Stu

Yep, the 31mm Nag is 42mm, I guess a 28mm 82 degree is mid 30's perhaps, given that a 27mm 68 degree is 30.5mm All good experimental stuff

Sounds good! You've clearly done lots of thinking about this . Does the eyepiece field diameter mean the focal length or the field stop? I'm sure the end result will be good, I've used systems which I know are vignetting but in practice it is barely noticeable. I do like the way you've found a system which allows you to mount the new focuser but still keep the ability to collapse the tube. I look forward to hearing how you get on with it under the stars! I love widefield scopes

Very interesting mod Craig, quite ingenious! Do you know whether you get full illumination of a 2" eyepiece? That would be my only concern as I assume the secondary was optimized for 1.25" to be as light and cheap as possible. You might get some vignetting on longer focal length eps?

Some more pics of this little gem. Having found the collimation to be out, I am keen to sort it so decided to dig out the rather hard putty which was covering the screw heads. Amazed to see a proper push-pull mechanism so I should be able to get it spot on; at f7.7 it shouldn't be too sensitive though. I might even go the whole hog and flock it at some point. Fitting a finder shoe should also be possible and will make life easier than grubbing on the floor trying to sight through the front and back holes! The date in the book say 1992! That's about the same age as the old Vixen 150ED I used to have.

Thanks Robert. I do like different scopes, and ones which are nicely engineered. It's certainly not lightweight! The box feels like it weighs a ton!

Lovely looking setup Tony, bet the LS80 is pretty spectacular! Nice to see the TV85 looking so good

Thanks Jules. Thought the optics would be better in the AS150?

If I understand the post on CN by EdZ correctly then I don't think you've quite got it. There is an effect going on which changes the convergence of the light beam which shortens the light path. I am hoping someone will come along with a ray trace comparison between the two to help clarify it.

Yes indeed Jules. Don't trust them myself, but thought I'd give this little'un a go

I think I might qualify for the smallest newt on this thread now TAL Alkor 65mm Built like a tank and lovely optics

Mark, I'm beginning to think I'm at the limits of my knowledge too What I do know (or think I know) is that a light ray hitting an air to glass interface at 90 degrees will not be refracted at all. I think that a parallel beam of light passes into the prism without deviation, and is then reflected through 90 degrees by the internal surface which is at 45 degrees to the light, and it then exits the prism without further deviation as it passes the glass to air interface at 90 degrees. The case for a scope is different as the light beam is not parallel but is converging towards the focal point. As I now understand it, the prism affects the angle of this convergence and so shortens the light path through the prism and increases the back focus available. This part I am still a bit unclear about and would like to see a ray trace through a prism showing this changes in convergence vs mirror to be clearer.

Hang in there! We will get it resolved. As said, I'm no expert and may be wrong but Yong's post was not my understanding so I thought I would check!

Yong, I'm far from an expert on these things, but I thought the light path through the prism still reflected off the back internal surface by total internal reflection? As I understood it from EdZ's post, it's the refraction of the converging light beam that actually shortens the light path. As said, and this emphasises the beauty of SGL, I'm learning again today as I've never considered this stuff before, all very interesting.

It's hard to understand quite what is so attractive about the short ugly billionaire with relatively few years left isn't it? This is actually what came to mind when I saw your scopes lined up

Well, it's not a light bucket is it This could quite possibly be one of the smallest Newtonians around at 65mm. Bought for less than the price of an Ortho off eBay, I was delighted to find out that the seller was none other than Dave T (f15Rules). It took me some weeks to manage to get some time to drive up to collect it, but when I eventually did, it was great to finally meet Dave and have a nice Astro natter . The TAL Alkor is built like a tank! It comes in a very nicely constructed wooden case, with a secure place for everything. The components screw together quickly and firmly to give a very simple to use, and rugged little scope. The mounting is AltAz, with the ability to set a coarse position, then have an amount of slow mo control once clamped in position. The 'finder' is just a couple of sighting holes front and back, enough to get you on target at the lowest power. The focuser is a simple single speed, very smooth but possibly needs a tighten up if I can find out how. There is a little bit of image shift which may tune out with a bit of fettling. The system of eyepieces is beautifully simple and easy to use. I say eyepieces, there is only one, a 15mm I believe. There is also a Barlow and an extension. On its own the eyepiece gives x33, with the Barlow it gives x88 and with the Barlow and extension you get x133, all you need really. There are no locking screws or compression rings, just simple, precise engineering so the components slide easily into each other with no slop. The engineering is beautiful but simple. There is a tiny secondary mirror on a single stalk and the little primary is in a fully adjustable cell. The adjustment bolts are covered on my scope, but a quick star test showed that the collimation is off so at some point I will dig out the putty and have a tweak. I do like trying to see just what you can achieve with very small scopes, which is partly what attracted me to the Alkor. So, the views? I tried the scope fairly quickly last night before it was fully dark. I picked up Saturn first at x33. Lovely crisp view with the rings clear. At x133 the separation between rings and planet was clear. I could also see the difference in shade between the A & B ring and a band on the planet's surface. Quite amazing for such a small scope. With better collimation who knows, the Cassini division may be possible on a good night. Moving on to Mars, the views were less impressive as you might expect. I could detect the phase, but otherwise it was a washed out pale orange colour with no surface markings at all. Mizar was next. At x33, which was all I used on this target, the split between Mizar A and B was clear and the whole system, including Alcor (appropriately given the scope name) and Sidus Ludoviciana the eighth magnitude star between them, looked lovely. A fabulous view. I had a peek at Izar, and would class this as a definitely probable split . The miscollimation showed here, the diffraction rings not symmetrical around the primary, but there was a fairly clear view of the blue gray secondary amongst the rings which were jumping around in the poorer seeing. Finally, I was trying to find Vega (doh!) which should be trivial but it was in the 'dob hole' which affects AltAz mounts too, so I ended up somewhat fortuitously on the Double Double by accident. At x33 they were a fine pair, but at x133 I was delighted to see a clear split in both pairs. Each star was beautifully resolved with a lovely diffraction ring around it. The more difficult of the two pairs was still resolved, with clear black space between the stars. Overall I'm delighted with this little scope. Hopefully it should be fun to use on the moon with Lorna (my 2 year old) as she gets older, and it is rugged enough to stand up to a bit of rough and tumble. I will also enjoy using it for fun quick sessions to see what I can see with it. It will sit on the EQ platform too which will make planetary and lunar observing much easier. A few pictures of my new 'Light Thimble'

Derek, I bet you were like Bernie Ecclestone lining his mega motor homes up in the paddock with those scopes, weren't you? They look millimetre perfect

Nice one quaoar, looks great. Your cat is going to get a shock any minute though!

Well that's a very different statement to the one you made, and EdZ's version makes sense. The shortened length is due to the refractive index of the prism changing the convergence of the light beam and meaning that the optical path is shorter. It's nothing to do with the light travelling more slowly through the prism, but is everything to do with the refraction of light as it enters and exits the prism, which ultimately is caused by the slowing and speeding up of light at the interfaces.

The speed of light is constant in a vacuum but it does change (slow) in other media such as glass. I think astronomers only normally consider the speed in a vacuum because that is what it is for the vast majority of its journey to us from the stars. The shorter light path on a prism however has nothing to do with the speed of light, it is simply due to the geometry of the prism vs the mirror i.e. The prism has a physically shorter light path. I'm not totally sure why, but putting my Baader Zeiss T2 prism next to the BBHS T2 mirror, the prism is clearly a smaller package.

It is worth considering the Baader T2 diagonals or prisms as they give a larger clear aperture but still have a short light path helping you to reach focus.

Having briefly had a couple, they do have a kind of rugged beauty to them Derek, all very solid. The thing that confuses me is the plastic dew shield which seems to fall off very easily and doesn't seem to match the rest of the build. Nice and lightweight though I guess.

The scope looks awesome Mike, really cracking stuff. If it were me I would just let them get it right, the scope will be around for a long time, best have the best mirror possible!

Thanks Chris. Now back to the APM 130...

Without wishing to take this thread off topic, I just checked, and the primary is listed at 3500 K, and the secondary 20900 K, which seems quite blue to me . I've never had a decent chance to try with Antares, must give it a go sometime. Would the FC100 do it under excellent conditions?

Wonderful image John. That's one I've never managed to see, must try harder! I didn't realise the colour contrast was so lovely!