12 replies to this topic

[Grounddweller]

Hello all,

I have purchased (and sold) a fair number of telescopes and every time the same question pops into my head. When considering an in-person purchase of a telescope, what is the best way to judge if the optics are good or not? Is it as simple as carrying a 25-26mm plossl and popping it in the focuser, viewing a faraway object and moving it around the field of view? Or is there a way to estimate what the wave might be (on a mirror) or if the lenses are pinched. I look forward to reading your comments.

[bobzeq25]

No.  At this point you should just use it and see if you like it.  Next time, think about getting a scope/vendor that provides an optical analysis of your specific scope.  It's not easy, and requires expertise.

 

There is no magic here.

Edited by bobzeq25, 21 October 2021 - 07:36 PM.

[ButterFly]

Artificial stars.  If it is too close, it can look like spherical aberration.  Two mirrors, with the light entering at an angle, can increase the apparent distance.  A ball bearing also works.  Cooling the optics can still take a while.  Pinch is easy to tell.  Interferometric analysis is not.

[photomagica]

A star test - either real or a distant artificial - is the quickest way that I know to get an idea of the quality. Day time objects seldom tell much about the performance but they will tell you if there are really big problems or not.  For day objects I use a power of at least 30x per inch of aperture and for night as much as the seeing will take, 50x per inch or better if possible. Look for a clean, textbook diffraction image of the test star. Dick Suiters book Star Testing Astronomical Telescopes will give you an idea of what good and bad looks like.

Best regards,

Bill

[maroubra_boy]

As Bill said, star testing is the best way, and this means high magnification under good seeing conditions.  This can also entail resolution testing with trying to pull specific details from objects such as the Moon and Saturn.  Splitting double stars is another test, but it is a very technical challenge double stars as splitting them does not need to have a hairline split between them, but how small a figure 8 it can display.  Magnitude disparity between the two stars can also be problematic.

 

A very quick and easy first up test with the Moon is if the scope will resolve the Apollo 11 trio of craters if the scope is 4" or larger .  I have had 8" SCT's totally failed to resolve these and I have also had a 5" Mak have no problem showing them.  A C5 I had was able to show Armstrong but not Collins or Aldrin, and an old orange tube C8 I had could show all three but only under very good seeing conditions but it was still a struggle, and a mate's C8 had no difficulty at all.  The 9" Mak I have has no problem showing smaller craters around this trio...  If the scope in question cannot show this trio, then it isn't doing its job.

 

The Apollo 11 trio is only one of many features.  Apertures 7" and larger you can try for the rille that runs down the centre of the Alpine Valley.

 

Star testing will also show many aberrations, such as pinched optics as you asked, astigmatism (takes all of 5 seconds to work this one out with collimated optics!), quality of collimation, and a few other aberrations.  As for a quantifiable assessment of wave quality, this requires an interferometer test.  You can approximate this from testing using empirical testing such as above or with ronchi testing or foucault testing.

 

Alex.

Edited by maroubra_boy, 21 October 2021 - 08:07 PM.

[luxo II]

Star test - and I take a few eyepieces along and I want enough magnification to clearly see the diffraction pattern at focus.

 

If the focal ratio is F, this is best done with an eyepiece focal length (in mm) around 0.7F to 0.5F. So, for say a f/15 scope, around 8mm is ideal. For f/10 SCT's, 7 or 6mm. For f/7 refractors... 4mm.

 

Moon or planets are not a valid test IMHO.

 

In a nutshell, what you are looking for is depicted in figure 96 at this page: https://www.telescop...aberrations.htm

 

Poor collimation, pinched optics and tube currents can be dealt with as DIY issues, but not spherical aberration, astigmatism or coma, nor longitudinal or lateral chromatic aberration (refractors).

Edited by luxo II, 21 October 2021 - 08:26 PM.

[PKDfan]

Hi Grounddweller!

I think the fastest way to check a telescope, a refractor especially, is the focus 'snap-to' test.

If the scope comes to a satisfying focus, a travel from in to out focus should exhibit a 'snappiness' that says yes your optics are good.

Test on anything a fair distance away at anytime of day or night.



Clear skies & Good seeing

[photomagica]

In a nutshell, what you are looking for is depicted in figure 96 at this page: https://www.telescop...aberrations.htm

 

Poor collimation, pinched optics and tube currents can be dealt with as DIY issues, but not spherical aberration, astigmatism or coma, nor longitudinal or lateral chromatic aberration (refractors).

 

That link is great - thank you! It does take some practice to interpret the diffraction pattern. It is one of those things that one gets better and more confident at the more telescope you test.

Bill

[luxo II]

Practice that on whatever scopes you have, too.

[markb]

+1 on the snap test.

 

A 'focus snap' test is surprisingly telling. Really good optics come to a distinct focus, often described as snapping to focus.

 

It works very well in daytime, possibly even easier to interpret than at night.

 

Every optic I've had that snapped to focus tested extremely well by other methods.

 

I also, assuming a daytime viewing, use bolt threads on signs and telephone poles as a nice evenly spaced linear test target. Tiny print is also excellent, if either falls within view.

 

Bare wire runs and straight edges of shiny objects show CA pretty easily 

 

For more formal testing, an artificial star (Hubble Optics or a chromed ball bearing in the sun or laser lit) and Suiter intra/extra focal comparision, and/or a Ronchi tester.

 

One warning, if too close (and SCTs are particularly sensitive to enough distance), artificial star can give false indications on SA, IIRC. The other aberrations will show up, at least with a Ronchi.

 

The Suiter is hard for most to interpret, but a near match in and out of focus means a good figure.

 

A Ronchi tester is much more straightforward to interpret and easily identify many issues. They are a bit hard to find at the moment, but you can download files than can print on printer acetate.

 

But I like the snap test as a first step 

[Grounddweller]

No.  At this point you should just use it yand see if you like it.  Next time, think about getting a scope/vendor that provides an optical analysis of your specific scope.  It's not easy, and requires expertise.

 

There is no magic here.

It wasn’t a query due to a bad purchase, but rather a question that I have had when contemplating a scope and had not really found a prescribed method to follow when looking for quality of the optics, other than manufacturers reputation. As with any purchase of a preowned item, one likes to have some measure of security and typically a scope purchase takes place in an environment and time that is not conducive to a thorough operational test. I have not seen this subject discussed so that is why I posted the question.

[luxo II]

In addition to the appearance of a bright star at focus (per the figure I linked) thee is another test worth doing especially on an ATM scope.

The appearance of a star at high power and slightly defocussed inside focus vs outside focus is revealing, with respect to the correction for spherical aberration. The website I linked has a diagram showing what you may see.

Edited by luxo II, 21 October 2021 - 10:37 PM.

[speedster]

Several solutions aimed at cameras that would work just as well with scopes although the distances could be farther.  One example is If you are just comparing scopes, you an use a camera lens test pattern or even make your own.  That would let you compare sharpness, resolution, contrast, etc.