Where do achromatic refractors figure in today's world?

[John]

2 hours ago, vlaiv said:

In theory barlow can reduce CA - but not because of shape of the light cone, or "longer focal length". It can do that with fast achromats if inserted deeper inward because it can "vignette" light cone acting like sort of aperture stop - blocking outer parts of lens.

This will depend on barlow used, its clear aperture and speed of scope. For F/5 scope, at 100mm from focal position beam is 20mm wide. In theory barlow with small clear aperture and that needs to be more than 100mm from focal position can introduce this sort of vignetting and act as aperture stop - lowering CA a bit.

Stopping down the aperture with a cardboard mask will have a similar effect. Over the years a few refractors came ready "stopped down" through either smaller than optimum light baffles or the end of an overlong focusser drawtube cutting into the light cone. The manufacturers were not always up front about this !

 

 

[Cosmic Geoff]

On ‎01‎/‎06‎/‎2019 at 11:47, Ags said:

Gotta be paper for me. I'll run the Archive.org text through a publishing platform and get it perfect bound.

You could, but considering you can still get a hardback without paying the prices asked by some chancers, I would not judge it worth the effort.  Preparing a text for printing can be a lot of work, and the printer may charge a setup fee.

[Ags]

44 minutes ago, John said:

Stopping down the aperture with a cardboard mask will have a similar effect. Over the years a few refractors came ready "stopped down" through either smaller than optimum light baffles or the end of an overlong focusser drawtube cutting into the light cone. The manufacturers were not always up front about this !

 

 

I remember the supermarket refractor I had as a teenager. I remember the big (6-7cm) front lens, and the huge baffle directly behind it cutting aperture to maybe 4cm.

[mikeDnight]

 

10 hours ago, Cosmic Geoff said:

Interesting that somebody else found the Startravel 102 worked well with a Barlow. Mine seemed to work well with a kit Skywatcher X2 Barlow which appeared to counter the chromatic aberration and give good planetary views.

 

10 hours ago, John said:

The CA is still in the system though - using a barlow lens does not actuallty reduce it, despite the longer virtual focal length.

 

To be honest, I have no recollection of CA in the ST102 on the night I observed Mars. I'd be pretty confident it was present, but I was focused on the view of Syrtis Major which was much more memorable. CA in an F5 achromat is just the nature of the beast so I'd have expected it anyway. I think what the experience proved, is that some of the Star Travels have reasonably well figured lenses. My friend Philip eventually sold the little Star Travel to my younger son Daniel, and we used it to observe the transit of Venus later that year. A couple of years later Daniel sold the little Star Travel back to Phil, who was willing to pay over the odds to get that particular scope back. Daniel was 11 years old in the attached pic, and was a better businessman at that age than I've ever been!

 

Transit of Venus June 8th 2004. 

Edited June 2, 2019 by mikeDnight

[Ben the Ignorant]

15 hours ago, John said:

a few refractors came ready "stopped down" through either smaller than optimum light baffles or the end of an overlong focusser drawtube cutting into the light cone. The manufacturers were not always up front about this !

It's not just a thing of the past, this 72ED was accidentally stopped down to 69mm by a mispositioned baffled:

http://www.astrotest.it/test-reports/apochromats/skywatcher-evostar-72ed-ota/

Very good scope in most of its features but the single baffle is not enough (how much would a couple more plastic or aluminum rings cost?), and the main tube is not blackened properly (how much does a little black paint cost?). My 80 semi-apo and 80 full-apo both lacked baffles, I made some extra ones.

By the way, this thread is read by many nostalgics and collectors who might want to see that japanese GOTO 80/1200, sold as tube and finder only:

http://www.dark-star.it/vendesi-e-cercasi/