First time builder

[CANISMINOR]

Hay there peeps,

My name is ash and only just recently got in to astronomy and have found it breath taking but all iv had a look through is a 3" cheap thing, now im finding my self rattaling my brain day and night to try and build my own. I opted for the dobsonian style and so far have got my hands on a 6.10" mirror, this may be a bad thing i dont know but its one from a shaving mirror you know the one's. its like 2X magnification or something. also im going to get an 200mm diameter by 1200mm -1300 length tube (cardboard) just for the test run if you will. I think i will need a 47mm scondry mirror as well unless im told other wise?

i kind of know what to do finding the focal point so i know where to put the eye piece and all that and getting my base sorted for me, so really im just looking for some advise or ideas manly help with dimensions and stuff so if there is any one out there that can help i will love you for ever.

so there you have it you've heard my plee

Thanks

Ashley Bond

[sbooder]

Hi Ashley,

I think you have had little response because no one wants to give you the bad news. I am afraid a shaving mirror will not give you any images at all.

Telescope mirrors are not made in the same way and are of a much higher surface quality to mirrors made for everyday use.

A standard shaving mirror will be glass with a backing of some reflective metal, probably a tin and silver.

An astronomical mirror is glass with an ultra thin coating of Aluminium on the surface, the class is ground to a specific shape (either spherical or Parabolic).

You will need to source an astronomy mirror set to start with. I would suggest an 6”- 8” for your first telescope if you are intent on making your own.

Good luck and I am sure that many here will be happy to give you all the advice you need along the way.

[Gina]

Yes, I'm afraid that's quite right. Good luck with the build I haven't attempted anything like that yet but I expect to one day - I like making things.

[Squats]

Hi Ash, try googling `make your own dobsonian`. There`s lots of info around. Also, google `telescope mirrors`- a blank, which you`ll need to grind yourself, will go for about £30 upwards. Ground ones go for three figures....

Or try E-Bay, there are bargains there. I got a 4`` TAL for £25.

I`ve looked into building my own Dob, but price it up carefully- there are plenty on E-bay for about £100, if you can collect it.

Good luck

[CANISMINOR]

cool thanks guys thats a shame tho, i know this might sound silly but my mirror does have a curve in it and is round and has any one tried useing this sort of mirror befor? ok so iv seen some Parabolic mirror's and they look quite thick and also cant it take years to grind a mirror? and this Aluminium coating how do i get my hands on it and could i not coat the mirror i have with this?

my budget is low, iv done my research but if any one can point me in the direction of a seller of these mirror's or the equipment used to make one im willing to give it ago all the way i love building stuff iv made all sorts. this is far more technical. iv also seen the method to grind the mirror too so wont be to hard just time consuming.

so if any one can help me on this front that would be fantastic

thanks again

Ashley Bond

[marcopolo]

I've used that kind of mirror before but it was for burning wooodlouse, not looking at astonomical objects. It's about 4 orders of magnitude too innacurate for astronomy. To give you an idea of how accurate a telescope mirror is, if it deviates from the corrcet shape by even a few millionths of an inch it is rubbish. Astounding that the human hand alone by sheer repetition and revolution can grind something so accurate!!

Sorry almost forgot. Aluminium is evaporated onto the mirror surface (I think in a vacuum chamber) It a fairly specialist process so not one for the home I don't think! I wonder how The pioneer John Dobosn did it / had it done??!! He was some dude that lived in a Monestary and made telescopes from scrap wood and ship portholes. He was quite good at it too.

Good luck.

Marco.

My advice for people wanting to build their own scopes is don't touch mirror grinding with a barge pole unless you are very patient and have lots of free time on your hands. Theres so many mirrors out there in the 4-6in range that can be gotten for less tha £100 second hand or posibly new. Try get 6in if possible. Look no further than ebay.

[Gina]

Sound like good advice to me Marco. I doubt grinding your own mirror is as cheap as it sounds. You need a turntable and you need a way of measuring how you're doing and how accurate the shape is. With that level of precision, I can't see that the testing equipment is going to be very cheap. That's how it looks to me but I've never done it and don't really know so I suggest asking in one of the many threads on the subject. You may not get a reply in this thread.

[CANISMINOR]

thanks marco and gina well it looks like im out of luck using a shaving mirror lol but no worries, so really my main options would be. A, buy second hand mirror off line for cheapish. or B, get a blank, spherometer, turn table and all the grinding tools then send off to specialist after like 3 years of grinding lol to be coated.

ooh man i realy want to make my own preferablely from scratch but it seems it will cost me more to make one than to buy a dobs new haha.

well i would like a 6" or bigger really to observe stars, planets and systems rather than the moon so ok if any one can direct me or knows anyone selling mirror's that would be great

thanks again guys

Ashley Bond

[Gina]

ooh man i realy want to make my own preferablely from scratch but it seems it will cost me more to make one than to buy a dobs new haha.

Yes, that's the trouble! I often find it costs more to make something than buy it. Sometimes I've bought something and later made my own having found where I might improve on the bought item. That's the case with weather stations. Bought the cheapest wireless one that connects to a computer and learnt the ins and outs and now building my own - which is great fun. I'm a keen DIY person.

[sbooder]

It is always the way, telescopes like computers used to be cheaper to build yourself. I remember when I first started in computers, It would cost about half to build your own, now it costs double.

[Gina]

It is always the way, telescopes like computers used to be cheaper to build yourself. I remember when I first started in computers, It would cost about half to build your own, now it costs double.

I did manage to build my own computer for a bit less than a bought one of similar spec. But it helped that I already some of the bits, like optical and hard drives, keyboard and trackball, and I used the free OS, Linux. One advantage of building your own is that you can use exactly the parts you want.

[FraserClarke]

Sound like good advice to me Marco. I doubt grinding your own mirror is as cheap as it sounds. You need a turntable and you need a way of measuring how you're doing and how accurate the shape is. With that level of precision, I can't see that the testing equipment is going to be very cheap.

The test equipment can be really amazing simple -- you literally don't need anything more than an LED and a razor blade. If you have some hand tools, a very functional test set-up will cost you about £15 to make. You don't need a turntable -- you just walk around the table.

Making small mirrors is really not very complex. You do need patience, and the right balance between "well that's not good enough, I'll have to that bit again, and again, and again" and "good enough is good enough, I'll leave it now". You might not get 1/10th wave mirror on your first go, but it more than likely be a perfectly usable mirror.

HOWEVER, making any mirror less than ~12" will cost you more than you can buy a finished mirror set for. And I suspect making a telescope less than 8" will cost you more than the finished thing will. If you're worried about cost -- buy 2nd hand. If you like making things, go for it.

[Ptarmigan]

you need a way of measuring how you're doing and how accurate the shape is. With that level of precision, I can't see that the testing equipment is going to be very cheap.

Frazer beat me to it !

The Foucault Test

Link to Wikipedia image :

and then there is the more complex, but still doable by amateurs, Ronchi test.

[CANISMINOR]

lolz went off track a bit there with the computers hehe but i totaly see what ya'll saying well iv kinda worked out my cost to build my own

.blank mirror: 6" will cost £28.99

.grinding tools: for 6" will cost £26.99

.mirror coating: £68

.secondry mirror: 44mm £46.99

.tube: 1200mmX206mm £24.95

.focuser: £35.99

.eyepiece: £31

.finder scope: £30

.all the bits to hold it together: £10 (i guess)

.stand: free

drum roll please for the grand total: £302.91!!!

so it turns out £100 more than a sky-watcher 6" dobsonian at £199

well i know what im gonna do now then.

just buy one new wait a good few years get some more exp then build a BIG one

from scratch but please feel free to use this thread for those that are building for the first time maybe some one knows a cheaper way if so i wanna know

thanks again

Ashley Bond