I've had an idea, would this work?

[Doc]

I've just had a thought....

To eliminate the spider vanes on a secondary mirror mount how about similar set up to a mak-newt where the front of a normal dobsonian is covered with a sheet of glass with a hole exactly centred.

The hole is slightly larger then the secondary mirror mount so to enable centering of the secondary. Then when the exact positioning has been obtained the secondary mount is secured in place.

This is not a corrector plate so should be able to be removed to be clean easily.

Would this work, and what would the pro's and con's be?

Would the sheet of glass disturb the view you will get?

[GazOC]

I think getting glass of a sufficent standard to not degrade the image would be pretty expensive at larger apertures.

[kniclander]

isn't a sheet of glass just like a lens in the sense that it would cause chromatic aberrations:icon_scratch:?

but then how does the sheet of glass on the front of my mak not do that (or maybe the corrector plate corrects that?)

[JohnC]

Interesting idea Mick

Would the glass still need some sort of figuring due to the refractive index? Not sure.

You seem to be heading towards a project of one sort or another

[Doc]

Those are the questions that I have no answer for. Surely the glass on the front of a mak newt is a special type.

As for size it's maybe 7" diameter.

[FraserClarke]

Like an astroscan;

Astroscan Plus Telescope - beginner telescope | Edmund Scientific

The problem is that the window is a 'corrector' whether you want it to be or not; you'll put as much effort into making it flat (so as to not distort the image) as you would into making it a specific shape to do some correction. So you might as well use it to improve the images in some way.

Curved spider vanes are a better way to go to get rid of* diffraction spikes and give you refractor-like star points.

* of course, they just smear the spikes and make them less noticable

[Glider]

Would it need to be at least anti reflection coated?

[Naz]

As with photo filters, the better quality ones that don't degrade the light path or change the colour cost a bomb. Optical quality filters for cameras cost as muc and more than kit lenses.

B+W 95mm MRC Clear UV Haze (010) Filter (BW45110) - Warehouse Express

And that is not 4", what would it cost for say an 8"? plus putting a hole in it.

The idea is good, I think if I understand your idea, but the cost alone would probably be against it. Both surfaces would have to be optically flat and parallel.

Archie

[Bizibilder]

It would have to be an optical flat AND have both surfaces optically parallel - hence it would be very expensive to make. Secondly not only would you have to collimate the secondary but collimate the optical flat as well!! It also closes the tube end to give the typical problems of currents in catadioptric scopes.

[beamish]

My Cape has a flat optical grade glass front, about a 1/4 "" thick, it has a broad band coating to cut unwanted reflections but has NO effect on the light coming thru re magnification/correcting etc and basically does exactly what you're proposing Doc !

[Doc]

Oh well not that good an idea then

Back to the drawing board....

[beamish]

I have a spare

[Doc]

I have a spare

You keep that warm for me Karlo if you wouldn't mind. I still have alot of items to get and of course money to save.

But it's worth a try I think.

[beamish]

It's not going any where ! let me know if/when

[Doc]

Someone has thought of it before me

Telescope Reviews: Glass window on Newt.

[Llamanaut]

wouldnt the glass just turn the newt into a mak?

[johninderby]

Here you go

16"F/5 Aries Newtonian with optical window build in observatory class quality , include tube rotating ring system, mountplates, tube balansing weight system, cooling fans, dew shield, 2" Starlight focuser, Astrosital mirrors, fully broadbandcoated optical window, very small central obstruction of only 15%, interf. Tesreport stating above 98 % strehl on system and better then 1/8 wavefront p.t.v. Manufactoring cost today it would cost about $ 50,000. Total weight about 130 kg

Price: 22.990,00 EUR

Teleskop online Shop , TMB, APO , Dobson , Professionelle Teleskope, Kuppeln , Teleskopabdeckungen , Teleskope , Ferngläser , Teleskope online kaufen , APM Telescopes - Verkauf Beratung Herstellung

John

[Naz]

wouldnt the glass just turn the newt into a mak?

No, totally different configuration of the optic train.

Archie

[Bizibilder]

I said it would be expensive but I didn't think quite that much!!!!

[Peter Drew]

I've made several Newts with optical "windows" in the past for clients who did'nt want to see any form of spider diffraction.Quite frankly the only benefit IMO was the protection of the aluminising. On the negative side, the component gave the same dewing problems as SCT correctors and added considerable weight as they have to be relatively thick to maintain the optical quality. Oh, and they were very expensive!. I still have a high quality 12" aperture window made by David Hinds if anyone has a special project in mind.