161 replies to this topic

[Princess Leah]

Why are stars a little tighter looking through my 80 triplet than my Synta 80ED?

Is this mostly due to correction of spherical aberration?

 

Or can the human eye detect a tiny amount of CA,  that doesnt manifest itself in false colour, but yet alters the shape of stars?

[Daniel Mounsey]

You first would want to explain which telescopes specifically you are comparing because there could be several reasons. Are the focal lengths of the two the same? Are you using the same eyepiece in both scopes? Do you have astigmatism yourself or maybe you do and you may not know it? Often at lower magnifications as you described in another post could contribute to this because the exit pupils increase. False color becomes less evident at lower magnifications, so something seems off. Spherical aberration is something you notice more at higher magnification, not lower. 

[db2005]

It's probably a matter of optical design and optical quality.

 

My Synta 80 mm ED exhibited some spherochromatism which was very evident at around 150x magnification. It made stars look less tight than they do in better apo doublets.

[Princess Leah]

No this is a general question relating only to optical quality.

 

Like everyone else on CN, I can distinguish how the sharpness (most point like - trying to find the right words!) of stars vary dependent on the optical quality of a telescope.

 

We can notice this at even modest magnifications.

For examples stars are not as point like in a ST80, compared to a 80 'APO'.

My question is can this be due to CA?

Or would CA always reveal itself in false colour/halo.

 

What would make the stars less point like / pristine in a ST80?

If it is CA, why don't we see false colour/violet halo?

[Starlancer]

I believe this is an area where newer cheaper APOs are sacrificing for price.  If you look at some of the spot diagrams you can see the size tends to be larger, they seem to be relatively well corrected across the spectrum but have larger spot sizes for focus.

 

My TOA-130 has the tightest stars I've seen from a scope and that's reflected in the spot diagram for it.  

 

I am not an optical expert, Just an observation I've made

[Princess Leah]

I believe this is an area where newer cheaper APOs are sacrificing for price.  If you look at some of the spot diagrams you can see the size tends to be larger, they seem to be relatively well corrected across the spectrum but have larger spot sizes for focus.

 

My TOA-130 has the tightest stars I've seen from a scope and that's reflected in the spot diagram for it.  

 

I am not an optical expert, Just an observation I've made

Thanks, that's interesting.

So even when we are not seeing any colour at low magnification, is it still having an effect on how tight the stars are?

 

What does it mean 'larger spot size for focus'? What makes the spot size bigger?

Edited by Princess Leah, 24 July 2024 - 02:09 PM.

[jrazz]

FWIW I think my FC-100DC (a fluorite doublet) matches my TOA (an ED triplet) for "pinpointiness". Ultimately at some point it probably doesn't matter anymore and your eyepiece and diagonal start making more of a difference.

 

I should say that for those they match with the same eyepieces meaning the exit pupil is the same. If you try to match magnification then the greater aperture of the TOA means it can reach farther for the same "pinpointiness"  (a term which I shall trademark henceforth!)

[Wildetelescope]

My comparison of my TMB F6.2 105 and my Tv 102 saw no real difference in sharpness of stars. Just a slight difference in contrast in features on Jupiter. I suspect what you see is due to variability in sample quality.

Jmd

[ris242]

My 60ED shows pinpoint dots.  I wouldn't expect a 60mm triplet to be any better.

 

then again I don't run a 40mm+ eyepiece in it with a 7mm exit pupil and fire glare at me.

[Daniel Mounsey]

Thanks, that's interesting.

So even when we are not seeing any colour at low magnification, is it still having an effect on how tight the stars are?

 

What does it mean 'larger spot size for focus'? What makes the spot size bigger?

 

 

I hate to tell you this, but you are working your way into a rabbit hole because you are already necessarily assuming these are the reasons why you are seeing this. You really should begin asking yourself some very basic questions first. You'll understand all this better if you just answer some basic questions. Sometimes the simplest answers elude end-users. Like the others have mentioned, you can also get pinpoint stars with doublets. 

Edited by Daniel Mounsey, 24 July 2024 - 03:47 PM.

[RichA]

Why are stars a little tighter looking through my 80 triplet than my Synta 80ED?

Is this mostly due to correction of spherical aberration?

 

Or can the human eye detect a tiny amount of CA,  that doesnt manifest itself in false colour, but yet alters the shape of stars?

The triplet must be better.  Star images are  the same size in any 80mm scope.  Boost the power high enough, use a filar micrometer and measure them.

Edited by RichA, 25 July 2024 - 09:37 AM.

[Princess Leah]

I like rabbits, so no problem there.

 

So I often read reviews where this is mentioned. For instance in one review of a Skywatcher Evolux they compare it with the Televue 85. They mentioned how the stars in the Televue were just a light bit tighter/sharper - a little more sparkle.

 

I've looked through a couple of Televue's and I know what the reviewer meant.

 

So the simple question is what is the differential factor here. CA or spherical correction, or a combination of many things.

[Daniel Mounsey]

I like rabbits, so no problem there.

 

So I often read reviews where this is mentioned. For instance in one review of a Skywatcher Evolux they compare it with the Televue 85. They mentioned how the stars in the Televue were just a light bit tighter/sharper - a little more sparkle.

 

I've looked through a couple of Televue's and I know what the reviewer meant.

 

So the simple question is what is the differential factor here. CA or spherical correction, or a combination of many things.

 

Again, you are making assumptions about things using two different scopes. Many scopes have slight differences when you compare them, but there may be simple reasons why you are seeing this in the first place. Why don't you just clarify what eyepieces, diagonals and the actual scopes you are using specifically? If you knew for sure how to evaluate your own scopes, you wouldn't have to ask others about diffraction patterns here right? There's ways to test this but you really should provide others with better information so they can assist you. The story about the TV85 could be a number of factors. 

 

https://www.cloudyni...pattern-better/

Edited by Daniel Mounsey, 24 July 2024 - 09:30 PM.

[SandyHouTex]

Ahh, no.  Light is a wave from an Airy disk perspective.  It's size is defined by the wavelength of light you're looking at, and the diameter of the objective.  Nothing else.  The equation is here with more information:

 

https://en.wikipedia.../wiki/Airy_disk

[Echolight]

Ahh, no.  Light is a wave from an Airy disk perspective.  It's size is defined by the wavelength of light you're looking at, and the diameter of the objective.  Nothing else.  The equation is here with more information:
 
https://en.wikipedia.../wiki/Airy_disk

Ha!ha! That bottom pic looks like Sirius in the yard cannon!

[Oldfracguy]

Why are stars a little tighter looking through my 80 triplet than my Synta 80ED?

Is this mostly due to correction of spherical aberration?

 

Or can the human eye detect a tiny amount of CA,  that doesnt manifest itself in false colour, but yet alters the shape of stars?

I think you are probably on the right track there, in addition to your 80mm triplet being a good copy of one with a very high Strehl.  Most of these ED Doublets from makers like Synta (Sky-Watcher, Celestron, the older Orion) and Kunming United (Astro-Tech, TS-Optics, Altair Astro, William Optics, etc.), even the ones with better "SD" glass like FCD-100 or FPL-53, will exhibit a little Spherical Aberration.  That can be detected by comparing the diffraction patterns seen whilst defocusing inward and outward slightly.  However, others here on CN have reported that the better Doublet reftactors like Takahashis will show hardly any evidence of Spherical Aberration, and provide pinpoint stars that match those of good Triplets.  I have yet to try one myself, but I believe what these fortunate folks have witnessed.

[Princess Leah]

Thanks Oldfrac. 

As you say it could just be the Strehl value. (so a combination of four).

I found the following very useful.

https://www.telescop....net/Strehl.htm

 

 

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[Highburymark]

I like rabbits, so no problem there.

So I often read reviews where this is mentioned. For instance in one review of a Skywatcher Evolux they compare it with the Televue 85. They mentioned how the stars in the Televue were just a light bit tighter/sharper - a little more sparkle.

I've looked through a couple of Televue's and I know what the reviewer meant.

So the simple question is what is the differential factor here. CA or spherical correction, or a combination of many things.

I thought your questions were perfectly sensible - largely because spherical aberrations aren’t well understood, certainly by people who are still learning about refractors. And it’s a valid question to ask if correction for spherical aberrations is generally better in triplets than doublets.
As for the above comparison between an Evolux and a TV85, my understanding is that the TV85 may display a little CA on the brightest objects, but it’s extremely well corrected for other aberrations. That shows in the star test of my TV85. A tiny amount of CA under the most testing conditions is irrelevant, unlike spherical aberration which is far more detrimental to views through a refractor.

[Daniel Mounsey]

And it’s a valid question to ask if correction for spherical aberrations is generally better in triplets than doublets.
 

 

There isn't even any relevance to the question in the first place. All telescopes usually exhibit some form of spherical aberration. It doesn't matter if it's a doublet, triplet, quad, Newtonian, SCT, MN, DK, CC, etc. 

[PKDfan]

Why are stars a little tighter looking through my 80 triplet than my Synta 80ED?
Is this mostly due to correction of spherical aberration?

Or can the human eye detect a tiny amount of CA, that doesnt manifest itself in false colour, but yet alters the shape of stars?

Hi Leah !!

All telescopes vary in quality some will be 1/5wave - 1/8 - 1/12th wave etc and no two will ever be 100% alike even two units made side by side at the same time.

All eyepieces focus their light to varying degrees of pinpointiness too.

My old type2 Naglers are maybe not so great for throughput but as far as making TINY spotsizes well they are very very hard to match.

16.5XW does a good job Almost equaling the 20mm T2 Nagler for tightness.

My two small apos have such perfect control of spherical aberration that its effectively nulled out and quite frankly snap-to focus at or greater than 100X/" remains and is the final derivative or outcome of that excellent control of essentially ZERO SA seen at the eyepiece.

I think that great control of SA is paramount to the experience.



Clearest Skies
Lance

[Kim2010]

All my scopes exhibit pinpoint stars at lower power and Airy discs at higher power when seeing is excellent.

[Princess Leah]

So this is the kind of thing I often read on CN.

 

'Apos provide particularly beautiful images of some objects that are hard to match by other designs. In particular, they give extremely good views of double stars and open clusters. In these cases the pinpoint stars and high contrast will often give a more aesthetically pleasing image despite not necessarily showing as much as a larger scope.'

Other contributions on CN mention 'star tightness ' when doing low power sweeping. So what I'm talking about is widely mentioned on CN. Of course that doesn't mean it's fact.

It would seem a few people here disagree strongly with the pinpoint stars assertion!

However it is something that seems pretty obvious to me.

 

For anyone who does see /believe in the difference - my question is as to it's cause.

CA? Spherical or a combination of many aberrations.

When doing a star test on my Equinox 80 doublet I don't see it having worse spherical aberration than my triplet.

However it does have a little more CA. 

[Highburymark]

There isn't even any relevance to the question in the first place. All telescopes usually exhibit some form of spherical aberration. It doesn't matter if it's a doublet, triplet, quad, Newtonian, SCT, MN, DK, CC, etc.

Ok, well that’s of relevance to the original question. So you’re saying the third element in a triplet does not correct for SA in any way? Or is the third element purely there to control CA? Or is there no common purpose for that third hunk of glass, as every telescope is different? Surely those are relevant questions to people trying to understand the performance difference between doublets and triplets?

[PKDfan]

All my scopes exhibit pinpoint stars at lower power and Airy discs at higher power when seeing is excellent.

Yes me too ! Its truly great to witness that !!

As wonderful that to discover there is also that computing two and two together and finding occasionally an answer greater than four is possible (and egads finding sometimes less than four !!).

The awful for scope eyepiece and its polar opposite.

With the 'rightest' eyepiece that one which concentrates BEST the encircled energy through it i.e. most tightly will Magnify stars inherent properties (slightly) More and essentially squeeze more juice out of the orange AKA brighter and more individuality in star temperatures seen at the exit pupil.

Thats the #1 reason i'll never part with Als monstermasterwork. The double cluster at altitude B1 skies are hundreds of individualities floating serenely and is forever etched in my brain especially the red giants but also all the bright O B stars too all due to its vanishingly small spot size creation ala an answer of 4.25 lol.

For better nebulae transmission i'll use my new 16.5XW after sweeping at 45Xs Nagler then scrutinize deeper & with very similiar field characteristics @56.5X edit 54.5X typo-- pinpointiest of stars & resolute pupils of 2.22mm and 1.83mm while losing little true field 1.8° to 1.5°.

Perhaps that will be the superiour experience IDK but both fill a critical niche for me.

Thats not stopping me from getting a 16mm Brandon for 5X work with my 62mm apo as well as some RKEs maybe TPLs.

I'm sure alone the Brandon will offer up a slightly superiour view vrs the 16.5XW and that unknown excites me being an optical lover.

The Right eyepiece is usually not found right away and just good enough is unfortunately somewhat lame for them which got superlative optics so Know your scope and do some research before jumping in and then pray, do your homework and it'll surely be a homerun but a triple is a success too and money talks vividly here !


--LOL i thought i'd get away with spending about 500$ for them yet i'm at 4000$ & I'm not done still looking at another 1000$ BUT I do have two great scopes to contend with.--

The stars in my 30XW and Speers waler series 2 31.5mm are DRAMATICALLY different.

Its my line filter sweeper eyepiece with its 90° afov. where it excells being only 11ounces and has high transmission and has a super easy pupil.

So even a failure has a golden lining.



CS

Edited by PKDfan, 25 July 2024 - 02:47 PM.

[Daniel Mounsey]

 

 

 

The OP admits in another thread being unfamiliar with the star test. That's okay, there's nothing wrong with trying to get some tips. The issue is that the OP, like countless other OP's is still not sure what's causing the differences being seen, also admits not being sure about star testing, yet necessarily wants to go on this path this thread is now on. 

 

Look at he OP's question.....

 

Why are stars a little tighter looking through my 80 triplet than my Synta 80ED?

Is this mostly due to correction of spherical aberration? Or can the human eye detect a tiny amount of CA,  that doesnt manifest itself in false colour, but yet alters the shape of stars?

 

Just look at this from a simple point of view before going on. There's important matters that need to be known first, yet this thread is already on a wild goose chase about doublet vs triplet. There are literally thousands of posts on doublets vs triplets, they're a dime a dozen. I realize simple matters are such a difficult thing for others to fathom these days because they just love pontificating about what they want others to believe they know about optics. 

 

If an eyepiece is used in two different telescopes, it's often the case that the views may appear slightly different. So, what kind of eyepieces were used? Were they plossls, orthos, corrected wide-fields, Erfle's? What's the focal length/focal ratio of each scope? These factors can cause differences including off-axis colors using eyepieces. Eyepieces can produce lateral color. You can't just put eyepieces in different telescopes and expect everything to look exactly the same. What kind of experienced observer would expect this? It's absurd.

 

These are just basic questions, yet the OP just wants everyone to ignore this information like it's not important, while others go on another doublet vs triplet goose chase. If the OP also admits they are unfamiliar with how to star test their telescope, it's fine, but it's also a red flag because it indicates their experiences are limited. Do enthusiasts even look through telescopes anymore?