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Baffle vignetting in small scopes

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#1 Glassthrower

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Posted 01 October 2006 - 12:58 PM

This is a good test to see if there is some restriction blocking the aperture. When this was discussed somewhere else, Roland Christen suggested this test:

With the diagonal in place and the telescope approximately focused to infinite, (focus it and then remove the eyepiece) look through a pin-hole and see if you can see the edge of the lens cell, if you can, then there is no restriction to the aperture and the baffles are fine where they are. If not, you can place a ruler/scale across the front of the scope and see how much of it you can actually see.


Using this method, I have determined that my 60mm scope is vignetted down to slightly less than 2 inches. Getting an exact measurement was difficult - this was done on a whim without much setup. But I definitely do not see the edge of the objective, I distinctly see the baffle ring.

So, now I am wondering, should I move the baffle to remove the vignette effect, or should I leave it alone? Given the cheap nature of this scope, the vignette is likely intentional to compensate for poor lens figuring in the outer edge of the objective.

Now I am questioning whether this little scope is worth the trouble of tweaking. I'm not sure whether I want to dissect it, relube the focuser, flatblack the inside, and flock key areas, if I am going through all of that trouble for a ~48mm scope.

I believe it was fellow CN member PCAD (Peter) who tested a similar scope and found it stopped down to 42mm. I think that is what mine is stopped down to.

Might explain why this scope tops out at just under 100x.

Clear dark skies...

MikeG

#2 Darenwh

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Posted 01 October 2006 - 02:16 PM

Try moving the baffle back to get full use of the lens and see how the optics stad up. If they did this on every scope without testing the lens to see if it is necessary then it is possible you may have a good lens. It's not something that once done cannot be undone so give it a try.

#3 NewAstronomer

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Posted 01 October 2006 - 02:34 PM

The stop down nature of these baffles is probably an attempt to hide the poor objective but ALSO to remove even more false color on the moon/jupiter/saturn/etc

#4 Glassthrower

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Posted 01 October 2006 - 03:06 PM

Ok, bear with me a moment here....

If "60mm worth of light" is entering the front objective (assuming actual 60mm clear glass aperture), but only "42mm of light" is actually going to the eyepiece, then what exactly happens to that other 18mm of light? Does it become off-axis scatter and bounce around inside the tube, reducing contrast? Is it simply "blocked" and removed from the tube for visual purposes? I understand what the intended benefits of the vigenette are, but is there some side effect to this process : like reduced overall contrast in exchange for reduced overall color?

Or am I over-analyzing here?

BTW, My objective cell seems permanently attached. It won't unscrew or "pop" off. I get resistance. I'm afraid to break it, so I have aborted for now. Has anyone tried to remove the objective cell from this scope before (or one like it?) - Meade 60mm AZ-A2

Clear dark unvignetted skies...

MikeG

#5 Jon Isaacs

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Posted 01 October 2006 - 09:33 PM

>>Or am I over-analyzing here?---

Mike: I think you are over analyzing here. There is a lot of light that enters the OTA through the lens that does not make it to the eyepiece. Some of it is blocked by the baffling, some by the eyepiece field stop...

OK so here's the deal here...

I keep trying to give away my Tasco 59T. This is a Japanese 60mm F/11.7 Tasco with a 0.965 inch focuser. It is looking for a home.... Maybe not the greatest scope but it is one that has more potential that the Meade.

jon

#6 Glassthrower

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Posted 02 October 2006 - 07:21 AM

Jon -

Wanna trade OTA's? If you want to give away that Tasco, I could use it and I could give you my Meade OTA in exchange?

CLear dark skies....

MikeG

#7 Selecter

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Posted 19 October 2006 - 08:45 AM

is the Tasco a complete scope or just a ota? Got a paypal account? I'll paypal you some bucks for shipping. I'm at the stage where any scope is a kewl scope. :thewave:


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