First mirror - eight or ten inches?

[mapstar]

Well, I've been thinking and thinking and I have decided on 200mm f6. I want to make the mirror as good as I can as it is mainly for planetary work, so there's no sense in making the curve too challenging. Is there much point to choosing low expansion glass? http://www.galvoptics.fsnet.co.uk/telescope3.htm

Probably not much point for the spec's you have mentioned

Plate glass up to 25mm thick is relatively easy to pick up you should be able to get hold of it quite cheaply too.

Hope we see it soon ;-)

[Ajohn]

Looks like you wont get an answer. Some reckon you would gain using it. Some such as a comment on Oldham Optical's web site point out that people used to make 12in mirrors out of plate glass but wouldn't these days. This ties up with Texereau's comments - up to 12in as well. He mentions that the thermal effects even at that size would be scarcely visible in actual use. He also mentions that it's easier to work - it is compared with pyrex. On my first I tried to hog a pyrex blank out once with a plate glass tool and gave up. The wear on the glass was impressive, the effect on the pyrex was minimal. This is probably why video's like the sky at night one show a blank being hogged out with any old piece of steel that is available. Followed by a tile tool. Some use a tile tool straight off or one that uses steel nuts rather than tiles on very hard materials. I'm having a bit of a problem with a 220mm low expansion blank with an F3 curve that was pre milled for me and a tile tool. Not sure why yet but it doesn't seem to be correcting the fact that the usual pre milling doesn't produce a true curve. Probably just need coarser grit.

Where you might run into trouble with plate is warming the mirror up for hot pressing a lap - too hot to quickly and it might crack. On the other hand many people have done it in the past and pored hot pitch onto a plate glass tool. Texereau mentions warming up at a distance from a gentle source of heat and water at 35C.

I've never used plate but he is usually a reliable source of info. I've managed with glass tools via things lying about all except that very 1st plate glass tool.

It's a tough call really. Plate same thickness tool and mirror are likely to work more quickly. For my 1st I should have given it a try. Instead after the pyrex problem I bought matched same material low expansion premilled blanks and it was easy until it came to figuring - that took me a while. I don't do enough of them to do that quickly.

I don't know if you have texereau's book - it is well worth a read but some aspects are dated. His aim was a telescope capable of general use with it's max magnification capabilities given most peoples viewing conditions. Seeing often limits resolution. So still true. Shape in nebulosity etc can be seen with a scope of this size if light pollution isn't a problem. As he puts it - good views of nebulae. Big mirror light bucket designs aren't considered at all - it's a book for starters. Another one not so high quality working wise is Muirden  Beginners Guide To Astronomical Telescope Making isn't too bad of it's type. That may be on the archive as I don't think there are any commercial interests. Might be about on Abbe Books too used. His telescope design is dob like after a fashion mounted on a simple alt az stand

Actually I have a scan of Tex...... book 1 which is well out of print and not available any more. I can try and upload it somewhere over night - it'll take ages. If I do I will post a link. It was on the internet archive. Trouble is commercial interests removed it and there may be some like minded people around here.

John

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[Ajohn]

Good some one did answer - I ate dinner mid way through.

One thing I meant to mention see what Beacon Hill have. I think they still do mirror blanks and grinding materials etc. There is also that German supplier I linked to. His prices aren't too bad for what he offers.

John

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[Ags]

Thanks, that's a lot to think about. I think I'll go for a plate glass blank and matching glass tool; that will be the same as for the 6" mirror of my teenage years.

[Ajohn]

Might be a bit late but Beacon Hill provide what I feel is a better range of abrasives than Galvoptics. They are perhaps a bit over the top on the finer grades. As a company they have been around for a very long time too.

http://beaconhilltelescopes.org.uk/

 Strange site - it's looked like that since they went on the web. Maybe it's "if it isn't broke don't fix it". I tried to link to the correct page, just hit continue and then prices to see what they have. If anyone wants to follow the old worm wheel should match the diameter of the scope this is probably the only place to go - up to 14in.

The matching tool with the blanks may be thinner. If concerned ask. This comes from early suppliers. For 8in the actual blank would be around 30 -35mm thick and the tool 25, sometimes with raw edges rather than ground. 

John

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[Ags]

After enjoying RAC's thread about his first mirror (18" F3.3) and learning more about testing possibilities, and after learning more about what I can do with aluminium pipes and plywood, I have changed my mind and I will go for a more ambitious 250mm F5. I will pick up the glass blanks at the end of the month.

[Ajohn]

Oddly that's what I have decided to go for.  F5 should give good wide field results with a coma corrector and also hopefully wont push eye pieces needs too far and  10in makes a fairly compact scope for equ mounting.

John

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[Ags]

Looking at 3D printing the secondary holder in stainless steel or nylon. What gets me is how to construct the vanes.

I think I will go for a 20% central obstruction, +-14mm fully illuminated field stop.

[Ajohn]

Some use steel tape or pieces of a steel rule, strips of the same or aluminium. You will probably find some suitable metal strip in B&Q warehouse outlets if all else fails.

John

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[Ags]

Thanks... I am thinking of using strips of sheet metal under tension somehow.

[Ajohn]

 When I suggested B&Q I didn't notice Netherlands but maybe they or similar outlets are there too.

Most "metal" suppliers will cut it to what is required and sell to anybody without any problem. There are some large scale suppliers who will also cut to suite. I just go there and ask for what I want. Probably the easiest way to find ones that are local is to google things like what area you are in plus the word metal, none ferrous or what ever. Stainless steel will usually come up with pots and pans etc but none ferrous supplier and the others will also often stock it. Best ask for free cutting - the other type can be a pain to do anything with. The free cutting ones will generally be slightly magnetic. Free cutting mild steel is easier to cut with a decent finish as well but the bright drawn variety that is about now isn't so nice as good old fashion black bar. It tends to have a tough skin. Suppliers quality varies in this respect too or did. Since things went "standard" that's probably not so common now.

John

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[Ags]

The trouble with living in Holland is the correct Dutch technical terms escape me... Makes googling a bit awkward sometimes.

[Ajohn]

 I suspect it would be worse if you lived in Germany. I just couldn't believe my eyes when I eventually found the translation for a left hand tap - tap, in English can mean a number of things, in this case a thing to produce threads in holes to accept screws etc. Threads itself can mean different things. Worse still it had to be a left hand one. No translation tool helped me. I'd guess one might with terms as simple as metal and maybe ferrous, how none is added to that is likely to vary from one place to another and there might  be a specific word.

John

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[Astrobits]

A 10" f/5 will be noticeably more difficult to figure than an 8" f/6 but not too much.

As far as the thickness of the main mirror goes, I have made both 8" f/6 and 10" f/5's from 19mm plate glass with excellent results. Indeed,many years ago my 8"f/6, ( with a 19mm plate glass mirror and held in place with three blobs of Silicone -- gasps of horror from some quarters ) out-performed an 8" Schmidt-Cass ( a well known American brand ) standing next to me when a very experienced planetary observer used both to view Mars.

The only thing to watch with 19mm thick glass is to avoid over grinding with the coarse grits so as to leave the thickest mirror you can from the blank. I had a pre-curved steel tool and ground to curve with 80 grit but stopped when the curve was about 6mm from the edge bevel. I would then switch to 120 grit and continue until the curve was about 2mm from the edge. Then onto 220 grit and grind up to the edge of the bevel. At that point I had the thickest mirror that I could get from a 19mm blank. The finer grits removed very little glass after that.

So don't agonise over the material, just get what is readily available to you.

Nigel

[Ags]

I wrote a little program today to calculate field illumination for different sizes of secondaries. Based on the numbers I think I will go for a 63mm secondary after all. It will also be easier to construct a secondary holder if the secondary is larger :-)

[Ags]

I started making my secondary holder today - it looks like the most difficult part of the construction aside from the mirror, so I thought I would start with that. 

I calculate that a 46mm (18%) obstruction will be enough for my planet killer. I wrote a program to investigate the options:

http://opticsandalgorithms.blogspot.nl/2015/05/building-telescope-central-obstruction.html

If my program is right, I would still get at least 9" of aperture across the field of a 16mm Nagler, which sounds good enough to me.

[JRM]

Hello Agnes,

  most dob secondaries run the range of 16% - 20%, I have seen some smaller and also some though few larger,

the seconday covers the middle of the mirror which is also the smallest area, it really does not affect the image by

much either way, though some are very defensive on this issue.

there is a program called Newt for windows  that has been in use for years and work well for figuring out all the different sizings 

of telescopes, secondaries and such.

this link is helpful as well for learning your mirror spec for a given focal length

http://stellafane.org/tm/atm/general/scope-calc.html

as far as ease of grinding and figuring, the difference between your two suggested focal lengths is really miniscul for a 10", though

much harder to make then say a F-6.

just my thoughts, hope it helps

Rick M

[Ags]

Hi Rick, part of the fun for me is doing the calculations myself. I know about Newt, but using it would be like an imager just downloading hubble pictures :-)

Also, I thought 25% was more typical (e.g. OOUK 10" dobs).

[Ags]

By the way, the link you provided is a lot of fun:

http://stellafane.org/tm/atm/general/scope-calc.html

[Ajohn]

People often mention % area of the 2ndy mirror Agnes and don't appreciate the % diameter effects. 25% by diameter is generally considered ok for general use but planet people often try for less than 20%. I posted some screen shots of what happens as it goes up in the F12 4in thread. Past 25% it goes up rapidly. I showed the coma on a double sized scope too, F6 8in, up like a rocket but forgot to change the size of the 2ndry mirror otherwise it would be worse. I could post some more shots of the effect against no obstruction if any one is interested - when I have 1/2hr to spare.  Size of scope doesn't matter. It just changes the numbers on the resolution axis and faster Newtonians have rapidly worsening off axis performance. SCT / compound scopes etc can help with that if they are designed correctly but usually suffer from having larger central obstructions.

John

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[Ags]

Yes diameter is the most important factor, and from my own calculations you are fine if you stay at 20% or less. I made an animation of the diffraction pattern evolving as the CO is increased, as well as a graph showing the effect on contrast. (The animation is a bit dim and I suggest turning up your screen brightness if you view it - one day I will regenerate it with a different stretch on the data).

http://opticsandalgorithms.blogspot.nl/2015/05/animation-of-central-obstruction.html

[Ajohn]

The contrast plot doesn't really get the point across Agnes.  What is actually happening is that increasing the size of the 2ndry diverts more and more light from the diffraction spot into the rings  which limits resolution, the spot is dimmer and surrounding areas are lit more than they should be by the rings, net effect reduced contrast especially important when viewing low contrast detail in objects such as planets but it also has implications on other extended objects. It might be more interesting to show a plot of Strehl ratio, not sure as it might have limited effect. The usual  99% etc ratio represents 99% of the theoretical encircled energy which even on a perfect optic isn't 100% due to some going into the rings. MTF curves are really the best way to compare various effects as they also show how contrast is what actually limits resolution. That is around 7 1/2 % at the resolution level usually specified on astro scopes - useless except for spitting double stars yet some people claim to have seen to this level on low contrast objects. 

A worse problem in many ways is the accuracy of the mirror figure that is needed on a Newtonian, Even just a 1/4 wave error causes the brightness of the spot to drop to 80% of it's peak value. I have no idea what this means in terms of energy encircled in the diffraction spot but I'd guess it makes that figure worse. It generally reckonned better than at least 1/10 wave is needed. This is why NASA shot for 1/30 wave on Hubble. Opinions vary.

One other factor that might interest some one aiming primarily at planets is the off axis performance of newtonians - others too really. I'm fairly sure a simple formulae for Shrehl down to 80% can be found in the Newtonian section on this site, the numbers get worse with faster F ratio's.

http://www.telescope-optics.net/index.htm

There isn't much talk in amateur circles about encircled energy. Strange really as this is how the pro big scopes tend to be specified. X% of light into a spot of such and such a size over some field angle. It's probably irrelevant on most amateur sizes but at some point as they get bigger and seeing conditions limit resolution/magnification anyway  it can be relevant as the conditions can swamp optical errors. One solution at some point that might be used is  to settle for 1 arc second resolution rather than the theoretical maximum of some bigger scope.

John

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