Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
sixela
Postmaster
Reged: 12/23/04
Posts: 9499
Loc: Boechout, Belgium
|
|
It's an f/4 scope. In an f/4 scope (and in f/3.66 scopes), many simplere eyepiece designs (including orthos, Plössls and TMB Supermonos) will generate quite visible amounts of spherical aberration, even on axis.
By the way, with "soft" I don't mean blurry; I mean that you can actually see more detail when you select a higher focal length eyepiece and barlow it, or pick a short T6 Nagler than if you use the ortho or Plössl.
TV Plössls are slightly better than cheap Plössls like the Orion Sirius Plössl, but not that much.
It depends *very* much on f/ratio - in a Paracorred f/4.5 scope, there's little spherical aberration to see in an ortho or TV Plössl. Even at f/4.5, it's hard to see in focus (though detectable in the star test when the seeing cooperates).
I'm not alone in my observations:
http://geogdata.csun.edu/~voltaire/roland/startest1.html
Quote:
Try different eyepieces if the refractor is faster than f10, you may see different contrast effects. Plossls and some Orthos tend to add undercorrection to the image. When you find the eyepiece that gives the most equal out-of-focus images, use it in your observing. Hint - multi element eyepieces like the Naglers have the best spherical correction for fast scopes.
--------------------
400mm f/4.46 David Lukehurst truss Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
|
Owen
sage
Reged: 06/21/07
Posts: 353
|
|
Quote:
you can actually see more detail when you select a higher focal length eyepiece and barlow it
This could in effect produce the same focal length (eg a 30mm EP with a 2* barlow = effective 15mm EP) as the shorter simpler EP....
The reason I'm asking is my mini-mak is fine with its 30mm EP, is lousy with its 10mm EP, but is great with the barlowed 30mm EP - the EP's are kit EPs that came with the scope. And here was I thinking that it was me 
Thanks, we'd better get back on topic now
Owen
|
sixela
Postmaster
Reged: 12/23/04
Posts: 9499
Loc: Boechout, Belgium
|
|
Quote:
The reason I'm asking is my mini-mak is fine with its 30mm EP, is lousy with its 10mm EP, but is great with the barlowed 30mm EP
The effect I'm talking about doesn't happen on most Maks (as they are slow enough for most eyepieces).
--------------------
400mm f/4.46 David Lukehurst truss Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
|
HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
|
|
Hi,
I was proposing using a green filter on a singlet, but it was pointed out the other abberations of a singlet.
However in the last reference:
One saving
grace for achromats is a set of filters. It does not "fix" the
chromatic aberration, rather it lets you see the image in one color
in monchrome very sharply.
So I was on the right track it seems.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
|
HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
|
|
Hi,
I have been looking at the telescope optics site.
I have realised that you could place a CCD Camera/Web camera in an off-axis position in a corner of the spider opposite the masked off aperture on a large Dobsonian.
I can get a 6.5" mask on my 16" F5 which produces a F12 telescope.
So I can add a one surface telescope:
One Element:
-Off-axis Newtonian @ F12 (Imaging only)
Two Elements:
-Newtonian @ F8
-Newtonian @ F5 and Coma Corrector at prime focus for imaging
Three Elements:
-Newtonian @ F5 and 2x Barlow for Planets
-Newtonian @ F5 and Coma Corrector for wide field or imaging
-Maksutov Newtonian @ F8
-Doublet Refractor and Star Diagonal @ F15
Four Elements:
-Triplet Refractor and Star Diagonal @ F9
-Maksutov and Star Diagonal @ F15
-Newtonian @ F8 and Paracorr
I did the calculations (Two calculators) for illuminated field of a 8" F8 and 2" flat and it would illuminate a 36mm field, almost the diagonal of a 25mm square CCD.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
|
|
9 registered and 6 anonymous users are browsing this forum.
Moderator: Don W, Joe Cipriano
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 1649
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
[Re: 
Edit
Reply
Quote
