25" whit a 3.1" diagonal

My progress whit the SDM 18” whit a CZ primary Quartz and a Antares 4” Quartz secondary is going just fine.
Carbon fibre trusses is ongoing etc, and the design has take some time but we are at the end of that now.
Peter Read at SDM is a great guy to work whit!
Positive and endless way of ball around the idea a customer wants.
CZ is soon done whit the optic's.


But over to more future telescope ideas!

My progress of what I want in future has change, but I had been into ZTE free glass since I got involved in this and I like cellular designs.
Of course, cast Pyrex is a good alternative. It quickly breathes the heat and is quite light ( ca ; 15 kg for a very stiff 25 inch at ratio 6;1 )
I have realized different telescopes the rules is different, it's economy, stationed or real portable etc, etc.
I think none of this is possible to combine today actually.
In general, however, the sky itself talks in the end, but if ..

I was very impressed by the depths of the optic’s and star quality in Jorge Peters 25” LOMO made mirrors at f/4.5 and a solid 70 mm thick.
Then the chassis was in completely ball-bearing and it was so smooth as steering it with a finger, really amazing.

-Then how good can a bigger mirror be and how much does it work to use it, plus the height.
All good questions.

I have realized a few things now.
I really wonder how good fast thin Pyrex mirror are in reality, and few show Ronchi or a star-test.
-Do they work well on galaxies etc, yes sure, but with such a large diagonal and thin mirrors added.
I’m sceptical. ( this is my personal believes )
Then the heat must come from them and before that, the figure will not be hold.
Here is an advantage of Zerodur or ULE ( or Quartz )
But even they will get warm air above the surface.
But it can be solved with fans and figure will stay no matter of temperature.

Jorge solved his idea with a small diagonal at 3.9" ( he had 16% )
So with short L-measure and without coma corrector ( f/4.5 ) and he just used it at at medium to high-power and used the Ethos eyepiece under 13 mm.
His mirror was at 70 mm, so ratio 9;1.

I could pocket a mill in ULE so ribs and the surface will be 1 "and then the heat will go out faster and the mirror becomes easier to get to work and handle.
Thin ribs are extremely expensive to get done in machining, so thicker becomes significantly easier, so less pricy.
This could AGI do in Rochester.
Ex than 85 mm thick ( ratio of 7.5 ) it would weigh just over 40 kg, vs 2/3 of a solid core.
( Ok it's double to a cast cellular ) but then in ZTE = 0 instead of 2.5.

I hope CZ could grind it at 25" in ULE on ex f/4.25 ( 2.700 mm FL or 106” )

I has never like the long set-up using a paracorr II or the SIPS, and I know more glass today don’t get a big lost, and they give one coma free image.
Anyway it all looks strange, and it can be a very long unit and with a barlow on the outside of the UTA can be around 12”..
I checked run-out of this before and its really terrible!
All loose ID and OD holes and this Crawford focusers on top of that.
Nice with a special build 'sag-free' focuser and just a eyepiece.

For example, I checked out diagonal size and has the eyepiece barrel in the primary light path, but then an extremely short L-measure i received.
With Ethos on 17 mm ocular ( 158X ) and a ok mag-drop not over 0.25, so one see that a 3.1" diagonal will work.
I tested it at a 3" vs the lip on the holder ( in reality a 3.1” )
Normally use EP will be Ethos 13 mm ( 207X )

This gives a telescope with ladder of 1 meter to zenith, but at only 12%!

If I use a L-distance at 14.37” and half FS clearance of Ethos 13 a 3.4” diagonal ( 3.5” ) that give a mag-drop at 0.2 and give 14%.
Might be a better deal as the eyepiece barrel will no be in the light path.


-Ok, there will be a little bit of coma on the outer edge ( so was Jorge’s scope as well, but it did not borrowed me )

With a welded chassis in round tube chrome molly steel ( super rigid ) and ball bearing and trusses and the UTA in CF and so light diagonal and stiff primary this would be a real 'killer'!
La Palma use of course ( vs the terrible Sweden weather & sky )

Ok 1 meter ladder is needed, but everything is always a compromise.
But okay, one is also rather relaxed on a ladder.

I know what a core of machined Zerodur ( round and ruff curve ) cost from Germany.
ULE is more expensive than Zerodur, but even a better glass for optic’s, but ok a overkill' for visual use maybe.
Quartz is at 0.5 ppm/C and chip less than Zetodur/ULE but its very hard to machine ( but price is way less )

Whatever ULE etc, what do you guys think of such a project?

-Does f/4.25 without coma corrector work ‘decent’ on medium or high-power?

I still remember 2 nights at IVT 2017 in Jorge's telescope when I went around in Veil, Crescent Nebula, M27 and M57 - that was big WOW!
Jorge said, you should use this scops in the alp’s or at LaPalma!

I asked CZ and he can’t do the "same" tolerance of course on 25” at f/4 as a ex 12” and f/5.5, but a ’great’ mirror in Quartz ( ULE = my request )
It will be a tall scope if one go to f/5, ...but it means a 3” ( 3.1” ) secondary at 12%.
Also at f/5 coma issue will be very small, but it will be there.
If one use a L-distance at 14.3” and not get the EP barrel to go into field stop clearance.
( Ethos 13 - 3.7 mm )
So no use of a coma corrector.
That mean Ethos 13 mm will be 244X and 10X / inch ( scoops ‘low power’ )
Trusses will be long but in stiff CF and UTA in CF, and the secondary is light.
Trusses can be made in 2 or 3 sections.
There are allot of benefits here from a shorter 25” in Pyrex and thin, fast and a huge secondary.

Will it be worth it ?
Gain?
Will coma be a issue in this powers, vs study object’s fit it ?
Will the ladder get one crazy ?

As I heard, but it's are many answers here one can intrude the paracorr tube 2" and don't see any spikes.
Then the same 3" diagonal is to be used, but the use of a coma corrector ( trusses need to be shorter )

CZ mean a coma corrector is ’really’ needed even at a f/5, and recommended a f/4 to not be 'counter-productive' as he said it.
He would prefer a 45 mm thick Quartz over a 2-2.5” in Zerodur or ULE.
Well at f/4 ( 100” FL ) and has a ‘correct’ position of the paracorr and use Ethos 21 mm FS clearance a 4.5” diagonal is needed.
Not that bad ( only 18% )

Well, as I’m a stubborn guy I’m still like to check out the ’non coma corrector’ use and a smallest possible diagonal and in a thicker plano..at a ZTE free glass.
I’mean I has self see it was no problem at f/4.5.

-Don’t be upset on me ‘twist’ ideas..here.
I has a plan whit my scopes in a good life in LaPalma/Canaries later on rent it out and be doing own great astro.

Of course a even bigger scope is prfered, but everything has a limit.

About Zerodur I found out it keeps the heat more than ex Quartz, ULE or AstroSitall.
Cal/g is 0.8 when the others are only 0.18.
As a ZTE free glass it hold figure exact but keep the heat harder.
-in that case ULE is better, but $$$.

I think CZ is right, going for a f/5 at 25”, it will be tall..
A 25” is a nice size and at f/4 it is 100”, but he can do pretty good tolerance.
If the paracorr barrel to mirrors edge a 4” diagonal will work.
Z is very cheap on his mirrors but going to a cellular that is way stiffer, way faster cool-off time but if so going faster, or step up to a 32” it will be MRF and a way other price.
My former idea was a lightweight SiC ( very light and very stiff ) for portable use at dark sky, but I kind left the idea of a pricetag at 3-400K and be portable on a 25” is hard..
It’s stationary use.
So if one is fast as a f/3, why not think of a 32”, still ok height.
But again a CZ Quartz 25” at f/4 will be my next move after the 18”.

Idea of a slow 25” whit a tiny diagonal is not bad, but it will be some coma and one will be on a very tall ladder.
I climbed upp 3.5 meter and realised I was on the wrong road.

I has come to the insight in this project that f/4 is as far as I can go on height of the structure.
But a ladder is not all that bad, as it can be relaxing to.
But for portable use it's negative of course.

I has had the idea on a medium or a high power scope ‘only’ at ex 25” and f/4 and not use a coma corrector.
As I said, the idea I got last year at Jorge Peters 25” f/4.5 Sitall/Lomo at 70 mm thick and he used a 3.9” diagonal.
We used Ethos 4.7 mm ( 1 mm pupil ) and 600X.
Marvellous !

I can get away whit a 3.5” ( C-A 3.4” ) if the EP barrel stick in at 1”.
( 14% obstruction and mag-drop at 0.05 = worthless for a human eye – maybe, but WAY cool ! )

Example on Ethos EP ;
8.0 mm 317X. ( pupil = 2.0 mm ) 15X /”
6.0 mm 423X. ( pupil = 1.5 mm ) 17X /”
4.7 mm 540X. ( pupil = 1.2 mm ) 22X /”
3.7 mm 686X. ( pupil = 0.9 mm ) 27X /”
( I can add a Powermate x2 )

Coma will not be a big issue here ( as I tested it )

I think deep-sky must be around zenith, and low-sniffing is nothing for me.
Also no good sky is nothing for my astronomy.
So whit a ladder ( as I don’t like ) is actually a steady rest.
But a ladder is bad vs be portable.
But a mirror at f/4 is my guess more easy vs a f/3 etc.

I hardly believe it is as easy to see a good star-test in a f/3 @25”.
( Around 25X per inch or 2 mm pupil )

I doubt ( but this is personally ) that very fast bigger mirrors will show great stars.
After all, f/3 or faster has many issues.
Only good thing about it ( as I see it and MANY others ) is shorter trusses and one can has a 25” and not use a ladder.
But the optic quality ?

Sweden has no sky for it.
-But LaPalma has.

-What kind of mirror ?

A cast Dream cellular at 15 kg and 6;1 ratio or a CZ Quartz plano at 44 mm edge.
I think the cellular might be the winner out in field.