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Telescope repair advice wanted: UPDATED! Answered

This question is more like 2 questions-
1) Am I crazy?
2) Is this feasible?

Background: I recently inherited an astronomical reflecting telescope, which I think has been dropped at some point.  The design is as below.

Where the eyepiece enters the side of the tube that makes up the body of the scope, the tube is slightly dented.  The eyepiece still points at the secondary mirror and the alignment is good enough that I can point the scope at an object and get an image, but the alignment isn't perfect so the image has a lot of what I'd guess is spherical aberration preventing a decent image.

Firstly- is having the eyepiece on-axis and perfectly aligned so important that I will never ever get even a half-decent image from this scope?  If the tolerance involved are 0.1mm and minutes or seconds of angle, I doubt I can restore it that well.

Secondly, is having the eyepiece positioned on-axis as important as having it parallel with the ideal axial light path?  As it's held in with three screws I'd imagine I can tweak the alignment with some use of spacers etc. but if it's off-axis I don't think I can shift it back to the perfect position.  In other words, for the mathematicians, is having the eyepiece "translated" as much of a problem as having it "rotated"?

(Thirdly- if the reflecting scope is trash, can I get a 150mm diameter 1000mm focus lens and make a straight-through scope with the bits?)

UPDATE: it's working perfectly, see pictures below!


Optics is not my strong point, but couldn't you just shift the secondary around? Since the whole purpose of it is to redirect the optical axis, it seems like you could just slide the spider up or down the tube, and tweak the angle to line it up. On a less extreme level, that's how you collimate any reflector. Kelsey? Help?

Amazing! Endless props for the tip on adjusting the secondary, it was just what I needed.  After getting the eyepiece roughly aligned, I spent a while tweaking the secondary and went from everything being a blur to seeing a better image than I ever thought I'd get out of it.

I did the adjustment by putting my bike light at the bottom of the garden pointing at the house as a fixed light source and tweaking the screws on the spider- by the time I'd finished I could see the electrode wire on the surface of the LEDs (0.1mm across?) from a distance of over 60 feet away.

Fortunately it was a bright full moon and there were a few gaps in the ever-present British clouds so I managed to get this snap with my phone's camera to show roughly what I was seeing.  Bear in mind that
a) my phone's camera is legendarily awful
b) it was still slightly hazy even in gaps in the cloud.

I'm over the moon, pun entirely intended, with this result. Thanks everyone for your input :D


Reply 8 years ago

UPDATE: I spent a while better aligning the secondary a couple of weeks ago, until I had an image good enough to see the rings on Saturn.  Last night there were some clear skies and a nice bright crescent moon visible from my window, and after a bit of fiddling with the tripod and my phone camera I got this.  Again, it's not quite representative of what I could see with my actual eyes, the clarity and detail was astonishing.

I've put these through an unsharp mask because my phone's camera is atrocious and can even wash out photos in good sunlight, but they give a better idea of what I saw that way.  Compare with the previous photo...


Nice! I love to see a good scope up and functioning!

Now lets see your photos with a good camera. :P

Sadly they mostly look like this at the moment, swimming in an enormous field of black.  I suspect that's because my camera's lens is quite big (not quite DSLR size but still several centimetres across) and the scope eyepiece is quite small so I only get a small dot of light in the camera.  That is the main reason I'm using the phone camera- having an aperture a millimetre or two across means it fits the scope eyepiece very well.

If you have any more genius suggestions as to how to couple a digital camera to a telescope without buying a new eyepiece or whatever, they would also be welcome- otherwise I''ll probably just have to build a mount to keep the phone camera steady and aligned properly.


I am not going to jump into this discussion (sorry, Cam!).  All of my optics knowledge is theoretical.  You three already know more about actual telescope assembly and operation than I do.  I have found the discussion so far quite interesting, and am learning good stuff.

Since you're guessing it's spherical aberration, can you describe it, just in case it's not.

Good call- I suspect I am abusing the term here.

What I saw was, looking at a point light source, ie the LED lights at the bottom of the garden, the points of light were distorted into a vaguely triangular blur.  (The angle and shape was somewhat reminiscent of a gingko leaf with the brightest point at the "stem"). 

These two diagrams (ray diagram and example) make me think what I am actually describing is coma.  The blurs were all aligned the same way, looking a little like the bottom of the attached image, not radially like this example.  I wish I could take photos to illustrate all this but my camera would go insane trying.  Could try the phone camera I suppose...

Cameron: I hadn't thought about it that way, it sounds sensible.  The spider holding the secondary has screw adjusters for the mirror, though I'm not how much travel they have, so will try that next time I have a chance.


Reply 8 years ago

(image failed on last post- this is the closest match I could find to what I was seeing)


I'm hardly an optics expert, but it does sound more like coma.  Do you have a second eyepiece?  if so try that. if not try rotating the eyepiece, if the coma rotates with the eyepiece the misalignment is in the elements of the eyepiece. If the coma doesn't rotate, then it is the spider that needs adjustment.