437 replies to this topic

[Ed Jones]

After a discussion a while back on partial obstruction I decided to build a partially obstructed F/5 Chief instead of the Newt I had started.  Even though it's not fully unobstructed it still has some advantages:

1. The Airy disk is still improved even partially obstructed

2. No spider or spikes

3. Excellent baffeling (>= any Newt)

4. One post with an easy breakdown

5. Long BFL for a bino-viewer

But not as easy as building a Newt, I use off-the-shelf lenses but must flatten the R4 curve (not hard).  The image has 1.5 degree tilt but isn't a lot.  I have the primary back from coating and lenses ordered.  Here is the design.:

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[RajG]

AP just announced a new refractor, the 110GTX, so, naturally, I had to try to model it in OSLO. It's a 110mm f/6, with Strehl >0.94 over 486-656nm (and better than 0.95 for most of that range). Glasses are not specified, but they do say it uses small air gaps.

 

The design below has comparable performance over 486nm-656nm, the e-line is virtually perfect on-axis. The g-line (435nm) is not diffraction limited but wavefront error is less than 0.5 wave PV (AP does not specify performance at the g-line).

 

Attached Files

•  TripletSFPL53SBSM81 110mmf6.len   2.4KB   65 downloads

Edited by RajG, 25 May 2022 - 06:50 PM.

[rflrs]

Surplus Shed recently sent me an e-advertisement that had Hoya
BACD16 (620603) pressings (L10336).  These are [close to, at

least,] isoconvex, 2.625 inches in diameter, 0.75 inches center
thickness and 0.1875 inches edge thickness.

 

What might one want to do with such?  So as to not lose either
diameter or thickness, it had to be designs that included this
diameter BACD16 as a positive lens.  Any additional lenses had
to be using an inexpensive glass type;  I arbitrarily chose to
stay within the Hoya catalog.

 

The first design that came to mind was a simple doublet, adding
a negative E-F2 mating lens.  With this combination, a Baker
lens would have a negative air space when coma and spherical
aberration were near minimal.  Hence, an air-spaced design was
selected.  A space of 0.5 mm at 33 mm off-axis was propitiously
selected; the C and F mean spot locations would only be about
0.13 microns from each other.

 

 dbl10-20.len   769bytes   40 downloads

 

The glass thickness for the BACD16 lens was chosen so that it
would fit within the pressing (leaving about 0.3 mm to spare
at the lens edge):

 

 

Another possible design would be a Cooke triplet.  Again, E-F2
was chosen as a mating element; BSC7 was chosen for the third
element.  To maximize aperture, the BACD16 lens would be last,
putting the BSC7 lens as first.  It ended up with 53 mm aperture
and 140 mm field diameter (about 21.4 degrees) with vignetting
starting at the 70% field.

 

 CookeT03-29.len   1.18KB   36 downloads

 

The maximal RMS spot size occurs near that 70% field point, nearly
matched near the field edge.

 

 

To continue this trend, I looked at Petzval designs.  I could not
find a good solution using BACD16 as the positive lens in both
doublets.

 

Richard

Edited by rflrs, 07 June 2022 - 04:11 PM.

[rflrs]

Of course, no sooner than I open my mouth publicly than I have to backup:

 

 Ptz26-77.len   2.63KB   33 downloads

 

The equivalent Schott glass N-LaSF46A than Hoya TAFD25 might be a better

choice, as it has the same TCE as Hoya BACD16.

 

Both designs may be subject to ghost images reflecting between the two

inner surfaces of the second doublet.

 

Richard

 

Edited by rflrs, 07 June 2022 - 07:20 PM.

[Ed Jones]

  Saved back in 2011 and forgotten I thought it might have been Scott Milligans's 32 inch relay design he helped make but it's a close variant.  It's an F/8 all spherical design with a large well corrected field across the visible spectrum.  It does have a little field curvature but it can be corrected with a plano-concave lens near focus.  Not something I'd ever undertake lacking a good site, dark skies, resources and lots of cash but it's a very interesting design and cudos to Scott.  

  Also here is an F/6 design with planos on 2 of the lenses to make it easier.

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[Mike I. Jones]

Got a request for suggestions on how to make an existing 12.5" f/5 Cassegrain primary mirror into a planet killer, keeping it simple (no Cass or Greg secondary).  This design mates a 1.3" Antares diagonal and the Edmund 63766 negative achromat (FL = -120mm) to give an f/16.6 system with minimal central obstruction and nice sharp imagery over a 1.1" diameter field.  The 2.5" hole in the primary is bigger than the diagonal obstruction!  The Edmund lens and 1.3" diagonal are mounted together in a single barrel.  The orientation of the Edmund lens is critical and should be installed as shown.

 

 12_5 f5 Primary + Edmund 63766.len   1.27KB   84 downloads

 

[lylver]

AP just announced a new refractor, the 110GTX, /.../

Another try, with smaller air gap. r2 & r3 are less curved, edge gap would be around 0.6mm.

Second inner surface r4 & r5 are more classic with 0.2mm, can be a 0.2mm spacer (recalculation needed).

Theorical strehl is excellent for all purposes : RajG is right for this combination, nice range with 0.95 strehl.

Note : my old Atmos does not give same results as OSLO, but the strehl curve is nice to show.

[rflrs]

I actually came up with quite a different solution:

 

   trp06-59.len   2.45KB   31 downloads

 

The Strehl is:

 

 

Richard

[lylver]

Another solution, nearer (over 0.94 at F & C) but I am not conviced about lens spacing.

Attached Files

•  AP-GTX-110f6v.len   761bytes   41 downloads

[RajG]

Well, A-P did specify small air gaps for the 110GTX. With large air gaps, this solution using N-BK7 as the mating glass is diffraction limited from 404-656nm.

 

 

 TripletSFPL53 NBK7 110mmf6.len   2.34KB   33 downloads

Edited by RajG, 28 June 2022 - 09:50 PM.

[rflrs]

The write-up on the GTX110 indicates that a design for the field-flattener had not yet been done.

In order to fit my choice for such a design into OSLO-edu, one of the air-spaces in the original

design had to be removed.  Keeping the requirement for minimal spacing expressed in the

write-up, I came up with:

 

 trp06-78.len   2.38KB   30 downloads

 

The two lens field flattener was designed to be 150 mm from the focus; is this enough to put

a diagonal in the train?  The difficult part was removing ghost reflections between the surfaces

of the  field flattener.  The design I ended with is:

 

 trp06-78a.len   2.59KB   27 downloads

 

That design may, however, be difficult to implement exactly.

 

Richard

[lylver]

Another one, three glass choosen, apochromat, with minimal space, central spot near to be included in Airy Disk.

Diffraction limited on approximatively 1.4° without supplemental correction or reducer.

Graphics with field curvature set to -200mm.

Edited by lylver, 05 July 2022 - 06:52 PM.

[rflrs]

Rajesh asked me to submit this alternate:

 

   trp06-80.len   2.45KB   27 downloads

 

The RMS vs field plot, with the chosen wavelengths weighted

photopicly, is:

 

 

However, the AP site indicates that a field flattener had

not yet been designed.  I came up with this design (cannot

fit in OSLO-edu):

 

Title: trp06-80
110 mm f/6 Triplet ala AP GTX110

Surf     Radius    Thickness     Glass   Diam

 STO   Infinity    -5.911803              110
   2      258.8            8   S-BSM81    115
   3      178.5           24   S-FPL53    115
   4     -249.6    0.2074086              115
   5     -261.8            8   S-BSM81    115
   6     -752.6     439.3985              115
   7      84.28            5      LLF1     70
   8      45.78         10.5    N-SK16     70
   9      61.91          7.8               70
  10      119.9            5     N-FK5     70
  11      73.75          8.5       LF5     70
  12      120.9          150               70
 IMA   Infinity                            44

 

 

The RMS vs field plot, again with the chosen wavelengths

weighted photopicly, is:

 

 

These are different wavelengths from those chosen for the

triplet, extending down to the g-line.

 

Richard

[zavich1414]

Apochromatic triplet 100/f5.8, diffraction limited in range 410 - 1670 nanometers. For fluorite was ised Reznik dispersion formula, for Schott glass - Sellmeier one.  Glasses were taken from glassbank.ru

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Edited by zavich1414, 23 July 2022 - 08:32 AM.

[lylver]

Japan COC (Carton) 80f/15 triplet. (and Fluorite doublet)

Classic M.Herzberger's method.

Petzval sum is good, nearly flat design, it don't need flatener for astrophotography.

I used 1mm spacer, may be thinner with different internal radii.

I was wondering if this could be used in the first Takahashi 1972's triplet ?

Any one knows ?

The fluorite doublet is flint in front, it may be at the origin of some known commercial (with fluorite in front, when the coating was hard enough) like the Vixen FS102 f/9  : O_SSL5 + CAF2

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a modern version, less performant, as enhanced achomat, cemented.

 mCARTON-TAK-80-1200.len   703bytes   36 downloads

Edited by lylver, 31 July 2022 - 04:49 PM.

[RajG]

Here is a 100mm f/15 Super Apo triplet using N-BK7/N-KZFS4/S-BAM12. It is diffraction limited from 400nm-1152nm. It uses small air gaps, but in principle this could be made as a cemented lens since the TCEs of the three glasses are nearly identical (it would need re-optimization, of course).

 

 

 100mm f15 triplet Super Apo.len   2.31KB   30 downloads

[zavich1414]

Triplet with poly-strehl pleasing to the eye.  130 f/7.3

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[rflrs]

In the thread https://www.cloudyni...optical-design/

there is a discussion of the "Pluto Discovery Telescope".  In "Telescopes Eyepieces Astrographs" there

is  prescription given for that design.  I have converted this into an OSLO design file:

 

    CookeT03-30a.len   1011bytes   36 downloads

 

However, James Baker suggests an alternative figuring of the Cooke Triplet designs he gives in

"Amateur Telescope Making", Book 3, in that the majority of figuring is done on the two inner surfaces

of the outer lenses instead of single figuring near the stop.  I worked up an example of this:

 

   CookeT03-30.len   1.01KB   33 downloads

 

The spots are reasonably small for an image of approximately 20 inches in diameter:

 

 

 

The Strehl ratio does not seem to be too good; on the order of 6 to 7 magnitudes down.  I sometimes

wonder how they ever took any pictures at all!

 

Richard

[lylver]

For informations on old designs : glasses have notably changed from 1880 to 1930 and more again now.

From what I found.

 

Some words :

F2 glass is a german Schott optimisation of "Dense Flint", it may differ from time to time.

Chance Hard Crown was also known as "english telescope crown", a very good glass close to german K7 but better.

The classic doublet was Chance Hard Crown + Dense Flint, dispersion matching is better than BK7-F2 or K7-F2

BAK1-F2 is the favorite combination of RF.Royce.

 

1880

 

1907 Source A.König, these are Schott/Zeiss ordinary's telescope glass just before Schott tuned the BK7 (known when it starts as the "white" borosilicate crown).

BK7 needed some years to be stabilized (bubble free) so I believed it was ok some time after WW1 and really used industrially by Zeiss around 1920.

In 1907 the Zeiss E objective was not F2-BK7, it was the littrow balk2/balk3 based doublet designed at Bamberg located Berlin-Friedenau by Czapski

 

1929 Source A.E.Conrady

 

1957 Source Corning Glass

 

by BGRE

 

Posted 06 August 2022 - 02:03 AM
The Cooke triplet used in astronomical refractors is not the same design as the Cooke triplet used in slide projectors or cameras.
The relative spacing between the lens elements is much smaller in the "Cooke" triplet telescope objective.
All elements usually have the same diameter in the "Cooke" refractor triplet.
At least one element of some photovisual variants of this triplet used a glass that became "cloudy" over time (typically decades) due to atmospheric attack.

And to complete BGRE statement and to conclude : The Cooke Astro's Triplet was made with not durable glass, and reworked later in Zeiss B triplet by A.König later.

something like BALF4/KZF2/air/K7. looks like a reverse of the Taylor triplet.

Need some work I never conducted to reengineer the glass parameter for OSLO.

It should be a nice PNP triplet with low petzval sum (phi/n sum) : near flat so suitable for astrophotography, this was the goal of this work around exotic glass.

Only info I have is that one glass (front) may be a classic silicate crown without borate. (O.543 glass was stated in a "silicate" θ chart), and the second is a short flint O.658 that was not durable and soon replaced by O-164.

Remember that at this time, photographic films had grains around 50um so concentrating light inside Airy disk was not the ultimate purpose against reliably detecting a moving object between two shots.

An early glass catalog is welcome or Calcul des Combinaisons Optiques by Henri Chretien 1958 pp. 261-(262)-267 as quoted by Ceraglioli.

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How Taylor calculated is astro triplet : glass choice after normalize to match a) achromatization FC, b) null G' C delta for apochromatisation => spherical and spherochromatism (tertiary spectrum) remains to be handled with curvature and spacing. If ponderated mean of Abbe of the positive lens (here N=54.6 vs 50.2) is far from the negative lens : curvature are less strong => this is what we call the Maximilian Herzberger's triangle to minimize spherochromatism.

So apochromat designing (color part of the old concept) was known well before 1907, it was still hard to compute aplanetism and handle other high order aberration like the tertiary spectrum (spherochromatism)

ϕ is lens power (reverse of focal length in diopter), ν is Abbe number, θ as explained under.

Please note that theta value is like a P_G'F chart not the glass index : θ = (nG'-nC/nF-nC) = 1 + (nG' - nF)/(nF -nC) # 1 + P_G'F

G' is Hγ=434nm, not g ray that is Hg 435,8 nm, things changes ...

An early way to match the modern PgF chart to choose glass to reduce secondary spectrum and do apochromat.

Retro-engineering is much difficult without the glass index : you need to explore all the P_G'F value for glass and their v_D (and table with G' are no longer a standard today).

Edited by lylver, 11 August 2022 - 10:53 PM.

[lylver]

another quotes about the original Taylor triplet : may be easier to retrieve the glass properties. Danjon & Couder, Lunettes et Telescopes 1935

Couder said that it is quite difficult to built with short f/D, so pratically f/18 seems a good compromise and some sacrifice about the "crossing" aka the gathered wavelength on focus.

He wrote about blue + yellow-orange + infra-red. giving a result three times better than an achromat.

So the B objective with the "new" glass tuned by Schott (in fact the KzF2) gives a result of 6 times better than achromat correction.

So the well known comparison : Zeiss B, the reworked in red that last long till fluorite could be grown artificially and later fluoro-phosphate glasses.

If I remember there is a superb compilation about this in A survey of refractive systems for astronomical telescope by R.Ceraglioli

 

[Now it is enough, it will become off topic]

[Aljr]

Hello. Here is an "apochromatic RGB" Dialyte-Petzval. No need to grind the lenses. The objective lens is a PCX 150/4000 from Surplus Shed. The middle of the telescope is made up of two AliExpress 60/800 achromatic doublets, but in the second doublet, we use only the SF5 lens and discard the BK7 lens. In the third group of lenses, the corrector close to the focus, we have a DCX converging lens and a PCX converging lens, which must have their rays(of curvature) exactly as described, not more, not less. I'm going to build this telescope. It represents for me an insistence: a simple lens can be the objective of a telescope. This telescope is not bad. I don't want to convince anyone to build it. There are many telescope designs out there, but only a few succeed in popularity and performance.

 

http://www.dialyte-r...or.blogspot.com

Edited by Aljr, 20 August 2022 - 06:22 AM.

[jan.vantomme]

When reading the recent topic about baffling a 8" f/15 classical cassegrain, I remembered that long ago, (25y) we designed and made a 0.53x focal reducer for a large 62cm f/15 telescope based on an Edmund 50x200 achromat + a weak plano convex lens. (for the relative small KAF 1600 sensor)

Was in the pré-OSLO  ;  beam4 period ......  ;o)

 

For one reason or another, the combination of the Edmund BAK4-Sf11 lens (EACH45179) works quite well on classic f/15 cassegrains.

So for fun I tried it on the 8" f/15 cass design by Matt Paul.

Reducing from f/15 to f/8.8 gives spots smaller than airy disk in a field of roughly 0.5°

At the field edge of a 1.25" eyepiece (+/- 0.35° off axis) spots grow to 3x airy.

 

 

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[hwb3]

This thread's the most beautiful thing I think I've ever seen on CN (I mean except for sketches and astrophotos)!

I was grinning most of the time I was looking through it.

 

A handful of designs were posted under ATM Optical Designs at my invitation.

 

Index: post #2 - [three] element flat field Astrograph design with a 2.5 degree field intended... ; posts #7, 13 - [three] element all-spherical optics for a 12.5" f/6 scope [Schalck-Newtonian] ; post #14 - all spherical medial refractor ; posts #31-34 - Telescope catadioptric thin objective ; post #35 - unnamed

 

Right at the beginning over there, you'll see links to ATM astrographs-the designs and Comparison of astrograph designs for amateurs, reposted here for convenience.

 

[hwb3]

Here is a design for a 1000 mm focal length f/6.6 (150 mm aperture) apochromat with a 2 degree diameter flat image that has  diffraction-limited correction over that field and has just 4 lenses.  It is a short design that is just slightly longer than the focal length.
 
  https://www.slidesha...bjective-design
 
 
- Dave Shafer

Lest the link vanish away, screenshots of this design below:

 

For the curious: full screen > print screen > paste in MS Paint > crop > save as gif > open in MS Picture Manager > resize > save

 

(Note: I'm not connected to this design in any way.)

Edited by hwb3, 10 September 2022 - 09:17 PM.

[rflrs]

A Note:

 

  The patent number for the Bietry design is 9,588,331, not 9,588,332.

 

Cf:

 

   https://pdfpiw.uspto...View first page

 

Richard