OvidiuDanut
super member
Reged: 09/25/07
Posts: 126
Loc: Romania
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Hi!
There is a common wisdom, or so i've heard, that the optical virtues of a premium mirror compared to a medium mass produced one will only show when sky conditions are at their best. And on the other hand, little or no difference can be seen if sky conditions are the way they usually are , bad that is....
So is there any truth in this? Can anybody bust this myth?:D
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When we look at the immensity of the Universe we start to see ourselves smaller and smaller...
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Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
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not a myth just plain old common sense. If the seeing is bad, no single telescope will perform at it's capacities.
It is only at moments with near perfect seeing conditions, that the premium mirror will outperform a mediocre mirror. difference in performance will then be clearly visible on planet observing : the premium mirror will show more details with sharper images.
These moments may be ocasional, but they sure do exist. Clear steady sky (oftenwith a tiny bit of fog) is mostly a predictor for perfect seeing.
So the advantages of a premium mirror are realy to be taken into account
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Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2460
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little truth.
poor mirrors never even give you the ability to use some power on the moon. many of the 16" meade starfinders could not even reach 180x before the image would break down.
now the difference between a 1/10 and 1/20 wave mirror might only be seen on a night of good seeing.
most mass produced mirrors our club has tested lately are between 1/3 and 1/5 wave. MANY older ones were 1/2 wave even if they were labeled "research grade".
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
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Dave, I wouldn't consider 1/2 to 1/3 wave mirrors as medium, but as plain bad.
Medium to me is diffraction limited near 1/4 or 1/5, good as 1/10 or better
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Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10483
Loc: Amargosa Valley, NV, USA
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Even on nights of average seeing there may be frequent moments of clarity lasting a few seconds before the turbulence dominates again. So even if you see the difference only 10% of the time, that 10% will be frequent enough so that you'll know it's there.
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2460
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Quote:
Dave, I wouldn't consider 1/2 to 1/3 wave mirrors as medium, but as plain bad.
Medium to me is diffraction limited near 1/4 or 1/5, good as 1/10 or better
i think we might be using the word "medium" in two different ways.
i was using it as ' average: around the middle of a scale of evaluation; "an orange of average size" '
so if our club tested a bunch of mirrors this year and most of them came out to be 1/3 to 1/5 wave - that would be the medium range.
i completely agree that 1/2 and 1/3 wave mirrors are POOR quality, but that is what we often see on the bench.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Quote:
most mass produced mirrors our club has tested lately are between 1/3 and 1/5 wave.
Dave:
How were these tested?
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2460
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Quote:
Dave:
How were these tested?
they are tested on an interferometer.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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dave brock
member
Reged: 06/06/08
Posts: 42
Loc: New Zealand
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In my experience you don't have to have the best sky conditions to benefit from a premium mirror. Average seeing will affect a poor mirror more than a good one because there is already excess light in the diffraction rings with a poor mirror. If you have a very good mirror you'll often hear others complain about the poor seeing when you find it's actually not too bad. It's only in the worst seeing that a premium mirror drops to the performance of a poor one.
Dave
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16" homebuilt dob
6"watson refractor
building 20" truss dob
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johangw
member
Reged: 10/10/07
Posts: 97
Loc: Netherlands/Belgium
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Quote:
In my experience you don't have to have the best sky conditions to benefit from a premium mirror. Average seeing will affect a poor mirror more than a good one because there is already excess light in the diffraction rings with a poor mirror. If you have a very good mirror you'll often hear others complain about the poor seeing when you find it's actually not too bad. It's only in the worst seeing that a premium mirror drops to the performance of a poor one. Dave
I agree with Dave
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Discovery 17,5 f4,1 TD & DSC
Orion Optics 150mm F11 on GP-E
Meade 5K 8.8mm
Celestron Ultima 12,5mm
TV 31mm T5 nag.
GSO superview 40mm
TV Big-Barlow
TV Paracorr
Canon 400D
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