6"/f15 Achromat testing

[Ryachu]

Hello,

A good friend made this for me many years ago and it has never been used. I would like to test it before going ahead and constructing a tube assembly. It is mounted in a cell and it is made from BK7/F2 if I remember right. I don't currently have any optical test gear ( optical flats etc. ) only

eyepieces and diagonals from current scopes. Ideally I'm after a quick lash up so with my limited knowledge I can get a quick idea of the performance. Then it's off to B&Q for some brass and mahogany

many thanks,

[Astroman]

You should see some of the test jigs I've cobbled together! (Or maybe not. ) I've set the primary on a table and hand walked an EP across the room to view distant mountains.

As I tell those who are surprised at some of my constructions, (and there have been hundreds), a telescope is only some pieces of wood or metal that is designed to hold these pieces of glass a specific distance apart and keep them there. Anything else is cosmetic.

HTH.

[barkis]

You worded your post as though there was a picture to see. I expected to see a link but no.

It is quite obviously a lens, so you can use the sun to get the focal length, but dont focus on anything susceptible to heat. Once you know that, you can do as AM did and lash it to a length of wood 100 inches long, very securely mind you. Then find the C of G and pivot between two other pieces of wood,in the form of a fork, then using an eyepiece, the moon is in the sky to give you some Idea as to whether spending any money is going to be worth it. Bit of a codge up I know, but it's a quick and non cost way to do it.

Ron.

[GordonCopestake]

6" f/15!? You would need a AP1200 for that i'm sure hehe

[Astroman]

I should post some pics of the coronagraph I'm currently cobbling. 8) You may get a laugh, if no other helpful info.

[Ryachu]

Some good ideas there. Sorry for the lack of pictures but my current phone doesn't have a camera ( and the lead probably wouldn't stretch to the garage if it did ). As for the wording I must have got it wrong , should I have said an "Achromatic Doublet Lens" ?

So I could nail it to something. Do I need to worry about stray light , baffling etc. I seem to recall it is only going to reduce contrast without affecting the actual star image. Are there any good daylight targets to try it out on ? I did read somewhere that a sunlit ball bearing was a good point source.

thanks,

[barkis]

Yes, baffling will be required, but for your immediate need, which is to establish whether to proceed with constructing a proper home for the lens, you are better trying it on a an infinity object, and the best for that is a star. The most suitable for that, is of course Polaris.

Polaris, being quite close to the celestial pole, will stay still long enough for you at least to make a quick assessment of the lens, bearing in mind that seeing conditions will play a part to. That being the case, you would need to leave your Jerry rigged assembly in place to ensure you

give the lens a chance to prove itself, one way or the other. Another point to remember, is that this support mechanism you build, is hardly likely to result in perfect alignment of the optical train, and therefore coma, and other oddities in the image, could be a result of that misalignment, and not necessarily a bad lens, so be careful about condemning it prematurely.

I guess it is almost certain there will be chromatic aberration, but that should come as no surprise to you.

Ron.

[Ryachu]

Hmm , you're right there. A shiny BB is going to have lots of diverging light rays. A source at infinity is what is needed.

CA at f15 :shock:

thanks Ron

[Astroman]

CA at f15 :shock:

The 24" Clark refractor at Lowell Observatory is an F/16. It's a fine instrument, but I've always been taken aback by the amount of CA in it, even with not so bright objects. I suppose it'd be a good thing to expect some CA in a 6" F/15. Modern glass has done wonders to reduce CA, but can't completely eliminate it as far as I know. (Which isn't that far.)

[Ryachu]

Thanks again for the info. I probably would have put it back on the shelf if I'd seen any CA. I thought at that sort of f number it should be fairly well corrected. The problem now is finding something long enough to fasten it to !

[GazOC]

I think theres every chance you'll see CA @ f15 in a 6" achromat, I'd be amazed if you didn't.

[barkis]

I would try to look on the positive side, A 6" f15 refractor is a serious instrument. The planetary and lunar views alone, are going to be something to die for. If it is a well corrected doublet, and well baffled in its tube, then any CA may be minimal, and who knows, maybe none.

Your biggest task, if all's well, is making, or getting a suitable mount for it. I own a 6" f8 refractor, and I was under no illusions when I bought it, that Chromatic Aberration would be present, and it is. It places a lovely yellow ring around the lunar limb, but it does not detract from the excellent detail it renders. I am more than happy with it.. I would love to have an eyeball at the f15 when you have got it finished.

Good luck with it.

Ron.

[GazOC]

The 'old rule' used to be that the f# should be 5 times the objectives size in inches to eradicate CA. Thats not to say the 6" f15 doesn't sound like a cracking scope.....

[Astroman]

Ron makes an excellent point about the yellow ring. I tend to ignore or just fail to see the coloration when I look through the Clark 24", just because, it's 110 years old, it's made by Alvan Clark, it's HUGE, and Percival Lowell himself observed through it, for goodness sakes! A long refractor is a long refractor, and should yield some wonderful observing moments. I'm fairly sure you'll learn not to see the CA, if present.

[Ryachu]

Here's a low res. picture of the lens.

(click to enlarge)

Looks a bit plain compared to modern multicoated glass. Knowing the chap who made it those lens spacers are probably made by Mr. Rizla !

I've got some mica from transistor heat sinks to play with the spacing.

[barkis]

My late friend Dave Sinden, made some wonderful lenses, and as far as I know, none were bloomed.

Did your friend make the cell too?. As for the spacers, I don't think they will be fag paper

But, those are important, as the dictate the air space between the two pieces of the doublet. Not only that, the orientation of the two, will have a special posittion relative to each other, and could be marked on the edges of the discs. They should not be dismantled.