ScopeTech 80mm Fraunhofer Telescopes

[Stu]

3 minutes ago, F15Rules said:

I'd be interested to hear your opinion on the new 80/F15 up against the Carton 100mm F13 John..that Carton spec sounds to me like almost a perfect recipe for achromat heaven, and as a user of several vintage 1980s Carton eyepieces, I know just how good their optics were/are..

Obviously the Carton would be a good deal brighter but the comparison in the other key areas, sharpness, contrast, colour rendition etc would be fascinating.

Dave

I can’t see how a 100mm would not beat an 80mm when they are both of similar quality? F13 vs f15 is unlikely to tip the balance I would think.

[johninderby]

22 minutes ago, Stu said:

I can’t see how a 100mm would not beat an 80mm when they are both of similar quality? F13 vs f15 is unlikely to tip the balance I would think.

The only downside of the 100 f/13 is it is a big heavy scope more suited to the AZ100 mount. The 80 is a fraction of the weight and may work on a much lighter mount.🤔

Big Red is about 1500mm long with diagonal 

.

Edited May 31, 2020 by johninderby

[F15Rules]

2 minutes ago, Stu said:

I can't see how a 100mm would not beat an 80mm when they are both of similar quality? F13 vs f15 is unlikely to tip the balance I would think.

That was exactly my point Stu, sorry if I wasn't clear. What I meant was that a comparison of this new Japanese Scope Tech scope vs an older achromat of renowned high quality would be a real test of just how good the new one is (or isn't).

But such a comparison could not fairly include image brightness or resolution for example as you wouldn't be comparing apples with apples..but on the qualities I mentioned such as contrast and sharpness, I feel the comparison would be fair.

If the new Scope Tech matched or got close to the Carton 100mm in these areas, that would for me be a big endorsement of the quality of the new scope's optics - especially when you consider that a new Carton 100 back in the 1980s was around £1200, IIRC!

Sorry for any confusion.

Dave

[Mark at Beaufort]

This has been a very interesting and informative thread. When I first saw this advert I thought that it might be a useful scope for planetary and double star observing. However, I also enjoy solar white light observing and if I own a refractor again I would purchase a Herschel Wedge. I assume that because this scope has some plastic it would not be suitable for direct solar viewing with a HW.

[John]

I'd be amazed if todays manufacturers could not produce very good quality small aperture long focal length achromatic doublets. If these are anything other than extremely good performers, something has gone rather awry.

 

[JeremyS]

So what was the question?

🙂

 

[andrew s]

46 minutes ago, JeremyS said:

So what was the question?

🙂

 

Your so managerial @JeremyS , who cares as long as we are having fun!

Regards Andrew

[Stu]

28 minutes ago, andrew s said:

Your so managerial @JeremyS , who cares as long as we are having fun!

Regards Andrew

I think he was..... 

[Dave1]

I think where the confusion comes is the mix up between angular airy disc size and linear airy disc size. Like in the link John posted. If you compare the linear airy disc size of F7.5 to a telescope of F15. The F15 linear airy disc size will be double the size airy disc. That's according to Sedgwick. 

But don't ask me to do the math! 

David

[johninderby]

I see no Airy disc

[F15Rules]

6 hours ago, johninderby said:

Big Red is about 1500mm long with diagonal 

Now that's a proper long frac!

I remember fondly when I had Andromeda (5" F15 D&G USA achromat)..she had a focal length of 1920mm and the tube, with Moonlite focuser and diagonal was just over 2 metres long..

Andromeda was thankfully rehomed to an SGL member and now sits proudly on a heavy pier and beefy EQ6 mount..a stunning scope😊🤩.

Oh, and Andromeda puts up large, perfectly presented airy disks in keeping with her long focal length and massive depth of focus!😎

Dave

 

Edited May 31, 2020 by F15Rules

[John]

Anyone remember reviews which commented on scopes that "snapped to sharp focus" ?.

I suspect the reason was that the scope was fast so the depth of sharp focus was shallow.

 

 

[andrew s]

9 hours ago, Stu said:

I have no knowledge about this Andrew, but that’s not what the link Johninderby posted to said. Now confused!

 

That is the what you get a prime focus if you take an image. I said the same in an earlier post. It is not what you get if you view it visually with an eyepiece at the same magnification.

The difference is with the image you are looking at linear size at prime focus on the chip, while visually you are looking at the angular magnification as the eyepiece produces an image at infinity which your eye then focuses on. 

Regards Andrew

Edited May 31, 2020 by andrew s

[andrew s]

23 minutes ago, John said:

Anyone remember reviews which commented on scopes that "snapped to sharp focus" ?.

I suspect the reason was that the scope was fast so the depth of sharp focus was shallow.

 

 

I do remember it but more in the context of aberrations. Aberrations cause there to be no clear focus point while aberration free optics snap in.

Regards Andrew

Edited May 31, 2020 by andrew s

[andrew s]

@Stu, when words fail try a diagram. Not very good but I drew this to scale just using geometric optics rules. I hope it show  what I tried to explain in words above.

Regards Andrew

PS It occured to me that the fundamental difference between the linear and angular magnification is that when you scale a triangle all the lengths increase in proportion  but all the angles stay the same.

Edited June 1, 2020 by andrew s

[Stu1smartcookie]

Just looked at this scope on FLO ( trying to get an answer to whether it actually is in stock ) .. mounted on an HEQ5 pro i reckon this will be a handsome sight ... more importantly , will CA still be an issue on a scope with such a long FL ?. I was just about to order a 127 bresser  when i saw this little (long) beauty . Never owned anything like this before so i am a little in the dark about it , not too much info around either . really does look well made . Does anyone know if the focusser would be interchangeable ?

[johninderby]

An excellant scope and is well made. The focuser is fine and very smooth. If FLO shows it as being in stock then it is in stock.