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refractor terrestrial viewing to test primary optic quality

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#26 25585

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Posted 01 December 2019 - 06:32 PM

Snap-to focusing can also be seen with binoculars, in fact because both eyes are in use its more apparent. When trying out binos & scopes, its one green flag feature, the others being detail sharpness and colour rendition - wide angle are often inferior to standard.

 

Snap-to used for shorter FL scopes really means less depth of field & more critical focusing required. I prefer longer, but not too long. If trying out a short FL refractor, its worth using a good Barlow with the same eyepieces to see any differences at higher magnifications. How clear & well defined are small details. The best are like long distance macro images.


Edited by 25585, 01 December 2019 - 06:34 PM.

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#27 Jon Isaacs

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Posted 02 December 2019 - 10:16 AM

Realistically, I think there's only a certain amount of relevance between day light performance and night time performance.  

 

During the day, color correction is important but not as critical as it is at night on a bright, high contrast like Venus.  You never get the high contrasts during the day you do at night, color correction faults hide behind that lack of contrasting objects.

 

Resolution and star tests suffer as well during the day, they're never quite as demanding as they are at night when high magnifications can be used effectively.

 

Jon



#28 emflocater

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Posted 04 December 2019 - 09:36 AM

Great replies! Thanks.

Cheers

Don



#29 gwlee

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Posted 13 December 2019 - 09:59 PM

Sounds like your optics may be deficient in some way. After 50 years of making and observing with many telescopes, I know an excellent optic by it's snap to focus and the caliber of its spherical aberration correction. Since I'm only surmising about your experiences and optical quality, the argument is basically now one of subjectivity on your part versus decades of experience and observation on my part. My experience is contrary to yours and I've seen my fair share of optics that didn't snap to focus or provide a sharp image, both homemade and mass produced. I also know when an optic has excellent correction for spherical aberration and when it doesn't. Your seeing conditions that you've described elsewhere in this thread seem to suggest that this may be one of many factors to consider.

I have owned and used my share of premium scopes too, and I doubt they were/are all deficient. To me, the most likely explanation is that we are simply choosing different words to describe similar experiences. For example, I would say that all of my light switches snap on/off but wouldn’t say that any of my telescopes “snap to focus” although the very best do focus more sharply than the merely good, and the sharpest scopes are the easiest to focus well because the point of best focus is more distinct. 



#30 emflocater

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Posted 13 December 2019 - 11:17 PM

Would not "snap to focus" in a way be partly related to your quality of focuser as well?

 

Cheers

Don



#31 RichA

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Posted 13 December 2019 - 11:20 PM

Hi Folks. Just curious question...with Black Friday and Christmas sales here, many Folks may purchase a small or high quality refractor telescope. Most Folks being anxious to try the scope out ASAP and running into days if not weeks of cloudy night skies, will try some terrestrial viewing. With that said and many of us refrator owners have done this...what advice can be offered of what to look for when doing some terrestrial viewing that can be good or bad indications of the quality of the scopes primary optic lens, until a star test or clear night sky viewing can be had..

 

True part of this will be the quality of eyepiece used as well as checking for CA (Chromatic aberration) around terrestrial objects such as tree branches, leaves, roof tops and towers, but what other things should one check for visually that are good or bad initial signs of the scopes primary glass lens quality?

 

Cheers

Don

In a store,  I used to test them by focusing on cobwebs at a distance.  They are so fine that if they were  rendered sharp, I knew the scope would be decent.  I also used this find some scopes that were expensive turned out to be duds.


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