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Comparing Telescopes - exit pupil or magnification ?

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#26 Jon Isaacs

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Posted 04 September 2020 - 08:54 AM

Just from a short comparison (under full moon conditions) evaluation of the AT 102 ED.

Lasted 20 minutes after twilight ended and before the clouds rolled in.

 

The scope is a winner !

fantastic focuser - smooth inside/outside focus with no image shift

APM 30mm UFF was perfectly flat to the edge - 23.8x FOV 2.73° Surprisingly dark background considering the full moon interference. Double Cluster was perfectly framed - beautiful pinpoint stars.

used the same eyepiece and located NGC 7789 with averted vision with just a hint of a few stragglers 

M13 was nicely resolved with an 11mm DeLite and easily took a 5mm DeLite (143X) with snap focus 

 

Nest clear night the shootout vs the Orion 120mm f/5.

 

I think you're gonna like the AT-102ED.  The ST-120 is a low power, wide field specialty scope.  Sure, you can do planets with it and double stars but when you have a scope like the AT-102ED, it's the one you'll use.  

 

My friend Jack bought an AT-102ED before the Pandemic.  The focuser, the build, it's just a nice scope and an amazing bargain for $600.

 

Jon


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#27 Marcsabb

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Posted 04 September 2020 - 09:19 AM

gnowellsct gave the most complete and reasonable answer to this thread.

 

For example, I've a Vixen NLV 2.5mm eyepiece at home which I use only with my ED114SS (a F/5.25 scope) to reach 240x. On any other scope with a longer ratio, this otherwise fine eyepiece, is useless to me. Contrast and color degrade rapidly, no matter how good the scope is. That tells me that my eyes limit is an exit pupil around 0.5mm small. Anything smaller than that and I actually see less than with lower magnifications.

 

Usually I try to stay in the 0.8-1.0mm range for moon and planets as I noticed that my eyes tire less and I can concentrate more on the picture.


Edited by Marcsabb, 04 September 2020 - 09:19 AM.

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#28 daquad

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Posted 04 September 2020 - 02:01 PM

gnowellsct gave the most complete and reasonable answer to this thread.

 

For example, I've a Vixen NLV 2.5mm eyepiece at home which I use only with my ED114SS (a F/5.25 scope) to reach 240x. On any other scope with a longer ratio, this otherwise fine eyepiece, is useless to me. Contrast and color degrade rapidly, no matter how good the scope is. That tells me that my eyes limit is an exit pupil around 0.5mm small. Anything smaller than that and I actually see less than with lower magnifications.

 

Usually I try to stay in the 0.8-1.0mm range for moon and planets as I noticed that my eyes tire less and I can concentrate more on the picture.

Agree completely.  But as a reminder, you can push the power beyond 50X/inch (< 0.5 mm exit pupil) for close doubles.  In fact, it is often necessary to do so when the doubles are at or slightly less than the resolving power of the telescope.  The magnification is not "empty" because all you are doing is emphasizing the separation of two bright discs against a dark background.  You are not trying to tease out low contrast detail.

 

If a separation of 4 minutes of arc is desired (about the distance between two mm lines on a ruler seen from a distance of .89 meters or 33 inches) then a magnification of about 53X/inch is required.  For 6 minutes separation the magnification needed is 80X/inch, etc.

 

I arrive at the figure 53X/inch by dividing 240" (4' of arc) by the Dawe's limit for the resolving power of a telescope and then normalize the result by the diameter of the scope.  This amounts to dividing 240/4.56 = 52.6X/inch, the magnification required for a 4 arc minute apparent separation.

 

Higher magnification is often required, especially if the components are unequal in magnitude, which they usually are.

 

Dom Q.


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#29 Jimmy462

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Posted 15 September 2020 - 08:26 AM

Just from a short comparison (under full moon conditions) evaluation of the AT 102 ED.

Lasted 20 minutes after twilight ended and before the clouds rolled in.

 

The scope is a winner !

fantastic focuser - smooth inside/outside focus with no image shift

APM 30mm UFF was perfectly flat to the edge - 23.8x FOV 2.73° Surprisingly dark background considering the full moon interference. Double Cluster was perfectly framed - beautiful pinpoint stars.

used the same eyepiece and located NGC 7789 with averted vision with just a hint of a few stragglers 

M13 was nicely resolved with an 11mm DeLite and easily took a 5mm DeLite (143X) with snap focus 

 

Next clear night the shootout vs the Orion 120mm f/5.

Methinks the metric here for comparing these two scopes (e.g. 120 f/5 Achromat vs. 102 f/7, um, "APO") will be when (read: at what magnification?) does chromatic aberration become apparent? Well, at least that's the metric I'd be using.

 

Comparing to either either Sedwick or Conrady's CA criteria as found here...

 

CA-ratio-chart-achro.jpg (JPEG Image, 797 × 426 pixels)

http://www.cityastro...chart-achro.jpg

 

...the 120 f/5 will exhibit an "Unacceptable Levels of CA" ratio of 1.06.

 

Now, were the 102 f/7 a mere Achromat it would already find itself in the "Filterable Levels of CA" zone with an approximately ~1.7 ratio!

 

However, the 102 being an "APO", I should think, will push its CA ratio much-closer-to or into the, um, green "Minimal or No CA" zone. (Perhaps someone here knows or can provide the mathematics for determining an accurate CA ratio number for APOs?)

 

The comparison I'd be most interested in finding out at the eyepiece would be, "at which magnification did each instrument begin to reveal objectionable levels of CA?".

 

My guess here (based solely on my own observational experience over the years with a smaller 102mm f/5 Achro) is that the larger Crown + Flint 120mm will best best suited for rich-field viewing at magnifications less than 0.5x its aperture (in mm)...120mm x 0.5 = 60x-and-under before CA bespoils the view.

 

The wildcard here will be how well your FPL-51 + unknown-mate 102ED is actually corrected for CA. Based on my reading user comments here on CN regarding that model (and its clones) it would appear that (depending on a user's sensitivity to CA) the scope should be able to provide relatively CA-free views up to around 1.5x its aperture (in mm)...102mm x 1.5 = ~153x-and-under, making it, in my book, both a rich-field scope and a decent planet-viewing scope.

 

As an RFT the 120 f/5, I should think, will prove preferable for its brighter views. And, as a planet scope, I would hope to be able to pull out the 102 f/7 for some .7mm exit -pupil viewing.

 

Anyhoo, I'll be curious to read, and look forward to, your comparisons and assessments.

____________________

 

Then, of course, there is the question of image contrast when comparing the two scopes! lol.gif

 

smile.gif




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