5 replies to this topic

[MeridianStarGazer]

I figure with 1.1x coma corrector and 28mm Astrotech 82 deg, 54mm secondary, that is a 3.6 deg field of view, 22.75x, and 6.06mm exit pupil.

How nice are the milky way star clouds, or Pleiades, or M31, or the double cluster in that? I bet they look beautiful.

Two tubes could be made from one 48" quikrete tube. A short tripod is needed.

Edited by MeridianStarGazer, 07 May 2024 - 09:12 PM.

[gstrumol]

 

But this 4 1/8", f/4.2 Newtonian is a lot of fun at a dark site:

 

 

and it's as rugged as a tank! 

[ButterFly]

54mm secondary

That secondary obstruction shouldn't be too imposing.  Off the shelf 6" f/4s have larger ones for imaging.

 

With that secondary at about 35%, it's about the same as the Quattro 200p, 8" f/4's 35%.  Its secondary doesn't get in the way too much for me visually.  Just use higher power for the Moon, and the like.  My 200p's focal length is about the same as my 120 f7/5, but its added reach is definitely noticeable.  With the same eyepieces, the magnification is the same, but everything is brighter due to the nearly halved f/ratio.

 

Just watch out for too large pupils on bright targets with lots of direct vision detail.  35% is still a lot.  It's rare to see an actual spot at night because our brain fills things in.  But, that obstruction is over your fovea, so if there is detail to be seen directly, it will be impacted, and the brain can't fill it what it's not getting in the first place.  M31's dark lanes from a dark site are an unexpected example of that.  There is enough contrast there for its dark lanes, including the thinner ones, to be direct vision from a dark site.  Star color is another example, though a little less surprising.  Our color receptors are in our foveas.

[Keith Rivich]

I have a 6" f/4 Newt mounted to my 25". Its great for those really wide FOV targets such as the Veil (which I can see the entire nebula with a 35mm Pan and an OIII. It also handles power pretty well. Over the eclipse week I used it on several NGC galaxies using my 4.7 and 10mm eyepieces.

 

 

While it doesn't replace the big scope it does have its place in my observing. 

Edited by Keith Rivich, 08 May 2024 - 09:13 PM.

[davidc135]

My second ATM mirror was a 6'' f4 which I put in a tube without a mount but just supported on my lap. That was 45 years ago with a 2'' diagonal and a 25mm Kellner ep.

Wonderful low power views of the heavens, the relatively limited fov of the Kellner probably helping to make up for the lack of a coma corrector.

I must get the mirror out and check its figure which I expect to be pretty bad.

 

David

[MeridianStarGazer]

But this 4 1/8", f/4.2 Newtonian is a lot of fun at a dark site:

astroscan.jpg

and it's as rugged as a tank!

Because what good is 3.6 degrees if one is slowed by Dobson's hole.

Looks like fun, though I don't know how to make that sphere. Mel Bartels found another solution.