big scopes

Thanks guys for the advise.

Eugene, the reason I asked about a collimater is, I had to clean my mirror and when I went outside last night I could tell the mirror was off, but if collimaters are that cheap I might get one.

Dave the mirror I was going to get was at Oldham Optics and they sell 14 inch mirrors at f 4.1 or f 5.Do you think a f 5 be much better than a f 4.1?I might rethink the size.

Damon I live in fairly dark skies except towards the east(Cork city).The scope I was going to build was something like a truss design with most of the scope made out of aluminium.I was going to have a pier in the back garden and just disconnect the scope from it when going inside.I hope to make a mecanism so it would only take a few minutes to connect and disconnect the scope.

Thanks again for the advise.

Interesting idea Brian.
As for focal ratio, from what I understand, focal ratio is proportional to coma so the slower the telescope the less coma you'll have. Also, the slower the scope, the smaller the secondary and the contrast will be a little better. Also, slower mirrors are usually a little cheaper.
However, this must all be balanced up with the size of the scope. Also, a corrector lens (eg. Televue Parrcor) is also available which corrects for coma. This is inserted in the focuser and then you insert the eyepiece.
For a 14" f5, the eyepiece height will be about 70" or 5foot 10inches above the ground when pointing at the zenith (or mabey a little less if you have a low profile miror cell/box)
For a 14" f/4.1, the eyepiece height will be about 57" or 4foot 9inches above the ground.
Unfortunately, I don't own a large scope like this so can't tell you if coma is bothersome or not. Apparently, some find it a intrusive, others take no notice.

BTW, have you considered getting a mirror from the US??

As for the pier, were you polanning to equatorially mount it or just use it in alt-az?

If just alt az, why not put a couple of wheels underneath the mirror box, get a couple of handles and roll the socpe out of the garage/shed like a wheelebarrow?

Hi Briano,
I've seen the globular cluster M13 in hercules through many different sized scopes, and the differance between a 14" and a 6" is literally astronomical !!
It'll be much brighter and more defined in that it you'll see its stars more clearly. You'll never forget you're first view of the orion nebula through it where you'll be able to see the nebula as if it was cloud in the sky.

You need to be realistic, you really cant carry out a 14" tube every night you want to go observing and hook it up to a polar mount, you'll do it for a while, but eventually you'll get sick of carrying it out.
If you going to polar align a 14" scopes, it would be a BIG setup, you could leave it permanently mounted and have a roll away shead, it literally rolls away when you want to use the scope.

OR, if you use an altaz/dobsonian rockerbox on wheels of some sort, like Michael suggested, it would be much easier to handle.

As for F4 and F5, any optical defects will be more pronounced in a F4 system, things like spherical abberation is harder to get right in a F4 scope. You'll also need better eyepieces, for instance the regular meade super plossels work fine on a F10 and F6.6 scope, but they might start to show problems with F4, you'll need to get some nagler eyepieces to make the most of it.
Also objects like planets will be smaller in a F4 system, but then again you get wider fields of view.
I'd bet the F4 mirror is more expensive then the F5 also, but you'll be able to reach the F4 eyepiece without using a small ladder.
Make sure you know the wave front error of the mirror , i.e. is it 1/4, 1/6 wave accurate, this is important.

Collimation gets more critical as the F number gets smaller, this is important if you want to look at planets etc...
I used a dobsonian reflecter 12" F5 scope for a few years, it was great fun, but if wasn't half a beast ! definitely not portable.

Well, thats just my 2 cents.

The cost of the mirror is 1450euros for the f 5 and 1740euros for the f 4.1 and that 1/10 wave lenght with a second mirror.

The pier would be an equatortial, machined out of mild steel and aluminium with drive motors.I intend to make the tube out of 2mm thick aluminium with hollow bars along the scope for support.The tube itself shouldn,t be too heavy, the rest of the scope would be left outside.Its not going to be a dobsonian or there will not be any wood used.

I have designed a very small amount of the scope on my pc but most of it is in my head, I am also waiting on a book from amazon.The mechical side of things I can do either at home or at work, but it is the optical side of things I am not sure about.

I might just get a smaller scope.

Thanks again guys.

Briano,
Do you mean that the telescope tube itself will be all metal and be 2mm think, if the tube is complete, do you need these hollowed bars? are they going to run the length f the outside of the tube ?
Better yet, why not get rid of the tub and just make it a complete truss design, saves on weight.

Just make sure that this is not too heavy and that your mount/motors can handle the weight.

The mirrors good, Sounds like its going to be a great setup.

Briano,

You asked about a laser collimator, it is possible to make one. I have, following (roughly) the design in www.starcrwzr.com/laser.htm

The most difficult part to source is the 1 1/4" OD tube. I used a plastic PVC pipe used in indusrtial water systems. The lasers pointers can be bought for about 5 euro or less now. I find it useful as a quick check after transporting the scope. Total cost 10E and an hours work!

I have also used the "Barlowed laser" technique and find it very useful.

P.S when you are on that website, have a look at the 10 commandments page.

Michael (cheapeskate ) Scully.
www.kerryastronomyclub.com