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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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#32 j.gardavsky

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Posted 16 December 2019 - 08:46 AM

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.

Spider webs and lichens are good test targets, I am also using these in my optics tests,

JG



#33 John Fitzgerald

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Posted 16 December 2019 - 09:01 AM

Terrestrial objects can be distorted by bad seeing, especially after mid morning on a sunny day.   Test on power line insulator glint in the early morning sun.  A quarter to a half mile distant is good enough for a rough test.  Too distant and it will likely be distorted by seeing.  Avoid looking over buildings or stretches of pavement. 



#34 emflocater

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Posted 16 December 2019 - 09:49 AM

What is the most important thing above all else that one should look for and notice during a terrestrial viewing test that the main objective is very bad (other than CA for an Achro) and return the scope or look for another purchase?

 

Cheers

Don



#35 Auburn80

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Posted 16 December 2019 - 10:06 AM

What is the most important thing above all else that one should look for and notice during a terrestrial viewing test that the main objective is very bad (other than CA for an Achro) and return the scope or look for another purchase?

Cheers
Don


IMHO, its contrast; both in seeing subtle shade differences and in very fine detail.
Example: I put my SV80A against my Oracle on a cross atop a church steeple about 1/4 mile away. Located as it is, there were stains on the white paint, a rusty nail head and spider web between the upright and cross member. The new 80 was just a tiny bit better than the Oracle on that target that day. I chalked it up to new vs 30+ year old coatings and possibly better CA. Could be a better figure also but can't make that call yet.
With that said, it will take a lot of experience to make a judgement on just one isolated example. One will need to be comfortable knowing what one should see at that aperture on that specific target on that day.
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#36 emflocater

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Posted 16 December 2019 - 10:25 AM

Does not a poorly made focuser hinder the true faults or obscure a great optic lens?  It seems that the quality of an eyepiece can hinder the true faults or obscure a great optic lens as well? I guess the thought is...is it a bad objective lens, a bad focuser, or a bad eyepiece? Trying to find the culprit seems a bit complicated.

Cheers

Don



#37 barbie

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Posted 16 December 2019 - 02:49 PM

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. 

The best optics snap to focus. Poorly made optics don't, it's that simple. All else being equal(good seeing conditions, thermal acclimated, etc.), a well made optic, in my experience snaps to focus without excessive fiddling with the focuser knob and remains in focus. Poorly made optics, under the same conditions don't. That's been my experience, period!!


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#38 peleuba

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Posted 16 December 2019 - 04:52 PM

The best optics snap to focus. Poorly made optics don't, it's that simple. 

 

 

From my perspective "snap to focus" is an interesting topic.  I hear a lot about it and in my experience, its not practical to use snap to focus as singularly being an arbiter of good optics. 

 

Really, snap has more to do with focal ratio and corresponding depth of focus then it does a good/bad lens.  As an example, a short focal length telescopes even if mediocre will have a more definite "snap" then a long focus telescope even if perfect.  In a long focus instrument, there is a small range of best focus.  At short focal lengths there is a definite singular point of best focus.

 

So, not having a "snap" does not mean the lens is bad.  And, conversely, having snap does not always mean the lens is good.  There is also a magnification aspect as lower magnifications can give the illusion of better focus snap where at higher magnifications, in bad seeing, one always seem to be hunting around for best focus as the seeing cells wreak havoc on the image.


Edited by peleuba, 16 December 2019 - 04:59 PM.

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#39 barbie

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Posted 16 December 2019 - 05:39 PM

First off, when I say snap to focus, I'm talking strictly in the realm of high magnification(under good seeing), so let's be perfectly clear about that point. Second, I have decades of experience designing and making optics, both professional and amateur and have seen my fair share of scopes that didn't snap to focus produce absolutely lousy images( at star parties). I also used to be of the same school of thought regarding focal length and depth of focus but I no longer subscribe to that because I've seen otherwise. My two "shorter" apos snap to focus with just as much depth of focus as my F11 4". Also, you fail to understand that correction for spherical aberration is one of the paramount factors at work here. As for the "depth of focus" issue, you are simply parroting what others have been saying!!grin.gif I've stated in other forum threads that a good optic(well designed and figured) will snap to focus while a bad optic won't, AT HIGH MAGNIFICATION.  I've seen that proven time and again and stand by that statement!!


Edited by barbie, 16 December 2019 - 05:54 PM.

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#40 Steve Allison

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Posted 16 December 2019 - 06:30 PM

In an article in Astronomy magazine many years back on how to test the optical quality of a telescope, the author suggested that if objects "snap in to focus", the optics are likely to be good. The author was Richard Suiter.

 

To me, "snap" means there there is a very definite point of best focus. It may be easy to focus in or out through this point, but when reached in an excellent optic, it is quite obvious.

 

On my best telescopes, I sometimes achieve an image that is beautifully sharp but when I adjust the focuser the tiniest bit more, it suddenly becomes even sharper. This is what I am talking about.


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#41 Eddgie

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Posted 16 December 2019 - 07:01 PM

In an article in Astronomy magazine many years back on how to test the optical quality of a telescope, the author suggested that if objects "snap in to focus", the optics are likely to be good. The author was Richard Suiter.

 

To me, "snap" means there there is a very definite point of best focus. It may be easy to focus in or out through this point, but when reached in an excellent optic, it is quite obvious.

 

On my best telescopes, I sometimes achieve an image that is beautifully sharp but when I adjust the focuser the tiniest bit more, it suddenly becomes even sharper. This is what I am talking about.

The problem with this is that it is only valid for reflective optics and not totally applicable to refractive optics.

 

A non Apo refractor  will not have an exact focus.  The user focuses the telescope somewhere along the elongated color crossings of the caustic cone.  The red and green lines in even an f/10 achromat are not all that close.  Even at f/10, the red Airy Disk is twice the size of the green Airy Disk.  The user tends to focus a somewhere between the red and green blur but the exact point where they focus would be based on how much red there is, and how sensitive the observer was to red (not very). 

 

Also, some depends on the observer's visual accommodation.  A young observer with excellent visual accommodation can have their eye correct for close focus, meaning that they might see a telescope in focus over a tiny range and conclude that the telescope does not have an exact focus, while an older observer with limited accommodation might look at it and say that it has a more exact focus. 

 

I would then not put snap to focus using the human eye as a indicator, and using a camera, you are dealing with focusing on the smallest blur diameter in refractive systems because in all but the most highly corrected Apos, the blur size changes with focus.

 

As it turns out, the human eye is not particularly good at seeing these differences. This means that it searches for the smallest blur diameter but again, not all colors will be in perfect focus. Camera would catch the defocus though and this is why people that want the best possible image use Apos. 

 

 

By comparison, the reflector will aways bring all colors to the very exact same focus point, so if the scope does not have a single point of perfect focus, then that is a potential signature of spherical aberration. For refractive optics though, it is not a slam dump. 

 

I would expect the Apo to have the potential to snap to focus, but with a fast ED scope, the user is trying to find a position on the caustic cone somewhere between the close red/green focus and the best blue focus. They will usually settle on a point where they are a bit closer to green than blue, and hence, they see the blur fringe but the green is now slightly defocused form its best focus.  What they are then looking for is a smallest blur diameter somewhere between the red/green crossing and the blue crossing. When you throw in visual accommodation and color sensitivity differences, I would say that it would be difficult to call a non-Apo bad because it did not seem to have a perfect focus. 

 

Complicated, isn't it..

 

(And Suiter talks about all of this in his book.  A lot of people selectively quote Suiter, but I think it is best to have read the book so everything is seen in context.  I have read both first and second editions several times each, and I would say that it is a sad thing that most of the people on CN have not... )


Edited by Eddgie, 16 December 2019 - 07:09 PM.

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#42 Steve Allison

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Posted 16 December 2019 - 09:23 PM

Well, Suiter did not qualify his suggestion about snap focus in any way in his Astronomy article, which was written for beginners. It was just one of several tips he gave to roughly assess optical quality.

 

I am not sure why you think it is sad that most of us have not read Suiter's books in their entirety. Most of us are in this hobby for fun and have enough years of experience to tell a good optic from a bad one. Being able to precisely quantify optical aberrations is really not necessary to enjoy observing, although it may help provide bragging rights.


Edited by Steve Allison, 16 December 2019 - 09:24 PM.

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#43 Steve Allison

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Posted 16 December 2019 - 11:58 PM

Both of my excellent long-focus achromats have a definite best focus point. While maybe not as critical as with my F/7 and F/8 apos, some careful focusing is still required to get the sharpest image.

 

Lesser quality achros, especially long focal length models, can indeed have a vague or mushy focus, but this is easily distinguished from the much more precise focal point of the best achromats.

 

I would advise potential refractor purchasers to look through several high quality examples to learn the differences in focusing (and everything else) between the good and the bad.


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#44 emflocater

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Posted 17 December 2019 - 12:11 AM

The only "Snap" I have seen and had was on my Jason 311 60 mm f11.6 refractor when I was carrying it outside and the plastic finderscope hit the edge of the door fame and "Snapped" right off! bigshock.gif

 

Cheers

Don


Edited by emflocater, 17 December 2019 - 12:14 AM.

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#45 beanerds

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Posted 17 December 2019 - 06:37 AM

This is probably the toughest test , looking at tree branches against the daytime sky with the sun getting high .

 

Here is my 110mm Long Perng f6 APO at about 130x , not to bad  I think ,  but this is an unforgiving test ,,, a lot like Venus .

 

Beanerds.

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#46 emflocater

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Posted 17 December 2019 - 09:11 AM

This is probably the toughest test , looking at tree branches against the daytime sky with the sun getting high .

 

Here is my 110mm Long Perng f6 APO at about 130x , not to bad  I think ,  but this is an unforgiving test ,,, a lot like Venus .

 

Beanerds.

I have done the same thing with all my refractors just like your picture of trees and branches against a bright clear blue sky! I can say that I was very satisfied with what I saw through the eyepiece, at least terrestrial wise. I would also crank up the magnification a bit to see the veins in leaves as well. Once a clear night was available after the terrestrial tree branch and leaf vein test, I was delighted to see the scopes performed well astronomically. I did notice that when I used some cheap eyepieces for the terrestrial viewing the sharpness and contrast was off. Once I replaced with a expensive name brand eyepiece the terrestrial view was very sharp and contrast. This is why I posted my question about if a bad focuser or poorly made eyepiece can make a good objective lens seem like a bad objective lens, thus confusing the observer.


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#47 peleuba

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Posted 17 December 2019 - 10:04 AM

Barbie

 

 

First off, when I say snap to focus, I'm talking strictly in the realm of high magnification(under good seeing), so let's be perfectly clear about that point. 

Understood.  I was the one who brought up magnification - you should have clarified that earlier.

 

 

 

Second, I have decades of experience designing and making optics, both professional and amateur and have seen my fair share of scopes that didn't snap to focus produce absolutely lousy images( at star parties).

Yes - We've seen you state this several times on CloudyNights - that you are a "pro" and have "gobs" of experience but, your not the only one.  Your experience is no different then the next guy who has seen something different in his setup and is willing to share and support (when challenged) his results.

 

As someone who is/was a professional/scientist you should understand the notion of differing opinions/criticism as one of the tenets in which the scientific method is based upon, yes?

 

 

 

I also used to be of the same school of thought regarding focal length and depth of focus but I no longer subscribe to that because I've seen otherwise. 

Fine.  But depth of focus is not a myth.  Moreover, I did not, out of hand, dismiss your assertion, I merely gave some evidence that its more complex then your contention that "snap to focus equal good optics".

 

 

Also, you fail to understand that correction for spherical aberration is one of the paramount factors at work here. 

I don't fail to understand anything at least as it pertains to what you are saying.  I have an in-depth understanding of spherical aberration (high order/low order) and the effects they have on the image at the focal plane as well as the other Zernike aberration polynomials of coma, Astigmatism etc.  I have an optical bench and routinely test in double pass at 4 wave lengths - Red/Green/Blue/White.  I can demonstrate the change in spherical correction as a function of wavelength and the compromises the designer makes to achieve best correction at the central green wavelength.  I have a collimator device that generates a point source using parallel light so I can star test, indoors, with no adverse optical effects from the star being too close to the telescope under test.  I also have an interferometer - a Bath - and am building a Michelson which is a variant of the Twyman-Green.  I test optics for friends and for folks looking to sell their telescopes.

 

The failure to understand does not lie with me.

 

 

As for the "depth of focus" issue, you are simply parroting what others have been saying!!grin.gif I've stated in other forum threads that a good optic(well designed and figured) will snap to focus while a bad optic won't, AT HIGH MAGNIFICATION.  I've seen that proven time and again and stand by that statement!!

 

Barbie - I am speaking directly to you when I write this - I do not parrot anything.  I have seen you accuse others of this so its a common theme.  To be clear, I can defend (often with test bench photos) everything I write as well the opinions I give.  These are my own that I have come by honestly through years of experimentation and knowledge having gleaned on my own optical bench as well as the many hours of actual observing astronomical objects.  Moreover, I don't do this in a vacuum.  A friend who is a professional astronomer/optics guru and former team lead on the HST WFPC/WFPC2 is my sanity check when I have a question or doubt my result.  He'll run the same tests on his optical bench to verify/nullify what I see.

 

I get that you don’t like to be challenged on your point of view, but this is the internet and it’s a wild world out there.

 

Anyway to get back on topic, I stand by what I say that "snap-to-focus" is not the singular way in which to pass judgement on a telescope.  A good telescope may or may not (to a greater/lesser extent) have this characteristic.

 

 

 

 

 

***Edited for Grammar/Spelling


Edited by peleuba, 17 December 2019 - 01:09 PM.

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#48 peleuba

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Posted 17 December 2019 - 11:14 AM

Well, Suiter did not qualify his suggestion about snap focus in any way in his Astronomy article, which was written for beginners. It was just one of several tips he gave to roughly assess optical quality.

 

You sort of answered your own query.  Suiter probably did not qualify his suggestion in order to simplify it for beginners and for his publisher - Astronomy Mag.  Too many qualifications and the conversation gets diluted.  Optical testing is often the pursuit of nuance.  We each have greater/lesser abilities to judge nuances and beginners (Suiter's audience) probably have the least.

 

 

 

I am not sure why you think it is sad that most of us have not read Suiter's books in their entirety. Most of us are in this hobby for fun and have enough years of experience to tell a good optic from a bad one. Being able to precisely quantify optical aberrations is really not necessary to enjoy observing, although it may help provide bragging rights.

 

Suiter's book is about star testing.  And, its one of the easiest tests to perform and one of the hardest tests to interpret as there are almost always different aberrations that, when combined, can give results that are hard to sort out.  

 

Bragging rights are one thing, I guess.  But more importantly, for me, its the ability to understand how various abberations affect the image in the eyepiece.  Only rarely does one telescope of same aperture and design "blow away" another similar telescope.  Testing optics, qualitatively (star test, DPAC) requires an ability to judge nuance.  

 

Did you ever get an optical flat?


Edited by peleuba, 17 December 2019 - 11:38 AM.

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#49 jay.i

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Posted 17 December 2019 - 11:45 AM

This thread reminded me of a post I made that outlines what I do to evaluate optics during daytime, not just for optics but mechanics as well (say, if you were to visit a seller to try a scope before you buy it). I finally found it! https://www.cloudyni...t/#entry9313202

 

For ease, I will copy and paste it here, but there is some other chatter in that thread that might be valuable. Also, I didn't really talk about tree branches against a clear sky as a CA test, but that has been mentioned here, and is the most common way to see CA during the day. Move the branch(es) around the FOV to see how the CA changes from center to the edge.

 

Snap to focus has been a good test for me, personally. There is a lot that can be in focus during the day, so your best bet is focusing on something that doesn't have anything else near it, like a road sign that is a bit elevated above the observing position, so the sky is the background, or a roof several dozen feet away. I also like to look at areas that DO have a lot of things near each other, like grass, reeds, cattails, trees, etc. I rack the focuser back and forth and judge just how sharp the sharpest part seems to be, and how best focus drifts from one object to the next as I focus in and out. This is also a good way to see chromatic aberration. The out of focus areas will take on a tint, usually, in a doublet. My TV-85 displayed a slightly purple or green tint overall for out of focus objects, but also added a slight falsely colored rim to edges of things like grass or cattails or tree branches.

 

What to bring:

 

* Your best (or typical, I guess) mirror diagonal, and best (or typical) prism diagonal.

* Your best (or typical) eyepieces, at approx. lowest mag, second lowest mag, medium mag, and high mag. If you anticipate local seeing will be good, bring your best highest mag eyepiece as well. You can also include your favorite eyepiece(s) if you have one, even if it's in between those categories.

 

As far as success criteria goes, I would look for the following:

 

* Clear indication of focus. This is "snap focus" that people talk about. Even great optics with low SA p-v won't always OBVIOUSLY have a perfect point of focus, but you should be able to easily get something in focus without having to fuss too much with the fine focus knob. You should be able to rack far in, then rack out at a consistent rate and STOP when you think you're about to hit focus, and hit it. You won't get it every time, but I'm surprised how often I nail focus seemingly on a whim with my good optics. I just stop and bam, it's in focus.

 

* Impeccable sharpness at the center. We're talking "wow that's sharp" provoking thoughts. Not "that seems pretty good". This WILL depend on magnification and local seeing conditions, however, so compare it at low, medium, and high mag. You probably won't get stunning sharpness at high mag during the day with terrestrial objects; you're just too close to the ground and the air will prevent the clarity you want. Definitely try it at multiple magnifications. I would personally want a stunningly sharp center focus at a ~2mm exit pupil for daytime objects. At lower mag, almost anything looks sharp, but at medium to high mag is where you'll start to see a difference.

 

* Complete lack of fog or haze. At higher mags, the low contrast might be mistaken for a fog or haze affecting the lens, so use low-medium mag for this. The optics should seem transparent, with no edge brightening, no fuzziness to the image, and no cloudiness. There should be no un-evenness to the clarity of the image. If you notice any sort of contrast loss, across the whole image, or in one area, check eyepieces and diagonal for dirt/smudges. If it persists, check lens with flashlight. Likely not caused by even what some might call severe dust, but could be caused due to a residue left over from poor cleaning, or possible emerging fungal problem. I'd expect this problem to be evident just by looking at the lens and to be honest I would not expect it, period.

 

* No rattling or other noises in the objective. This is a mechanical thing. Some Takahashi objectives apparently DO rattle per manufacturer spec due to how they've manufactured the cells, but I would absolutely verify with the manufacturer first if you do hear such a thing, before putting any money on the table.

 

* Smooth focuser mechanics. No grinding, rubbing, or slipping. If you have a heavy diagonal and eyepiece, bring them and point the scope at 60deg altitude. If it doesn't slip without any tension, point it all the way up to zenith. If it starts to slip, try the tension knob. You may just have to put some pressure on it (to be expected). Most focusers will require some tension at zenith with a 2lb+ load on the back. If there is grinding or rubbing, the drawtube may be misaligned, but it might be fixable with screws on the focuser itself. This is something that would not prevent me from buying the scope after looking through it, sans any other issues, but I would probably ask for a discount considering I would have to fix it myself, and it might as well be considered defective in that state. Just because a crappy focuser is "good enough" for someone doesn't mean it's a properly functioning focuser, same as how just because mediocre or average optics are "good enough" for someone doesn't mean they're actually good optics.

 

* Smooth dew shield mechanics. It should stay in place even when approaching zenith, but some will slip. Wouldn't be a deal breaker for me as you can probably put some sticky tack or something behind the dew shield once you've extended it, and remove it after the observing session, if it slips easily. It is also possible to have a silicone ring created that would slip over the dew shield onto the OTA, and you'd roll it up behind the dew shield as you extend it, then roll it back down onto the OTA to retract the dew shield. If there is a locking screw then just make sure it works and doesn't mar the OTA if you accidentally try to slide the dew shield down with the screw engaged.


  • j.gardavsky likes this

#50 j.gardavsky

j.gardavsky

    Apollo

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Posted 17 December 2019 - 02:57 PM

Just  two comments on the terrestrial views tests

 

The snap-in-focus depends also on how fast the telescope optics is.

On my 6" F/5 achro this snap feels like within 0.05mm, and it helps to put a helical microfocuser on the prism or diagonal mirror, to feel the snap-in-focus more precisely.

 

The longitudinal CA (C-e-F deviation) in the eye relief space behind the EP can be also estimated.

Take the H-Alpha, the green Solar Continuum filter (close to the e-line), and the H-Beta filter. I place the filters on the eye lens of an eyepiece, snap the view in the focus and take the focuser reading.

When the C-e-F deviations are less or at least the same as  what is expected from an achromatic refractor in its focus space (i.e. focus length of the objective times 0.0005), then you have found a good match of your achro doublet, prism/mirror, and the eyepiece.

 

Best,

JG


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