big telescope

[Tycho Brahe]

I was out using a meade lx200 tonight, its sweet, makes me want a big scope... Also got a book by john dobson on how to build a dob...and as I'm skint...

I'm lucky and unlucky. Unlucky cause I'm Rubbish at DIY, but lucky cause I WANT to get better, and I have access to a variety of vacuum coating machines, so 'should' (not withstanding me being Rubbish at DIY) be able to silver the mirrors... Here are the first two of my questions;

1) most mirrors are aluminised right, is silvering better but more expensive, or do they use aluminium because its just as good?

2) If I could grind/silver a mirror at 12" with an F/6, that would mean I'd need a tube 72" or 1.83m tube, right???

Thanks lads, they'll be a lot more questions coming I'm sure...

[Demonperformer]

1) silver surfaces were relatively easy to produce by the amateur, but are far from permanent as they are readily attacked by impurities in the air (particularly sulphides) and so tend to need redoing regularly.

aluminium coatings produce a layer of aluminium oxide which protect the surface and so they last a lot longer.

reflectivity @ .59 micorns: aluminium 83%, silver 93%

2) Tube ID should be 1 - 1.5 inches greater than the mirror size and should be "an inch or two longer than the focal length of your mirror"

Info from NE Howard's "Handbook for telescope making"

[MG1]

Oooh Tycho, that sound exciting.

I hope that you can keep us updated with your progress once you get started.

Good luck mate.

Mike

[orionhunter]

check out the prices of mirror blanks first. its probably cheaper to buy a used lightbridge flex 10/12 inch dob.

[FraserClarke]

All very good advice so far -- particularly the part about saving money by buying rather than making! 12-inch is probably about on the transition where building/buying are about the same price. If it's you first mirror/telescope, it will almost certainly be more expensive. I'd probably budget ~2-300 quid to make a 12-inch mirror, once you include the blank, the grinding material, grinding stand, the test equipment, aluminising, etc... You could maybe do it a bit cheaper if you tried hard and scavenged where possible.

However, that may be good value to you if you want to spend the time learning how to make your own telescope. You'll certainly understand it better than if you buy a telescope. I did hear someone say once (possibly on here), that if they could make something for just twice the cost of buying it, that was good value!! I quite like that philosophy

Re: silvering vs aluminising. Do the latter. Far easier (pay someone else to do it!), and far more resilient (protected aluminium coating should last for years, vs months for silver). The only advantage to silver is if you have a special need to go below ~380nm -- which in the UK you don't, because hardly any of it is coming through the atmosphere!!

[EDIT: Other way around; Aluminium is better than Silver in the UV...]

Oh, and I'd also suggest that if you do decide to make your own mirror -- make a 6--8" mirror first. It will almost certainly save you money in the long run!

Good luck. We'll be here with advice if/when you need it

[Glasspusher]

I did hear someone say once (possibly on here), that if they could make something for just twice the cost of buying it, that was good value!! I quite like that philosophy

I recently did a rough calculation which revealed that to make a 14 inch mirror would cost about half the price of buying one!

Some good points Teadwarf, I think for a first mirror it is possible to go bigger than you suggest. I say this because there is a wealth of information on the web today that was not available just a few years ago, take this forum for example (please take this forum:)).

John

[Tycho Brahe]

' if they could make something for just twice the cost of buying it, that was good value!!'

Ha Ha I like that too!!! I prefer to try and do things myself to learn... I usually screw it up and get something that kind of works but like you say, I understand better what I have failed to make work right which makes me appreciate, understand and use better what I have to buy! But this times gonna be different... ITS GONNA WORK!!!!!!!

Two more questions already!

1) Could I use the ptfe pipe the water board use for pipes as a tube? Its heavy but it doesnt corrode and is very tough...and would you recommend a truss type dobsonian or a fully enclosed tube? Does the colour of the tube make any difference? (slightly abusing the definition of one question there!!!)

2) If a mirror is 1/6 pv what does that mean?

Thanks for all the help lads

Zane

[PeterW]

Books like Texereau

How to Make a Telescope: Amazon.co.uk: Jean Texereau: Books

or Kreig and Berry

The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes: Amazon.co.uk: David Kriege, Richard Berry: Books

will help.

Solid tube is fine (typically thick walled cardboard), any colour outside, but a nice matt black internally is preferred! >10" solid tube is not great as it becomes a pain to move/store/transport. If you get a laser collimator then a truss will be fine, as long as it hold firm you can align it.

1/6 pv is a measure of the degree of deviation of the mirror from the ideal shape as a fraction of the wavelength of light..... anything better than 1/4wave is generally accepted to be good, though there are lots of variations of what you are measuring and what the numbers mean.

Probably best to save up as much as you can, second hand 10" scopes change hands for <£300 which is a bargain, with optics better than you'd be lightly to make 1st time round.

Hope this helps.

PEterW

[FraserClarke]

I think for a first mirror it is possible to go bigger than you suggest. I say this because there is a wealth of information on the web today that was not available just a few years ago, take this forum for example (please take this forum:)).

Yes, you're probably right John. When I started 10-15 years ago there was a lot less information easily accessible (i.e. what do Foucault images look like when you get it wrong?).

I re-read Texereau recently; and whilst I love the detail it goes into explaining why mirrors are the way they are and the fundamentals of optics, I'm really not sure I'd recommend it to a beginner any more (controversial!!). Whilst the techniques will certainly give you a good mirror, they now seem quite old fashioned. No mention of tile tools/polishing pads/thinner mirrors/slitless testers/mirror cooling etc... (i.e. Texereau recommends you close all vents around the mirror to remove any airflow over the mirror -- the exact opposite of the current practice!). Having said that, I'm not sure what other book I would recommend...

I like the Kriege book; though it's quite focused on 'build the telescope that I like'. but it covers lots of the mechanics parts that Texereau doesn't.

I'd agree with Peter that a solid tube for a 12" is probably getting heavy if you want to move the telescope around. You'd also have to look at the flexure of the tube. I'd suggest White for the outside, to reduce heating during the day. Also easier to see at night, which is useful

Naively I'd suspect the PTFE might flex -- but I have no evidence to back that up. I'm sure there are numbers on the web you could use to calculate it. A truss tube is probably easier. You can put a cloth shroud around it if you're observing somewhere bright.

Re: 1/6 pv. Deviation from the perfect surface as Peter says. There are two definitions, which is important to distinguish: PV means "Peak-to-Valley", i.e. difference between the maximum and minimum deviation from the perfect surface. RMS is "Root-Mean-Square", the average deviation from the perfect surface. PV is a much tighter tolerance than RMS. A 1/6 wave PV mirror will be <1/10 wave RMS, and will be an excellent mirror.

[orionhunter]

another point worth making, is that you can buy a used 10/12 inch large scope and be out tonite looking at the heavens.

making one will take (say) between 6 months and 2 years.

you have to WANT TO MAKE a scope, to make it really sensible.

[Glasspusher]

I re-read Texereau recently; and whilst I love the detail it goes into explaining why mirrors are the way they are and the fundamentals of optics, I'm really not sure I'd recommend it to a beginner any more (controversial!!).

I agree, for exactly the reasons that you state.

John

[Glasspusher]

If a mirror is 1/6 pv what does that mean?

Peak to Valley (PV) Error

A telescope mirror may be described has having ¼ wave PV wavefront error. This means that the distance between the highest and lowest point on the mirrors surface is 1/8 wave. Remember that the surface error is halved at the wavefront due to reflection. On the face of it this mirror meets the established criterion for a good mirror. Consider this, we have two mirrors both ¼ wave PV (wavefront) 1/8 wave on the surface, one has a small bump ¼ wave high, the other has several bumps ¼ wave high. Both meet the established criteria but the first mirror will out perform the second mirror. Clearly, a PV rating is not giving the full picture it fails to take into account the size of the surface defects.

RMS (Root Mean Square)

With RMS the intention is to better characterise, in one number, the entire mirror surface (or wavefront) by accounting for the relative size of defects, this done by measuring the difference between the actual and measured surface in many different places. This is the same technique used to obtain standardised variation of a population of random variables, and is know as standard deviation. Importantly, for RMS to provide its potential, the mirror surface should be sampled by many equal, preferably small, increments of areas across the entire surface (or wavefront). Really, this can only be done with an interferometer, a device that compares the entire wavefront of the optic under test against a reference wavefront on know quality.

It can be shown mathematically that a mirror that is smooth but under / over corrected by ¼ wave at the wavefront calculates to an RMS value of 1/14 wave. (1/13.856), note that RMS value is approximately 3.5 times the PV value. Despite its obvious advantage in quantifying an optical system there is no accepted standard RMS number unlike the ¼ wave PV standard.

Strehl Ratio

Perhaps the best way to quantify the errors in any optical surface is by use of the strehl ratio. The strehl ratio is the ratio of intensity of the peak of the diffraction pattern of an aberrated image compared to the intensity at the peak of an unaberrated image, values ranges from 0 to 1, 1 being a perfect mirror. Strehl ratio (SR) is sometimes expressed as a %, 100% being an ideal optical surface with 84 % of the light in the central ring of the Airy disk. Although RMS and Strehl ratio are different, when the RMS is small they are related by an approximation such that an RMS value of 1/14 wave equates to a SR of .80.

Hope that helps clarify things a little!

John

[Brunty12345]

im interested in how this is going to turn out, defenatly keep us posted

[Brunty12345]

http://www.skyoptics.net/Barnes-MARS_AUG_30_0006_IGB.jpg if that shot of mars was really taken through a lx200, i defenatly want one of those

[Tycho Brahe]

That photos immense... we were looking at mars through the LX200 the other night, and I was really disappointed... we dont know what we are doing and only have two eye pieces, but it really didnt look much better that through my 3" reflector... that may well be down to us, but...

[Tycho Brahe]

I've got Texereau's book, so you dont think its too good for a beginner? What do you think of N E Howards The Standard Handbook for Telescope Making?

[FraserClarke]

Jean Texereau was an optician in the 1950s -- and the book reflects that. Definitely read the book; it should give you a good understanding of why you need to do what you do to make a mirror. However, the manufacturing techniques presented in it are outdated (they will still work of course). It also doesn't (in my opinion) discuss enough about the mechanics of the telescope...

I haven't read the Howard book. It's 60/70s vintage I think?? in which case I suspect it is similar in the way it goes about making the mirror. There would be a market for a new mirror making book now I think. Techniques have evolved so much in the past 10-20 years. I don't think such a book exists though.

Read Texereau to understand the underlying principles, and then read webpages about how people make mirrors these days. Best of both worlds

PS -- Adam; I can show you images of Mars through a LX200 that will definitely not make you want one

[FraserClarke]

The only advantage to silver is if you have a special need to go below ~380nm -- which in the UK you don't, because hardly any of it is coming through the atmosphere!!

D'oH. Brain fart... It's the other way around. Silver has very low reflectivity in the UV, whereas Aluminium is pretty consistent down to ~200nm. Silver is better in the near-IR.

Anyway, for all practical small telescope observing needs, Aluminium is a better choice.

[Tycho Brahe]

The things Ive read so far seem to indicate the aluminium has a protective layer of TiO2/SiO2 deposited over it to make it more durable... Should be able to do something similar with the silver... I'm sorely tempted to use gold! Be good for IR!!!!! Not so good for visible... depends how you rate bragging rights I supose!

[Tycho Brahe]

Okay, I'm going to order the mirror blank this weekend... Advice seems a bit split about grinding a first mirror... I'm not sure wether to dive in with a big blank or try a smaller 8inch grind first....

And the secondary mirror is just flat right? It just needs to be angled?

[FraserClarke]

And the secondary mirror is just flat right? It just needs to be angled?

Yes -- but the secondary is general considered harder to make than the primary! Most people buy them.

[weareborg]

Okay, I'm going to order the mirror blank this weekend... Advice seems a bit split about grinding a first mirror... I'm not sure wether to dive in with a big blank or try a smaller 8inch grind first....

And the secondary mirror is just flat right? It just needs to be angled?

Where are you purchasing your mirror blank from????

[Tycho Brahe]

I'm currently torn... I was thinking I should build a scale model, then I thought why not do it half, so six inch, if your going through the trouble of building a 'model' make it count...

If I do that, I'll get a mirror kit off galvo optics... its about 60 quid for the blank tool and grinding powder... not the end of the world if I screw it up.

If I decide to go straight for 'the big one' I'm not sure cause galvo seem to go up to ~12inch! There was a big old porthole on ebay... I got outbidded on a 12" dob mirror that need recoating just now! So I nearly escaped grinding one!

[pvaz]

Just a personal view on this:

If your building the scope for the pleasure of making it yourself, as a hobby, then do it. In that case I'm sure you'll be proud even of a 6" and then you have a better success chance for anything bigger you want to do next.

If you're doing it to have a big dob for less money, then I'm sure if you do some extra hours at something you have experience (as in your job) you'll get the money for a 12" dob in much less work hours then it will take you to make a scope.