Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Long-f/ratio instruments and insensitivity to defocus –– amendments to the "scientific proof"

  • Please log in to reply
60 replies to this topic

#51 KBHornblower

KBHornblower

    Viking 1

  • -----
  • Posts: 723
  • Joined: 01 Jul 2020
  • Loc: Falls Church, VA (Washington DC suburb)

Posted 10 April 2021 - 04:13 PM

You are supposed to know more than me.

I'd wager, having been where they track objects with  telescopes,

I know a little more. But....press on.

 

You are the one who claims to know what's happening

 with seeing...you constantly treat it as a  minor fluctuation in focal point.

 That, your premise, is not proven at all.

I have tracked objects with telescopes, specifically close double stars and planets at high magnification, where variations in the seeing are apparent.

 

You are more than welcome to go to post #14 and check out my work line by line.  If you think I am making mistakes at any point, tell us in quantitative detail what you think is wrong at that point.



#52 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • Posts: 19,042
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 10 April 2021 - 04:28 PM

The atmosphere will definitely change the focal point for a telescope.  Observe a planet close to the horizon, and you'll have to chase focus, even if the air at the surface is calm; or, a planet near the zenith, the first clear night after a strong cold front passes through your area.  Try Jupiter and/or Saturn right now in the pre-dawn sky.  Volatility, moisture content, etc. in the air column from surface to space causes other distortions as well.

 

ALL of my F15 & faster achromatic refractors show some degree of blue/violet fringing at the disk edge (brightest on the sunward side, of course).  I assess the air column volatility by how much that fringe expands, contracts, and/or oscillates after a few minutes of tracking & observing.  I'd say that's a strong indicator.  Will that effect be less noticeable in an F15 refractor than an F5?  Yes, as the F15 has less visible CA than the F5 to start with.  If this effect is what folks mean by my slow / high-F refractor does better in marginal seeing than my fast / low-F refractor, it should.  Before APOs, that's why we bought them.

 

Thus I stand by my finding that the shorter scope is no more sensitive to poor seeing, provided all other variables are controlled.

 

The F5 achro has more visible CA than the F15.  Defects in the air column that alter RGB foci will be more pronounced in the F5 than in the F15.  It's easy to see in side-by-sides.  Jupiter at 200x (50x / inch) in my F5, F10, & F15 achros:  Finer details like festoons in the NTZ will appear/disappear in the F5 more frequently than in the F10 & F15 achros.  And fluctuations in the CA fringing at the disk edge are much more noticeable in the F5.  That's one reason why I don't use my RFTs for serious planetary observing.

 

Why equivalent quality between same aperture refractors matters in these SxS comparisons:  In theory, my Edmund F15 should've had a performance edge over my Dakin F10 due to less visible CA.  However, with Jupiter & its lower-contrast details, the F10 routinely out-resolved the F15.  Why?  Both 4" achros had excellent DPAC patterns.  But, the Edmund had 3 paper-disk baffles glued to the ends of cardboard tubes; the Dakin has a single brass tube with 5 knife-edge brass baffles built into it, and this baffle assembly is held in place by screws.  IOW, much more care, time, & expense was put into boosting the Dakin's contrast than in the Edmund.  Again, at the extremes, these small differences matter -- and why field testing some of these ideas is less reliable.


Edited by Bomber Bob, 10 April 2021 - 05:03 PM.

  • paul m schofield and eblanken like this

#53 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 10 April 2021 - 06:59 PM

Seeing can  do at least these things (using a star):

 

 

1)---alter where (on the central axis) rays from a star arrive and converge,

        if and only if all the rays are deflected with  symmetry around the center

          ....a simple changing of the focal point along the axis

 

2)----cause a star to wobble all around in location, without affecting the star's convergence

            (extremely common)...that is, the actual focal point is wandering around

              up down left and right

 

3)---split a star into multiple stars (points), or even shapes like commas

       (you can see this more obviously in videos)

 

4)---cause a star to appear and disappear

          (fairly common: the rays diverge far away

          and are too weak by the time you collect them)

 

5)----disperse the star by means of 'poor lensing' before it gets to the telescope.

 

 

---------------------------------------------

 

2, 4, and 5, if they happen quickly enough , are mistaken for (1) often.


  • eblanken likes this

#54 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 10 April 2021 - 07:15 PM

Huge amount of observation, Bomber Bob.

 

I see how lower-frquency changes in the atmosphere could

   lead to your chasing focus.   Sort of a large mound type lens,

   much wider than your view.   Similar to domes and valleys

   in a temp. inversion.  That's almost another effect, say #6:

    distortionsthat effect the whole image the same....giant 'cells'

 

Other distortions? Oh yeah.

 

The blue fringe can be very long, even in long, tightly focused barrels.

I see it in one eyepiece vs. another and I'm not sure quite why.

All EPs are great in the center, but the pale-violet grey level spreads over things in some.

I can't quite trace it to the spot diagrams, unless it's more the violet spreading

out from itself than from other colors.

 

So...maybe for the F5, you are saying more CA to start with 

adds to seeing-related color?   That might provide a hint at

a simple modeling of the seeing as a common lens type (way out there).

 

You're saying the quality of the barrel, the stray light treatment, helps

  ...I can think of a few ways that could happen, but I think nearby

    stray light (and it's edge diffractions, for example) hasn't been looked into much.

 

One possibility is that by  getting the black level as low as possible,

you optical cortex is tracking and fixing things, more than you are conscious of.


Edited by MartinPond, 10 April 2021 - 07:17 PM.

  • eblanken likes this

#55 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • Posts: 19,042
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 11 April 2021 - 08:11 AM

What I'm saying is what's been said umpteen times on this Forum:  The final View is affected by the Seeing, the Object, the Telescope, and the Observer.  And, each of those 4 main factors has sub-factors.

 

As I understand it, the OP is proposing that DOF (Depth Of Focus) is a Telescope sub-factor that affects image stability.  Obviously, there's disagreement on this claim.

 

CA is a known sub-factor.  It can be predicted mathematically, and measured for a particular refractor.  And, we can see it without specialized equipment.

 

Seeing is a critical factor.  It can be not just volatile, but chaotic.  For most of us, our assessment of it is subjective, or relative to other sessions, instruments, etc.  

 

Given just 2 known factors, it's difficult to prove that DOF is a significant sub-factor.

 

FYI:  My CATs have many more instances of chasing focus than my refractors, and attribute it to inherent Telescope traits.  When the performance of my known good refractors is sub-par, I can almost always blame the seeing.


  • barbie and eblanken like this

#56 barbie

barbie

    Surveyor 1

  • *****
  • Posts: 1,532
  • Joined: 28 Jan 2013
  • Loc: Northeast Ohio

Posted 11 April 2021 - 08:07 PM

I've found the same to be true here at my observing locations here in N.E. Ohio! The optics in my scopes always perform well under steady skies when seeing and transparency are above average. I've noticed this with my long and short focus refractors, Newtonians and CATS. No matter how good the scope's optics are, poor seeing will always yield a poor image.


  • Bomber Bob and eblanken like this

#57 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 11 April 2021 - 08:35 PM

I've found the same to be true here at my observing locations here in N.E. Ohio! The optics in my scopes always perform well under steady skies when seeing and transparency are above average. I've noticed this with my long and short focus refractors, Newtonians and CATS. No matter how good the scope's optics are, poor seeing will always yield a poor image.

In scientific/legal/logical debate terms,

  you flag the whole debate on "relevance".

Seeing can devastate any kind of performance.

For me, that is especially true, in Northeastern Mass. 

I can't see any of the above digging me out of a bad sky.

Point taken.   A lot of soup up there.

 

I like the stray-light/baffling point,

   but that just helps with collateral light pollution, another matter

   that only compounds seeing problems.

So...why does some little difference at the highest power

 matter when the seeing is bad? 

 

Here is a case that extends your point at the cheap end:

long ago, before I did much with telescopes, I hiked up to

a mediocre 3500ft summit in Maine, not super high. 

I had a 20mm orthoscopic, nicely cleaned, 

  in a garage sale Jason Spacemaster scope,

  nicely cleaned (and nicely baffled!)  60mm by 800mm.

I still remember the images.  I have rarely seen sharper since.

(the mount had the rickets real bad, though).

 

I can't argue with your point..

We're discussing how many angels can dance on the head of a pin here.

 

"Scientific Proof" is not done by equations.

Theory drives equations that make predictions that are proven 

  by corroborating experiments. That all gets tossed if there is a

   common factor that makes the point moot.


Edited by MartinPond, 11 April 2021 - 08:39 PM.

  • barbie and eblanken like this

#58 gwlee

gwlee

    Mercury-Atlas

  • *****
  • Posts: 2,586
  • Joined: 06 Sep 2015
  • Loc: 38N 120W

Posted 11 April 2021 - 09:20 PM

Refractors with FRs longer than about f8 seem to be irrelevant to most observers these days for various reasons unrelated to insensitivity to defocus.
  • eblanken likes this

#59 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 11 April 2021 - 11:26 PM

Refractors with FRs longer than about f8 seem to be irrelevant to most observers these days for various reasons unrelated to insensitivity to defocus.

The long scopes have their fans, though.

Some long-FL scopes are popular and hiding in plain sight:

--There are even many F7.5 ED scopes.

---Lots of 80mm/F11s out for starter scopes....saves a lot making simple EPs shine.

   

 

What boat you buy depends on what you are fishing for.  

This is generally a luxury forum.  People have moved up 2-3 times.


Edited by MartinPond, 11 April 2021 - 11:27 PM.

  • eblanken likes this

#60 gwlee

gwlee

    Mercury-Atlas

  • *****
  • Posts: 2,586
  • Joined: 06 Sep 2015
  • Loc: 38N 120W

Posted 12 April 2021 - 01:12 AM

The long scopes have their fans, though.

Some long-FL scopes are popular and hiding in plain sight:

--There are even many F7.5 ED scopes.

---Lots of 80mm/F11s out for starter scopes....saves a lot making simple EPs shine.

   

 

What boat you buy depends on what you are fishing for.  

This is generally a luxury forum.  People have moved up 2-3 times.

Yes, but I expect that a poll of the visual observers in the refractor forum would find a much larger percentage of refractors below f8 in use than refractors over f8 despite their charms. The ergonomic advantages of shorter FLs are instantly obvious to most observers while the optical performance claims made for longer FLs are examined, endlessly debated, and ultimately passed over in favor of ergonomics by most observers these days. AP enthusiasts seem to have other reasons for passing over long FL scopes as do night vision enthusiasts. 



#61 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 12 April 2021 - 11:08 AM

For the most part, the excitement in the forum is about ED scopes.

High and low power in the same body, for an extra price.  It makes sense.

Polling the forum wouldn't reveal what's consumed in general.

Only if ED scopes decline a lot in price would they become the genl. market favorite.

Of course, that might just happen in a while.....




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics