Very fast 6'' Frac for £75 (just the objective)

[rawhead]

Found this on Surplus Shed on offer. (for a few days)

FAST 6.1" DIAMETER TRIPLET OBJECTIVE MULTI-COATED - Surplus Shed

6.1'' objective @ F1.25

It's got a focal length of 195mm.

An extremely fast manageable 6'' DIY frac.

All for £75 + postage.

Would the collimation have to be so accurate to make a home build unfeasible?

Any other obstacles I should be aware of?

Cheers

[theodore]

Hi,

like its says, OK for low power light buckets, but I imagine there will be a lot of aberations (chromatic and otherwise) at higher power.

Theo

[JamesM]

There is an old saying about free lunches and I believe this is a good example. Having read the details, I thought I would share some ideas about it.

First its an 'achromatic' triplet which implies to me that its a doublet with a further lens being used to shorten the light path. It is clearly not a 'triplet' in the sense that it uses three lenses to control refracted in order to bring different wavelengths of light to focus to the same point and so avoid chromatic aberration that would occur at this focal length. The sales script informs us that these lenses were made costing several thousand dollars and yet they are only being sold for $125, why so cheap? In fact if they were dearer it would appear more credible. This sounds like the price is about cheap enough to hook you in and for you to write it off as a loss, but not so expensive, as for you to feel more likely to want to get your money back or take it further up the line. The worrying statement is that the lenses have received "limited testing", surely having cost so much to make originally, they would have had to justify that cost and so documentation would exist to substantiate it and of course that would have to include some sort of test certificate of quality. The statement, "can't find any objective this fast and size at any price" might give us a clue as to why that might be, because to make this type of lens to the standard that is implied would probably make it not cost effective to even attempt it in the first instance and hence the reason why you can't find one. The best bit is the last bit, in that they will only send out periodic newsletters detailing feedback ONLY to those who purchase the lenses. A bit late to read that everyone else who was sold the same free lunch thought the lenses were in reality useless to use.

I wrote the above comments as a way of examining the detail of what was actually said in the advert. They are underselling an amazing triplet lens assembly which they haven't tested from an unknown company and at a price you will be happy to write off as a learning experience. This is no way a comment on your judgement, but as I started off by implying, that any offer that seems to good to be true, probably is and so they are trying to sell you a free lunch. I wouldn't touch it.

James

[rawhead]

Appreciate the feedback

[Gina]

I agree. I've always applied the rule "if it looks to good to be true it probably is". Stood me in good stead all my life.

[Tim]

Have to say, the gut feeling about these is a bad one :S

[Psychobilly]

Surplusshed specialise in ex military "money no object" kit...

Billy on the San Fran

.....

[ollypenrice]

Interesting though. What about a guide camera lens? No trouble finding a star!!!

Olly

[phzahn]

Hello everyone, this looks like a very nice place to spend some "down time".

I have information concerning this particular lens. The lens by itself is fairly useless, as most of you have guessed. However, the surplus shed also sells the complete lens assembly this objective comes from. The complete assembly sells for 575.00 dollars, and is probably one of the biggest bargains I have ever seen.

The complete assembly has many more lenses, and it is a zoom assembly. At the back of the main housing there is a precision beam splitter. At the back of the beam splitter is a shutter box with a very nice shutter.

Psychobilly is right about the money is no object statement. I purchased mine and recieved it only a week ago. The build quality of this unit is amazing. It is a very heavy assembly weighing 16 3/4 lbs.

As for the optics, there is the 3 element main objective. Then there is another lens cell behind the objective which contains one 90mm -78mm focal length concave lens and another lens which is 60 mm in diameter. This lens cell moves back and forth for the zoom.

Behind the 60 mm lens is another one in its own cell of approximately 45mm diameter (concave) and it also moves back and forth with the 90 mm one, but at a constantly varying rate to the 90mm. Then at the end of the housing is yet another lens of 28mm diameter which is in a fixed position. This is a convex lens.

After this the beamsplitter is attached, and there is a 16 mm concave lens at 90 degrees to the centerline axis. At the back of the beamsplitter is another one of the 16mm lenses going into the shutter box.

All of the lenses are coated. I have only taken off the shutter box and beamsplitter for now, as well as the last fixed lens before the beamsplitter. I am very reluctant to go any further than this. As far as focal length goes I do not have a lot to contribute other than to say if that last fixed lens is made to move with the lens preceeding it, spaced about 3 inches from that lens interesting things begin to happen.

For those whos interest I may have peaked I would like to infrom you of a discussion thread at Cloudy Nights Telescope Reviews. I do not wish to offend the owners of this site by putting a link to another site on their board. Do a google search using this " 6 inch triplet-F1.25? ", and it should be the first result. Again, if this offends the site owners I sincerely apologise, but the information is pertinent and not available elsewhere.

Looking thru the lens reveals a very nice image. I know this means nothing, but from what I can make of this, it should make an incredible astrograph or widefield scope, and have a useful magnification range from 22x to perhaps 100x or a bit less. This does not take into account the built in zoom feature, which seems to have quitre a bit of zoom.

I am now trying to get a better handle on the focal length range and how to proceed with the build. It will certaianly be a very fun project an dhopefully a fruitful one. I admit to being a novice when it comes to optics, I am only a mechanical/electrical engineer by education, but I do own a machine shop and can at least make anything that I can design so I will not lose any money on this save for the cost of the assembly. You cant take it with you when you die, so you might as well spend it on something, and I have all the fly fishing rods one man can ever use.

If I dont get banned from here I will post from time to time my progress with this little project.

Best Regards to you all,

Peter Zahn

p.s. please excuse any typos as I am not a very good typist.

Oh by the way, we have thought that this camera was used in a military airplane. The beamsplitter probably had a video camera attached and the monitor was in the pilots view so he could see what the camera was looking at. This is just a VERY big VERY wild guess. LOL

[rawhead]

Thanks for the info Peter and welcome to SGL

Now reading through the Cloudy Nights thread...

Do you have any pics?

Cheers

*EDIT*

http://www.surplusshed.com/pages/item/l10593.html

[phzahn]

Thank you for the welcome rawhead.

I do not have any pictures yet, but I will take some and see if I cant get them up here for everyone to see.

I've done a little more work with this unit over the weekend. It is too early to tell if I can make a useable scope from this assembly, but I will most certainly be able to make an effective widefield camera lens from it for use in tandem with a scope. The assembly does have quite a lot of zoom to it already.

My current dilema is figuring how to make it work with a focuser for visual work. I will have to acquire a few cheap lenses and play a bit with them to see what is possible, and what magnification range I will wind up with. I am not at all sure what sort of magnification range I will have for visual work once it is all said and done with. Right now I dont think there is enough room to put a focuser at the back.

In any event , it will make a camera lens I would have no hope of ever being able to afford otherwise.

[phzahn]

Well its been a while since I last reported progress with this lens.

What we have found out is that the lens will take some breathtaking wide field pictures. On page 6 of the cloudy nights thread is a very nice example.

Using the lens visually is also possible, but requires inventiveness and a lot of experimentation.

In my opinion, using only a few of the lenses and trying to make a scope from them is not going to work. You have to have the entire assembly including the beamsplitter, and all the lenses in it.

if you like a challenge, this is a good one, and at a bargain basement price, for the possibilities.

That beautiful picture is a single 5 minute exposure so its a really fast lens. F1.2 or faster depending on magnification? Maybe so.