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what is it that makes you believe you really do have good optics?

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#1 Daniel Mounsey

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Posted 08 November 2019 - 06:34 PM

What's your definition of good? How to you determine that?

 

If you are one of those observers who is perfectly content and just wants to view the night sky, then this is perfectly fine too.


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#2 RadioAstronomer

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Posted 08 November 2019 - 06:51 PM

I'm not an expert in optics evaluation here but this is what I look for:

 

1) Collimation, it has to be spot on. If a refractor is out of collimation I won't bother looking into anything else.

2) No astigmatism present in star test.

3) Minimal amount of spherical aberration. I want the diffraction pattern to look as similar as possible inside and outside of focus, giving special attention to the first outer ring.

 

My experience tells me that these three criteria by far have the biggest influence on how sharp and "refractor-like" a focused imaged of a planet or luna is going to be. And that's pretty much it. I'm purely a visual observer and I don'r really care about slight amounts o CA in focus. Couldn't care less about CA outside focus as I'm not interested in looking at things out of focus.


Edited by RadioAstronomer, 08 November 2019 - 07:02 PM.

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#3 wrvond

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Posted 08 November 2019 - 07:00 PM

What's your definition of good? How to you determine that?

 

If you are one of those observers who is perfectly content and just wants to view the night sky, then this is perfectly fine too.

I'm all visual as well, though I do have a NexImage 10 to use in my AT72ED - that's as deep into AP as I'm going to get, too!

What I look for:

Can I get what I'm looking at into sharp focus?

Is there any obvious CA?

 

I've put a lot of work into my XT10g, so when somebody I respect looks through it and says "Wow, that looks really good!", I'm pleased and confident it's as good as it can get.


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#4 Bomber Bob

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Posted 08 November 2019 - 07:01 PM

Before my DPAC rig:

 

Visually sharp planetary limbs (Mars / Jupiter / Saturn)

- Average = 50x / inch of aperture

- Above Average = 75x / inch

- Outstanding = 100x / inch

 

My problem with visual tests are my eyes -- getting older makes it tougher to assess view quality!

 

With my DPAC rig:

 

- Average = 3 straight bars at IF (Inside Focus) & OF (Outside Focus).  My 1970s Towa 50mm F12 achro.

- Above Average = 3 thin black bars @ IF & OF.  My 1964 Royal Astro Optical 76mm F15 achro.

- Outstanding = 3 thin black bars @ IF & OF that match (bar width & intensity).  My 1995 Vixen FL80S F8 fluorite doublet.

 

DPAC doesn't lie.  If I see a big issue in the patterns, I'm gonna see it at the eyepiece.

 

The Keepers in my signature block are either above average / outstanding; OR, there's something special about that refractor -- age, collectability, etc.


Edited by Bomber Bob, 08 November 2019 - 07:31 PM.

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#5 TOMDEY

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Posted 08 November 2019 - 07:05 PM

Ideally... it would be good polychromatic wavefront across the entire designed field: 0.015 wave rms single pass normalized to green was our common requirement at work, on almost everything we sold, but sometimes required much tighter. I was tasked with certifying that, using null-test interferometry.

 

In the context of a visual-use refractor, that would be over a flat field ~45mm across. That assures you will only be atmosphere limited; your scope will not contribute to degradation.

 

In actual practice, for visual home astronomy... it wouldn't have to be that good... and could still perform "goodly"...    Tom



#6 wrnchhead

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Posted 08 November 2019 - 07:19 PM

I wish I knew someone personally who could skillfully assess. I only have my own experiences. 8 in GSO newt, 4.25in Edmund from the 60s. 90s Ultima C8 OTA, Orion ED80 and now my nicest one yet, the WO GTF81. I definitely appreciate the sharpness of the fracs, but obviously a little weak visually on planets and DSOs. Did a side by side with my WO and my ST80, and wow was a difference that correction made on the moon! 

 

The biggest collimation lesson I learned about the c8 was learning how to use the Airy disk for collimation. Some well experienced astrophotographers with big youtube channels show themselves using the secondary shadow to collimate. I did that and the views were ok. Well I made a couple of very small adjustments using the Airy disk and holy cow, the difference on m13 was a lesson learned. 


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#7 Sol Robbins

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Posted 08 November 2019 - 07:24 PM

Other than being a stickler for really good collimation, I always critically look at the in focus image quality. Planets are a very good indicator of image quality in showing a range of high and lower contrast details. And I've spent a lot of time observing and sketching planets. I've been using and still use a 6" refractor with a Chromacor II for the lion's share of my planetary sketching over the years.

 

DSO's also have unique details to look for as well. For example, when observing various globular clusters the criteria I look for are star sharpness and differentiation, overall object brightness relative to sky background. How populated with stars the cluster is even with stars at the edge of visibility. The brightness glow of unresolved stars and the extent of the outside borders appearing either as an unresolved glow or as chains of minute stars.

 

I recently purchase an Explore Scientific 8" Dob, which is decent optically. Shortly after this purchase I lucked into a freshly coated R.F. Royce mirror. The Royce mirror does present the sort things I mentioned about globulars better by a bit in concert with my bias toward high contrast eyepieces with minimal glass.

 

M13 was a revelation at 310x using the new mirror and good seeing. The propeller was obvious. M64 showed a grainy kind of appearance rather than a uniformly diffuse glow. Very dim minute stars that are superimposed over M51, M27 and M57 held constantly with direct vision, without blinking in out. So, I got a bit of a boost with the Royce over the stock E.S. mirror regarding details you really have to look for. Besides, the Royce mirror came with test papers. wink.gif

 

So even though there's a lot validity and interest in trying to get an empirical read regarding optical quality, the proof is in getting critical when observing images at hard focus when conditions are good.


Edited by Sol Robbins, 08 November 2019 - 07:37 PM.

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#8 Nippon

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Posted 08 November 2019 - 07:34 PM

A clean airy disk at focus at high power on a steady night. Intra and extra focal star test patterns can be fine as a test if you know exactly what you are looking at. If you don't what you see can cause unwarranted concern. As an example I have a 4" ED doublet that has identical out of focus star patterns either side and a 4" ED triplet that has slightly different out of focus patterns either side of focus yet both scopes are equally sharp at focus showing the same amount of detail on planets and the Moon. And both have text book perfect airy disks.


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#9 John Huntley

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Posted 08 November 2019 - 08:07 PM

I guess I try and push the optics as far as I can using the most challenging targets that I can think of for the aperture. I tend to go for tight and unequal double stars, subtle planetary features (not the obvious ones), very fine lunar features, fainter planetary moons and that sort of target as tests. For me, it's important that I know the targets that I'm using as testers pretty well so that I can form a view on how well, or otherwise, my optics are doing. Obviously I also take into account seeing conditions and make sure that I repeat my test observations under a variety of conditions before forming any conclusions.

 

As soon as I stop concerning myself about how the optic is doing and just start enjoying the views, I know that I've got a good one smile.gif


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#10 bobhen

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Posted 08 November 2019 - 08:26 PM

What is your definition of good…
If an optic delivers “fine detail” on Jupiter’s disk at “high power” without haze softness or color error.
If the optic can deliver good/sharp views of the moon and planets even in average seeing and not be overly compromised when the seeing is average.
When observing a “bright” lunar peak, can the optic deliver a sharp view of details and tonality on that peak at “high power” without the slightest spray of defused light or obvious color.
Excellent star sharpness in focus in good seeing with no surrounding fuzz or haze or junk.

 

How good is determined…
Star Test evaluation in focus and defocused at high power
Fine Jupiter disk details at high powers
Contrast and sharpness evaluations compared to other scopes
Testing in different seeing conditions

 

Other considerations...
Did the optic come from a company with a “longstanding” reputation for "high" quality

Price

 

Bob


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#11 Jeff B

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Posted 08 November 2019 - 09:27 PM

This.  And a good star test.

Attached Thumbnails

  • 140ED Green Outside of Focus.jpg

Edited by Jeff B, 08 November 2019 - 10:30 PM.

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#12 Jon Isaacs

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Posted 08 November 2019 - 09:38 PM

What's your definition of good? How to you determine that?

 

If you are one of those observers who is perfectly content and just wants to view the night sky, then this is perfectly fine too.

what is it that makes you believe you really do have good optics?

 

 

Daniel:

 

The simple answer is this:  

 

All my scopes have essentially perfect optics, everyone else's have incredibly bad optics.  As soon as I buy a scope, the optics are magically transformed from average/mediocre into amazingly perfect surfaces.  It's just a special talent I have.  lol.gif

 

Now that we all understand that:

 

I basically look for aberrations like astigmatism, I look at the Airy disk, I look at resolution (see close double stars), I look for clean, crisp planetary views at higher magnifications, I do look at inner and out of focus images, I look at collimation and well centered diffraction patterns, I look at comparisons with other similar aperture telescopes.  

 

And in reality, good is the operative word.  I am not looking for great optics, I am looking for decent optics with no serious flaws.  I take care to prepare my equipment and accept the views for what they are.

 

I believe that my equipment is more capable than I am, I  believe that a better observer than I using my equipment would see more than I can see so as long as that remains true, I am happy with my junk.  

 

I feel blessed, fortunate, to have the opportunity to enjoy the night sky 150 or more nights a year. I feel blessed to have a nice little hideaway in the high desert with a wonderful friend for a neighbor where the skies are most often clear and relatively dark. This represents a far greater investment (and operating expense) than any of my equipment but with any luck, it's value is increasing so that when I am gone, when my wife is gone, the kids and the grandkids will have something more to help them on their way.  

 

For me, there comes a point when I say to myself, enough is enough.  Just how much does one man need to be satisfied?  I could buy fancier equipment but it's unnecessary..  I have lived a life that very few people on this planet can even dream of.. enough is enough.

 

Jon


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#13 Wildetelescope

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Posted 08 November 2019 - 10:30 PM

What's your definition of good? How to you determine that?

 

If you are one of those observers who is perfectly content and just wants to view the night sky, then this is perfectly fine too.

First, context.  I am over 50, have astigmatism in both eyes (2.5-3), live under the jet stream, in a red zone.  Also, I often see 10-20 degree temperature drops over the evening.  So, take anything I say with a grain of salt;-).   I am not much of a star test analyzer, beyond whether or not the scope is in collimation.  I typically look at contrast on Jupiter as one comparison.  I also look at how the image sharpness changes with magnifications over 50x per inch.  With my better scopes, I also look for the three dimensional feel looking at Jupiter and it’s moons.  I think all of the scopes in my signature are good examples of their design.  Jupiter and its moons really are a good test bed for telescopes.   Many other people have said this, so it is not a profound observation on my part.  I lean toward vintage scopes, so all my refractors show minimal color on Vega or Sirius.  Only the 120 mm achromatic shows color on Jupiter.  When collimated, the 10 inch dob gives the best combination of contrast, brightness and resolution.  The five inch AP triplet circa 1989, is something special for its aperture, among the scopes that I own.  On most targets it’s contrast is exceptional, and images are vivid and crisp.  It stands apart when compared to my six inch mak, and holds its own with my c8 edge on planets and dso’s.  Such is my very subjective assessment from my comparisons.  

 

In in the end I enjoy all my scopes, and loose sleep LOOKING through them, rather than worrying about their strehl numbers.  Right now i am having great fun looking at the planets with my son through a 60 mm Sears achromat for which  i paid 50 bucks.  It as actually pretty darn good.  And my Son loves using it.  I suppose that is not a bad definition of good. 

 

Cheers!

 

JMD


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#14 k5apl

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Posted 08 November 2019 - 10:39 PM

I find that there is something I cannot define when looking through a telescope with excellent optics. Maybe it is best seen when I have been looking through good optics, then switch to the excellent optics.  Or, visualizing my previous views through good optics then looking at the same object with excellent optics.  When I have seen that the scope is excellent, then all of the tests (not DPAC) mentioned before just confirm what I've already seen.  And, its more pronounced since I have had cataract surgery and see 20-20 or better with no filters (discolored eye lens).  Wish I could be more specific;  its sort of like I know when I see it.

Two scopes stand out in my memory:  The TMB 100mm f8 art-deco scope that weighed a ton, and a friend's home

made 8 inch Dob (made by an amateur in California is all I know).  I have the similar impression now with my new

92 Stowaway.  Another good candidate was one of the several SV 80/9Ds that I owned:  It was exceptional optically although it was battered and scratched and didn't look so hot.  I damaged by accident the TMB, didn't own the Dob,stupidly sold the 80/9D, but will hold on to the 92.

And, they all perform better than expected taking in account their aperture......

 

Addition:  When a refractor owner looks through my scope and says "What a good view.  Who made that scope?".


Edited by k5apl, 09 November 2019 - 09:27 AM.

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#15 PirateMike

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Posted 08 November 2019 - 10:39 PM

For me, there comes a point when I say to myself, enough is enough.  Just how much does one man need to be satisfied?  I could buy fancier equipment but it's unnecessary..  I have lived a life that very few people on this planet can even dream of.. enough is enough.

 

Jon

Yes, a person must imagine a line in the sand and say "When I am at this line I will be happy." and then be happy with where they are and what they have and not feel the need or want for more.

 

The ability to do this is the one thing that I can really respect. I feel that I am on that line, even with what little I have.

 

You can call it the "Happy Line".

 

 

Miguel   8-)

 

.


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#16 Joe Eiers

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Posted 08 November 2019 - 11:02 PM

  First, I MUST quote my favorite signature from a fantastic administrator here, Chuck Hards of the Classics forum.

 

{Optical testing has ruined more telescopes than any other cause.    

 

"Test an optic for someone, and you've disappointed him for a day.  Teach him to test his own optics, and you've disappointed him for a lifetime."  - C.H.}

 

  I cannot agree enough.  I've been involved with many gifted individuals that were master's of optical figuring over my 50 years of astronomy;   I've seen too many people disappointed over the tiniest flaws in their systems.  Not just disappointed, but rather frustrated and disillusioned over the whole thing.

 

  When it comes down to it, I know I have good (enough) optics if I can get a hard focus in good seeing on the planets (stars too), the Jupiter shows fine detail, and the background is reasonably dark for good contrast.   I won't even get into the whole eyepiece thing!  Sure I have lots of "Al's green goodies" but I spent two years comparing plossls/orthos and the differences were either too small to call or not at all.  I enjoy the wide fields of Televue.

 

  I hate to see good scopes get thrown to the dogs just because their not made by a "premium" mirror maker...

 

  Still, I love what Jon said a few messages up! Great advice!

   Joe


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#17 Jeff B

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Posted 08 November 2019 - 11:13 PM

Who was it here on CN's that said something to effect that "There is just a something, a lack of artifact, an intensity and you know it when you see it"?

 

In my audiophile forums we would engage in very similar discussions about our "systems".  One memorable quote was "There are systems that produce great sound and there are systems that produce great music.  When you get both in one system, that's when the real magic happens."

 

Kinda like when great optics and great seeing line up.

 

Jeff


Edited by Jeff B, 08 November 2019 - 11:21 PM.


#18 Mike Spooner

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Posted 08 November 2019 - 11:17 PM

Probably my best definition (no doubt I have several) is when I forget about testing and marvel at the observation. A couple of years ago I finished a scope with an 8" mirror that I couldn't see any way to improve by the bench tests (DPAC). I have access to several decent scopes but that 8" contributed to a huge amount of sleep deprivation that season with Jupiter and Saturn. Just couldn't get enough. Almost every view was exciting like the first time and I've been doing this over 50 years. Yes my seeing was great but that scope...

 

Mike Spooner 


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#19 Jason B

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Posted 09 November 2019 - 12:16 AM

What's your definition of good? How to you determine that?

 

If you are one of those observers who is perfectly content and just wants to view the night sky, then this is perfectly fine too.

If I enjoy the view, the scope is a good one. 


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#20 Illinois

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Posted 09 November 2019 - 06:50 AM

Able to see double-double star in Lyra at only 80 power and I can see E and F star in M42 at around 180 to 200 power then I am happy with my refractor! I am happy with my 9 years old 80ED and new SW150!


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#21 t.r.

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Posted 09 November 2019 - 07:14 AM

Simple...Star Test and then swing over to Jupiter...the image the scope puts up tells me all I need to know.

#22 Cloudystars

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Posted 09 November 2019 - 07:16 AM

A good star test, my ronchi grid eyepiece, and a nice snap to focus on a star.



#23 Asbytec

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Posted 09 November 2019 - 07:47 AM

Like many folks have said or implied, the first thing that comes to mind is a nice image in good (enough) seeing when thermally stable and well collimated. A nice snap to focus and a "sharp" image are good indicators. Most of us can recognize a nice Jovian view when we see one.

For better or worse, I usually put all my scopes through their paces using a star test over various nights under different conditions. Not that I spend all night doing so, but a few minutes to get a good look until I'm satisfied there are no gross errors or I have an idea of what is present. If there is some error, say astigmatism for example, I will work to minimize what I can by addressing the usual suspects.

When it's as good as it can be and provides sharp images and snappy focus, as above, with little or no gross errors including being as smooth as it's possible to determine, then I'm good with it. Such an optic does not have to be sensibly perfect, but the closer it is to being sensibly perfect the better off we'll be, of course. IMO, being good enough is, well, good enough. I've seen some wonderful things through a good enough scope when it is well collimated, thermally stable and seeing was very good.

My personal definition of good is the diffraction limit in terms of performance. By that, I mean a scope that can put close to 80% of it's light into the Airy disc. It's said this is the point where the image is "decidedly prejudicial". That's fine, I consider it good enough.

How one determines this probably requires "taking no prisoners" with an IF, or other reliable quantitative method. You may get away with some lucky and loose guesswork with qualitative methods (star test, Ronchi, Aberrator, Suiter, etc.) and using some theory and mathematics. If you arrive, derive, or contrive a final peak intensity figure of close to 0.8 or better, including any obstruction, you're good.

Edited by Asbytec, 09 November 2019 - 07:58 AM.


#24 CHASLX200

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Posted 09 November 2019 - 07:48 AM

When i can view planets at crazy powers and see unreal detail.


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#25 Alan French

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Posted 09 November 2019 - 08:26 AM

When i can view planets at crazy powers and see unreal detail.

I prefer telescopes that show only features that actually exist. ;)

 

Clear skies, Alan


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