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A Rangefiner is awesome for testing resolution

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#1 MartinPond

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Posted 15 March 2019 - 01:05 PM

(In the daytime, of course)

 

 

I took a chance on a "Yardage Pro"  rangefinder at the consignment shop

   ($14) and quickly replaced the battery that had a 2016 sell-by, so it wouldn't

    "juice out" out the battery compartment.  With new battery...shazam! Works great.

    I'ts a laser time-of-flight widget, so angles are no problem.

 

So it's easy now to place things like shampoo or Dorito ingredient lists at a 

  solid spot and get the distance, then figure the "total acuity", scope+ep+me....

 

As an example:

.

From an 80mm x 400m scope, focal-extended to 80x,
a 1.2mm font can just be read at 39 meters   (39000 millimeters).

 

Optometrists say you need 5 line seperations per-height
to read a sans-serif font, so the resolution is at least 1.2/5 = 0.24 mm.

 

The angle for this is   arctan(0.24 / 39000)
or 3.526e-4 degrees,   or (times 3600) 1.27 arc-seconds.

    (I assume I could do slightly better, but it was starting to blur)

 

According to several books, the Airy disk is:
                                          "For an 80-mm refractor, 1.4 arc seconds."

 

So while the eye can likely see better than the Airy disk

   (likely due to the brain knowing letters, and cortex processing),

    the results are pretty close.

 

----------------------------------------------------

 

One thing I observed:

     

   ----there was very little chromatic aberration at 80x  (1mm exit pupil)

        for an 80mm F5

 

    ---the thing is,  an 80mm scope gets fairly dim at night past 80x anyway,

        and the atmosphere here can having seeing wobble of 2-6 arc-seconds 

         fairly often.....

        I would need a bigger aperture AND a better sky to do more power...

 

      I do a fair amount of day observing as well, so the rangefinder will be

      valuable in looking at the ranges where the sometimes-brutal 

     "horizontal seeing" kick in.   Over a mile, the slushing.can get to 30--70 arc-seconds!


Edited by MartinPond, 15 March 2019 - 01:10 PM.

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#2 gnowellsct

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Posted 16 March 2019 - 08:53 AM


One thing I observed:

----there was very little chromatic aberration at 80x (1mm exit pupil)
for an 80mm F5

---the thing is, an 80mm scope gets fairly dim at night past 80x anyway,
and the atmosphere here can having seeing wobble of 2-6 arc-seconds
fairly often.....
I would need a bigger aperture AND a better sky to do more power...

I do a fair amount of day observing as well, so the rangefinder will be
valuable in looking at the ranges where the sometimes-brutal
"horizontal seeing" kick in. Over a mile, the slushing.can get to 30--70 arc-seconds!


In astronomical viewing many people assume that due to this or that limitation they won't be using a scope on planets. But when Jupiter is up off they go to have a look, and the wide fields end up taking a back seat.

One can say the seeing is bad, and I don't doubt you, but when the seeing is good, which DOES happen, you'll be the guy who forgot to bring his swim trunks to the beach party.

The range finder stuff is really interesting. But going to great lengths to verify your theoretical resolution contradicts your claim that you won't be using it!

Still, given the number of poor scopes that ship brand new, this is potentially a game changer in terms of helping us purchasers get reasonable quality control. Maybe we should all be running these tests, though ground thermals may compromise results.

GN

#3 MartinPond

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Posted 16 March 2019 - 09:28 AM

----------------------------

The range finder stuff is really interesting. But going to great lengths to verify your theoretical resolution contradicts your claim that you won't be using it!

----------------------------

 

In other threads, people mention that achromats can be very good or very bad.

As a practical matter, this is a way to see if the OTA I have salvaged from a crummy

starter tripod is worth putting on a nice bogen mount.

And.....using the rangefinder means I don't have to go to "great lengths" at all...it's easy.

Plus....knowledge is control: Am I going to get something I really can't use?

I realize it doesn't matter if you have plenty of money..

 

 

"---------------------------------------

Still, given the number of poor scopes that ship brand new, this is potentially a game changer in terms of helping us purchasers get reasonable quality control. Maybe we should all be running these tests, though ground thermals may compromise results.

------------------------------------------  "

 

Actually.....yes.

 You found the counterpoint to your prior point....stream of consciousness posting? 

    That's OK by me!

It would be great to identify the duds and the gems at the low end,

  those scopes worthy of trashing the tripod they come with and remounting 

   on a great tripod.    That is were I'm at.   Currently, many achromat scope barrels widely

   surpass the crappy jiggly moiunts they come with....but which?

 

I know this is "Road Warrior" stuff, but every other hobby ,  like ham radio or wood carving

    or guns or archery or whittling or model trains,  or even running or mountain biking,

    has a division of     "best you can do on a budget".   It's a challenge! 

    There are achievements to reach for.

   Being able to evaluate your optics cheaply and reliably is key to that.

   

Notice how I used tiny print at 40 yards.....that's how worried I am about 'horizontal seeing'.

If someone uses an 80mm ED spotter at high expense across 400 yards, do they have 

   any hope of being better than a 70x600 acchro?  So far, up on Holt Hill...the answer is: No.   


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#4 gnowellsct

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Posted 24 March 2019 - 07:58 PM

-

Notice how I used tiny print at 40 yards.....that's how worried I am about 'horizontal seeing'.

If someone uses an 80mm ED spotter at high expense across 400 yards, do they have 

   any hope of being better than a 70x600 acchro?  So far, up on Holt Hill...the answer is: No.   

By "horizontal seeing" I take it you mean ground thermals or daytime terrestrial seeing.  These tests are interesting.  I wonder if you are controlling for magnificaiton?



#5 MartinPond

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Posted 25 March 2019 - 02:13 PM

I like the term "daytime terrestrial seeing"...

it suggests both the horizintal angle and the thermal mixing issues.

 

I'm not sure what "controlling for magnificaiton" is, but the magnification

   is surely used to determine the threshold (in arc-seconds) where you can't

   see the font well.   I could make the font larger or smaller with 

   different magnifiaction, but I would see the same "wobble-n-slush" level..

   This is, I might be able to see the same font at 50x and 100x.

    Granted, the power has to be enough to see the font.

   

   You can tell the difference: acuity of scope limits are a steady thing,

    and the effect of seeing is a dynamic, wiggly, thrashing thing. 

    I remember the limbs of a pine tree wiggling about 4 times a second

    for a relative 12 feet (compared to the tree height of about 70ft.)

    ...miles away.




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