Star Testing (1 Viewer)

[Tringa45]

I am very happy with a low price Timex that is still more accurate than Harrison's chronographs.

David,

Harrison's chronometers were made in the 18th century and were accurate to around 1/3 second/day.
I don't know of any currently made transportable and mechanical watch or clock that could better that.
Your Timex must have a quartz movement, a technology unavailable in the 18th century.

John

 

[Binastro]

Thanks John for the correction.

I have been jabbed in both arms for Covid and flu reasons and am a bit groggy.
At least I feel boosted.
Whether they will manage a million Covid jabs a day I don't know.

For £20 the Timex lasts an amazing amount of time considering how much I use the light function.
Several years.
I usually just get another one when they eventually stop.

The mechanical watch I have has radium paint.
I'd rather have a quartz watch and the amazing light function.

I do have multiple digital radio controlled clocks with temperature and projection feature that cost £2 each new from the charity shop.
They give time to better than one second for meteor and satellite timing.
The batteries on these can last ten years.

Another one has sunrise/sunset, moonrise/moonset, humidity, barometer, weather trend etc. Inside and outside temperatures.
It was about £20 twenty years ago and still runs normally.

A mains Binatone green LED clock has been running non stop for almost 50 years.
Except for occasional power cuts.

Regards,
B.

 

[RandyBo]

Any lens can be hand aspherized. Glenn LeDrew at Cloudy Nights described hand aspherizing the objectives of a cheap Bushnell binocular to improve their correction (at the cost of removing some of the AR coating).

For the last 20 years I've used one of Roland Christen's hand aspherized scopes for birding (92mm F/6.5 Stowaway). I would prefer a lighter waterproof scope for field use, but so far I haven't found one good enough to replace the Stowaway. My wife's Nikon Monarch 80ED actually comes close in terms of aberrations, but I want more aperture so the search goes on.

I expect Roland Christen's criticism of the star test cult's pissing contests in amateur astronomy applies to much more sensitive testing at very high magnifications which may reveal tiny (and perhaps misleading) differences in unfocused star images that we can't see at the exit pupils of 1.3-0.8mm we normally use for star testing spotting scopes. Using the Stowaway as a reference I've never yet seen a scope with substantial star test problems visible at around a 1mm exit pupil that did't also have image quality problems as a direct result.

@henry link
Thank You, for your response.
Based on your experience, and owning a very good reference scope, can you view normally through a new scope, and then, kind of predict what the star test will probably reveal?
If so, have you ever been surprised?
Thanks,
Randy

 

[RandyBo]

@Binastro
@jring

I would like to extend the above question, for Mr. Link, to you guys.

Thanks,
Randy

 

[henry link]

@henry link
Thank You, for your response.
Based on your experience, and owning a very good reference scope, can you view normally through a new scope, and then, kind of predict what the star test will probably reveal?
If so, have you ever been surprised?
Thanks,
Randy

Sorry for the late reply. Determining how good a scope is just by looking through it can be tricky unless it's really bad. It's much easier if you have a reference scope of known high quality set up next to it to establish the best image that can be expected under whatever the existing conditions are. The role of the star test when you've already determined that there's a problem is as a diagnostic tool to show exactly what's wrong. Astigmatism and spherical aberration, for instance, might produce a similar looking soft image, but they will look completely different in a star test.

I often do a star test as the first step. If the aberrations or defects in the star test are bad the scope will invariably have bad optics. I've never seen an example of a good scope combined with a bad star test. Moderate aberrations in a star test will almost certainly affect the image quality, but do less damage and and mild aberrations might not be a problem at all at 60x and less.

 

[RandyBo]

Sorry for the late reply. Determining how good a scope is just by looking through it can be tricky unless it's really bad. It's much easier if you have a reference scope of known high quality set up next to it to establish the best image that can be expected under whatever the existing conditions are. The role of the star test when you've already determined that there's a problem is as a diagnostic tool to show exactly what's wrong. Astigmatism and spherical aberration, for instance, might produce a similar looking soft image, but they will look completely different in a star test.

I often do a star test as the first step. If the aberrations or defects in the star test are bad the scope will invariably have bad optics. I've never seen an example of a good scope combined with a bad star test. Moderate aberrations in a star test will almost certainly affect the image quality, but do less damage and and mild aberrations might not be a problem at all at 60x and less.

@henry link
Thank You, for your reply.
What do you reference to determine if a aberration is moderate or mild?
If a star test is not conducted properly (by someone like me who lacks the experience) could the test results reveal an aberration that doesn’t really exist or make a mild aberration look more like a moderate aberration?
Is there a rule-of-thumb for the magnification a star test should be performed at?

Thanks,
Randy

 

[RandyBo]

Sorry for the late reply. Determining how good a scope is just by looking through it can be tricky unless it's really bad. It's much easier if you have a reference scope of known high quality set up next to it to establish the best image that can be expected under whatever the existing conditions are. The role of the star test when you've already determined that there's a problem is as a diagnostic tool to show exactly what's wrong. Astigmatism and spherical aberration, for instance, might produce a similar looking soft image, but they will look completely different in a star test.

I often do a star test as the first step. If the aberrations or defects in the star test are bad the scope will invariably have bad optics. I've never seen an example of a good scope combined with a bad star test. Moderate aberrations in a star test will almost certainly affect the image quality, but do less damage and and mild aberrations might not be a problem at all at 60x and less.

I went back and re-read the first paragraph of your reply and please forgive my endless questions but, can I assume that you would not purchase a new scope if you did not have the opportunity to both: compare the view through the new scope to your astro-physics and also perform a star test with the new scope; regardless of how good the stand alone view through the new scope appears to be?

Thanks,
Randy

 

[Binastro]

I have had good star tests with not very good optics.

For me, I am used to astro telescopes.

I put a high power eyepiece without an image erector and if Saturn, Jupiter or Mars are available at a reasonable elevation, I use these.
Say 50 times per inch of aperture.

I also star test them.

If I had a scope long term, I would test it in excellent conditions at 75x per inch and sometimes 100x per inch of aperture.
My eyes are no longer good enough for high magnification tests.

When I was active I didn't really need a comparison scope.

Usually the scope is average, but occasionally the results are staggering.
These scopes are usually military in origin or have military connections, made in peace time, or made by a master optician as one offs.

I have had Soviet, British, Japanese and Dutch scopes that have really surprised me.

As to lenses, German, British, U.S. and French have been special.

I have not tested enough spotting scopes to give an opinion, perhaps fifty as opposed to hundreds of astro scopes and larger numbers of lenses.
Also few spotting scopes have been from top quality makers.

But the number of superb optics is about 1% of those tested.

As to the scopes that I have used, practicality was much more important than superb optics.
So long as the scope was good enough for the observation and practical, then that was what I used rather than a less practical super performer.

Sometimes the scopes were average or good scopes reworked by master opticians to a very high optical level.

Regards,
B.

 

[RandyBo]

I have had good star tests with not very good optics.

For me, I am used to astro telescopes.

I put a high power eyepiece without an image erector and if Saturn, Jupiter or Mars are available at a reasonable elevation, I use these.
Say 50 times per inch of aperture.

I also star test them.

If I had a scope long term, I would test it in excellent conditions at 75x per inch and sometimes 100x per inch of aperture.
My eyes are no longer good enough for high magnification tests.

When I was active I didn't really need a comparison scope.

Usually the scope is average, but occasionally the results are staggering.
These scopes are usually military in origin or have military connections, made in peace time, or made by a master optician as one offs.

I have had Soviet, British, Japanese and Dutch scopes that have really surprised me.

As to lenses, German, British, U.S. and French have been special.

I have not tested enough spotting scopes to give an opinion, perhaps fifty as opposed to hundreds of astro scopes and larger numbers of lenses.
Also few spotting scopes have been from top quality makers.

But the number of superb optics is about 1% of those tested.

As to the scopes that I have used, practicality was much more important than superb optics.
So long as the scope was good enough for the observation and practical, then that was what I used rather than a less practical super performer.

Sometimes the scopes were average or good scopes reworked by master opticians to a very high optical level.

Regards,
B.

@Binastro
Thank You, for your reply.
The only means of boosting the magnification on my spotting scope is with the factory 1.8 extender. This takes the highest magnification to 90X for the 82mm objective (0.91 EP).
Would this magnification be sufficient to star test this instrument?

Thanks,
Randy

 

[Binastro]

Yes.

But the extender might not be optically perfect, but should be quite good..

Regards,
B.