Unusual f/1.25 refractor

AstroApprentice (Jason) · #1 10-12-2016, 07:46 AM

Check out this for an unusual refractor on eBay:
http://www.ebay.com.au/itm/TELESCOPE...QAAOSwux5YR02s

glend (Glen) · #2 10-12-2016, 08:42 AM

There is a thread on the Cloudy Nights forum about an imagng version/ build of a 6" f1,25 achro triplet refractor. Can't link from my current device but if you search on CN or Google " 6" f 1.25 refractor". it can be found.

billdan (Bill) · #3 10-12-2016, 09:40 AM

I like the eyepiece turret, would be a handy accessory to have.

The coma must be awful at that F ratio.

tempestwizz (Brian) · #4 10-12-2016, 10:11 AM

The lens seems similar to those offered by the 'Surplus Shed' in the US a few years back. They went for under $100 from memory. I bought one but have not had much 'luck' getting it to do anything useful.
It is around 6" diameter and was advertised at f0.98 with 200mm focal length.
The Shed also offered a negative lens assembly that was purported to be made for this front lens which if proper placed turned it into a longer focal length assembly of around 800mm. I have tinkered with various combinations and have found that it appears to work with a tube length of around 700mm. The mechanics of getting a suitable tube to hold all the lenses is limiting me at the moment, so I've not progressed very far.
Maybe I'll give the wide angle as shown a try out. But if interested in the eBay version I'd suggest checking out the Surplus Shed first.

Stefan Buda · #5 10-12-2016, 07:26 PM

Quote:

Originally Posted by AstroApprentice

Check out this for an unusual refractor on eBay:
http://www.ebay.com.au/itm/TELESCOPE...QAAOSwux5YR02s

Is this some kind of a joke? Even if there were eyepieces that worked at f/1.25, the usable minimum magnification, for an exit pupil of 6mm, would be 26x, and that means maximum focal length of 6mm for the eyepiece. With the advertised 32mm eyepiece the exit pupil is about 26mm.

dannat (Daniel) · #6 10-12-2016, 09:18 PM

They tried to use as autograph in US, apparently it was from an aerial camera system. Focus point was a fraction of a mm, too hard to acquire focus, there was a second lens set that matched it to Make it useful

torana68 (Roger) · #7 10-12-2016, 10:01 PM

looks like $10 for a paper weight to me

The Mekon (John Briggs) · #8 10-12-2016, 10:14 PM

yes, the sum of the parts is surely worth more than the whole, and certainly nothing like the dreamer is asking

mental4astro (Alexander) · #9 11-12-2016, 12:49 PM

I can see some advantages that such a scope offers.

Yes a 26mm exit pupil is very large, but it's not like looking through a brick. Depending on the eyepieces being used, aberrations may be well controlled. Very much eyepiece design sensitive.

As a rich field scope, despite the effectively reduced aperture when using a 32mm eyepiece, it may well show subtle details that would elude most instruments. There's a lot to be said for a concentrated image.

It's all well to talk about the ideal exit pupil. But this is a reference point for optimal aperture effectiveness, but there is nothing to say that a larger exit pupil can't be used and does not have anything to offer.

Anyone bothered to look through this scope before dismissing it outright? This is a very unique instrument, and requires an open mind to see what it has to offer. I'd love to be able to spend some time with it. Shame it's in Brisbane and I'm in Sydney. Hugh, think you'll be coming down to Sydney any time soon?

Stefan Buda · #10 11-12-2016, 06:09 PM

It all comes down to those damn numbers..
You look at a star through this contraption, with dark adapted eyes and 32mm eyepiece, and 5.3% of the light will fit through your 6mm iris. Why would you bother?

mental4astro (Alexander) · #11 11-12-2016, 07:24 PM

Don't get me wrong. There is no denying "the numbers".

An instrument like this one falls into the realm of a curiosity. It will be very difficult to sell. But if the chance presents itself for some time with it, it certainly is worth the experience. The 5.3% translates to an effective aperture of 17mm. Still larger than our pupils. If the scope is capable of putting the whole of Orion through the eyepiece, I'll have a look. Purchase it, no.

It's easy to sit behind "the numbers" and crucify something. It's another to acknowledge the numbers but still have a look and understand what it has to offer. That's the only thing I've said all along.

Wavytone · #12 11-12-2016, 10:31 PM

Save your pennies and buy an 8x50 finder, it will work a lot better than this.

It's almost certainly the lens out of an old overhead projector, used in classrooms in the 1970s and 80s before digital projectors. From memory - having had one - these are usually triplets of pretty ordinary optical quality because when projecting an A4 slide the f-ratio mattered a lot (to make bright image in a room that might not be completely dark) but sharpness wasn't a huge priority.

As a telescope these are pretty much crap, and no good for photography either (too soft).

Yes you could put a 32mm eyepiece behind it, the result is that your eye iris will be the aperture stop, meaning that the resulting system (including your eye) is working at the same focal ratio as your eye - f/3 to f/4.

So... it's really only equivalent to a 50mm finderscope... and it makes a rather poor one at that because as indicated previously the focus will be very soft.

Stefan Buda · #13 11-12-2016, 10:34 PM

Sorry but this has absolutely nothing to offer other than a horrible optical mess.
The effective aperture with 32mm eyepiece and 6mm iris opening is 35.7mm, not 17. So if you use an aperture stop in front, of 35.7mm, you will clean up the 94.7% of the light that would otherwise just contribute to stray reflections inside the eyepiece, and your star would loose nothing from its brightness.
See, this is why the add suggests that it should only be used from a dark site.

N1 (Mirko) · #14 12-12-2016, 07:41 AM

There are other eyepieces than 32mm, you know

Stefan Buda · #15 12-12-2016, 08:22 AM

Yes, indeed, a 6mm one would give you full aperture with an exit pupil of 4.8mm, but what a mess of an image. Or maybe you know of eyepieces capable of working at f/1.25. And I'm assuming that this is some sort of miracle triplet that can produce a perfect image.

mental4astro (Alexander) · #16 12-12-2016, 10:02 AM

Well, I contacted the owner of this scope asking about its performance and no reply. So I take this as meaning they don't think too much of it, but only happy to ask for a pretty penny for it. Can only try.

FlashDrive (Poppy) · #17 12-12-2016, 10:39 AM

I'm sure this Scope has been advertised on IIS a fair while back .... maybe at least 2 years ago.

I'm sure it was ....

Col...

OICURMT · #18 12-12-2016, 01:29 PM

Quote:

Originally Posted by FlashDrive

I'm sure this Scope has been advertised on IIS a fair while back .... maybe at least 2 years ago.

I'm sure it was ....

Col...

http://www.iceinspace.com.au/forum/s...d.php?t=128062

LightningNZ (Cam) · #19 12-12-2016, 05:49 PM

From memory the one person on Cloudynights who got this to work ended up adding some negative lens components that reduced it to about F/3. It worked great, but only in a single wavelength at a time.
-Cam

JA · #20 12-12-2016, 09:11 PM

Quote:

Originally Posted by LightningNZ

From memory the one person on Cloudynights who got this to work ended up adding some negative lens components that reduced it to about F/3. It worked great, but only in a single wavelength at a time.
-Cam

Interesting Point LNZ.

At f/3 I don't know that anyone should bother, gone is the lure of the f/1.25 (although as others have pointed out maybe it was never there to begin with, in that it was unusable). Anyone that wants similar can just get a fast camera lens and attach an eyepiece via converter instead.

Best
JA