they are reasonable folks in my
    experience.

    jon isaacs





  5. #5
    Chuck's Avatar
    Chuck Guest

    Default New to refractors

    return it and buy an Orion 80ED...

    "Fred" <tackium@yahoo.com> wrote in message
    news:ae44bc9f.0312121848.441c5bf2@posting.google.c om...



  6. #6
    Fred's Avatar
    Fred Guest

    Default New to refractors

    Thanks for your help, Frank and Dave. I'll ask my questions below your
    avice:

    " First, make sure both elements of the doublet are aligned properly."
    How?
    "...you
    need to adjust the cell in the tube."
    How?

    "...Loosen the cell and tweak it so the rings are concentric."

    How?

    " If none of this works, talk with Apogee about a replacement."

    I might have to do that. I think this is a particularly bad one. But
    until they open Monday I want to try and save it.

    HAve fun,
    Frank, who's been very luck with his refractors to date.

    Thanks Frank.


    How can I tell if it is adjustable or not?

    Other than that, some adjustment may be available on the

    The focuser seem to be attached with three screws located not in the
    focuser but in the scope tube. I expect part of the focuser housing
    extends inside the OTA to where the screws are. loosening onbe or two
    of them don't seem to havce any effect.

    Having taken apart several Synta-clone f/5 'fast' refractors

    I was expecting some chromatic aberration but not this.


    It came with two diagonals. A 45 degree "erecting" one and an
    additional 90 degree which I ordered with the scope. The 90 degree
    feels and look of much better quality, but the same problem exists
    with both. So the telescope is highly suspect.


    I'm about to do this as I'm writing and get back to you.

    1.If they are concentric, is the only other possible source of comma
    the diagonal, or a badly shaped lens is likely too?
    2. If they are not concentric, what is the part I need to adjust? Is
    it by turning, pulling, pushing how and what?


    Stars are simply too bad at 40x and 55x. I'm quite sure what I'm
    seeing here is worse than normal. Even at 20x the defocused blob is
    shinier on one side, though it does look round.

    I will be able to diagnose it with the Chesire for sure (Why I didn't
    think of that?) but I still have no idea how to fix it...

    Thanks Dave,

    Fred.

  7. #7
    Fred's Avatar
    Fred Guest

    Default New to refractors

    One more question:

    If it so happens that it is both diagonals doing this, how could I adjust them?
    Thanks again
    Fred.

  8. #8
    Fred's Avatar
    Fred Guest

    Default New to refractors->Chesire results

    OK I can see three reflections clearly. Two large ones and a tiny far
    off one. The large ones form a figure 8 with the tiny one right in the
    middle. Pushing the focuser tube up with my finger succeds in
    superimposing them concentricly. In fact I turned the scope
    upsidedown, with the focuser knobs on top and the finder below, and
    the weight of the chesire alone fixes the rings. I somehow don't think
    this is the problem. The coma is much worse than this would indicate
    in my experience with my f4.5 Dob. Plus I did push the focuser up last
    night thinking presicely that it might me hanging a little, and I
    noticed no variation on the coma.

  9. #9
    Frank Bov's Avatar
    Frank Bov Guest

    Default New to refractors->Chesire results

    Fred,
    "How" is by loosening whatever screws you've got and moving the cell /
    focuser with respect to the tube until things look centered. But this
    assumes the elements are co-axial . . .

    Using a Cheshire is a good idea. If you think about it, you should have one
    surface that's concave toward you (first surface, first element), then two
    that are convex (second surface, first element and first surface, second
    element), then one that's flat (or maybe convex, I forget). Concave surfaces
    will make the Cheshire ring smaller while convex surfaces will make them
    larger.

    The tiny one in the middle sounds like the front surface is correct. The
    large ones forming a figure 8 means the spacing between the elements is not
    equal all the way around. Turning the scope so the optical axis is vertical
    and the primary is down would use gravity to bring the two elements
    together - it sounds like your lens is loose! Does it clunk if you shake it?

    In my ST80, the primary is held in a plastic cell with a screw-on front
    ring. I keep the ring a little snug, risking a triangular star pattern from
    pinched optics, until I'm ready to observe. Then I loosen it so it just
    touches but the elements are not able to move. I also once disassembled the
    doublet and forgot to mark their relative orientation. I had to experiment
    (the spacers left 3 glue marks) to find the right rotational orientation,
    but once I found it, star images again became classical Airy disks.

    Hope this helps,
    Frank

    "Fred" <tackium@yahoo.com> wrote in message
    news:ae44bc9f.0312131108.38e081cd@posting.google.c om...



  10. #10
    Fred's Avatar
    Fred Guest

    Default New to refractors->Chesire results

    "Frank Bov" <frankbov@rochester.rr.comic> wrote in message news:<amMCb.11390$JW3.7095@twister.nyroc.rr.com>.. .
    it sounds like your lens is loose! Does it clunk if you shake it?
    No, On the contrary it appears to be way too tight:


    This is what I find too. However the ring is either glued in place or
    screwed on incredibly tight. I'm unable to move it. I'm tempted to
    apply some heat to see if the plastic expands a little and the thing
    loosens up. I'm worried it might affect the lens coatings though. What
    do you think?

 

 
Page 1 of 2 12 LastLast

Similar Threads

  1. AP Refractors
    By Ed Majden in forum Amateur Astronomy Forum
    Replies: 17
    Last Post: 06-29-2006, 06:40 AM
  2. Refractors vs. binoculars
    By Tom Rauschenbach in forum Amateur Astronomy Forum
    Replies: 21
    Last Post: 02-17-2006, 10:22 PM
  3. 80, 90 mm refractors
    By Robert L in forum UK Astronomy Forum
    Replies: 2
    Last Post: 10-16-2005, 07:55 AM
  4. Why Refractors are Better!
    By Jeff R. Schroeder in forum Amateur Astronomy Forum
    Replies: 50
    Last Post: 04-29-2005, 03:45 PM
  5. ED Refractors
    By Eric in forum Amateur Astronomy Forum
    Replies: 12
    Last Post: 10-13-2003, 01:04 AM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Powered by vBulletin« Version 4.2.0
Powered by vBulletin«
All times are GMT. The time now is 03:45 AM.