Anonymous
Unregistered
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Hi there,
I posted in the beginners forum about whether this link: http://www.astronomy.co.nz/pub/GS980.asp was a reasonably decent telescope for the money, as over here, you need at least $1000 to purchase a half decent 8" Dobsonian telescope (don't even consider brand buying!), perhaps you can help me understand what "Wave error" is, I know it determines quality, but I'm unsure how it works out(I use a lot of ABC blocks to work things out (yes I should go to the library and read up on these topics  )).
I am also wondering about mirrors, what do they usually coat them with?, why do they deteriorate over time?, why do you not cover a newtonian with anti-reflective glass to avoid something like a bird that can't wait to remove that shine from that mirror... 
Edited by Tom L (06/01/04 05:59 PM)
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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Wave means that the surface is smooth and does not deviate over the average of the surface by that much distance....
On coating of mirrors, reflective surfaces cannot really be coated in the same manner as a lens where the light is passing through it and being focused. In other wordes coatings are made to NOT reflect. Wouldn't be so good on a surface that's entire purpose of existance is to reflect.
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4260
Loc: Atlanta, GA
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Okay first - the link you gave is to a Guan Sheng (GS)scope. They are considered quite decent for the money. The link shows them being sold out - if you find them elsewhere in NZ, go ahead and grab one.
As to wave error - light is made up of waves. When the waves from all over a mirror are focused to a single point, they interfere with each other (the waves add up). If they are all lined up perfectly, the peaks and valleys add up to each other. If they are not quite lined up perfectly, they may also subtract. At exactly 1/2 wave error, the peaks line up with the troughs and cancel each other out. 1/4 wave is considered the minimum criteria for a mirror to be "diffraction-limited". Most people list peak-to-valley (P-V) error, but a better way to measure is RMS error. This is the error averaged by area, for for example if a mirror had one half perfect, but one half was 1/4 wave, it would be 1./4 wave P-V and 1/8 wave RMS. Another mirror might be 90% perfect, but have one spot (10% by area) that was 1/2 wave. This one would be 1/2 wave P-V, but 1/20 wave RMS (and this one would perform better than the first example). You want to see the RMS number below 1/13 wave for a good mirror.
Of course, most manufacturere don't give you numbers anymore, they just say diffraction limited. Most also have satisfaction guarantees, so if you get a bad scope you ca probably return it. That said, GS has a decent reputation, so you will probably be fine with one of theirs.
The coatings on mirrors are usually some type of aluminum with a protective overcoat. The coatings can last over 10 years if they are well cared for (that means NOT cleaning them very often - dust doesn't affect the images much or the coating, so leave it...). Areas with bad pollution or near the sea (salty and damp) tend to cause them to break down sooner. Keep the scope covered when not in use (use shower caps over the ends, film cannister in the focuser) and they should last a long time.
Jarad
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jrcrilly
Refractor wienie (sort of)
Reged: 04/30/03
Posts: 24363
Loc: NE Ohio
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Quote:
As to wave error - light is made up of waves. When the waves from all over a mirror are focused to a single point, they interfere with each other (the waves add up). If they are all lined up perfectly, the peaks and valleys add up to each other. If they are not quite lined up perfectly, they may also subtract. At exactly 1/2 wave error, the peaks line up with the troughs and cancel each other out.
I think an easier explanation to understand is that they are simply measuring the error at a given point between where the surface should be (would be if the mirror were a perfect parabaloid) and where it is. It's just a distance measurement. Instead of specifying it in inches or millimeters it is specified in fractions of a wavelength of a particular frequency of light.
P/V measurements compare the point which is highest compared to the ideal to the point elsewhere on the mirror which is lowest compared to where it should be. Since an otherwise perfect mirror could have one high point and one low point, this isn't a reliable measure of overall quality. RMS is an arithmetical approximation of the average of errors measured at many points on the mirror - thus it gives a better picture of overall performance.
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John C
Urban Observatory
A&M/Astreya 76mm F/6 APO
TMB/LOMO 80mm F/7.5 APO
Tak FSQ-106N F/5 APO
Tak FS-128 F/8 APO
Meade 178ED F/9 "APO"
Meade ETX-125AT
C14
Teeter 20" F/3.8 truss Newt w/ServoCat
CI-700, NJP, GPDX/SS2KPC, SP/SS2kPC
ST-10XME, ST-2000XM
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4260
Loc: Atlanta, GA
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Isn't that what I said? 
Jarad
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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LOL, I was try'n to convey it in a way, somebody not particularly savvy with optics could understand.
You can go mad, try'n to explain wave front errors and their causes / effects... lol
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Tom L
Reged: 01/07/04
Posts: 30797
Loc: Sunny Oregon
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Jamie, thanks for posting this question. I took the liberty to rename it and pin it.
Jarad, that is what you said...but it is nice to have the same thing said differently. Thanks to all 3 of you.
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Tom 
Tele Vue 102mm f/8.6 on an EzTouch
Vixen 80mm f/5 A80SSWT on a grab-n-go mount
Edited by Tom L (06/01/04 06:00 PM)
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Anonymous
Unregistered
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RF Royce's website has an excellent explanation of the various method for testing mirrors and what they mean. He doesn't appear to like p-v wavefront measurements though...
This is probably the one that helped me out most.
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Anonymous
Unregistered
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Quote:
why do you not cover a newtonian with anti-reflective glass to avoid something like a bird that can't wait to remove that shine from that mirror
Then you end up with a dirty piece of anti-reflective glass instead of a dirty primary mirror. It doesn't improve the situation at all. A mirror coating isn't necessarily more fragile than lens coatings.
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jrcrilly
Refractor wienie (sort of)
Reged: 04/30/03
Posts: 24363
Loc: NE Ohio
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Quote:
Quote:
why do you not cover a newtonian with anti-reflective glass to avoid something like a bird that can't wait to remove that shine from that mirror
Then you end up with a dirty piece of anti-reflective glass instead of a dirty primary mirror. It doesn't improve the situation at all. A mirror coating isn't necessarily more fragile than lens coatings.
Yes; it's been done, but it's expensive to do with glass of the proper quality and surface. It also degrades cooldown characteristics and, no matter how well coated, wastes light. On the other hand, it's usually easier to clean than a Newt mirror way down at the bottom of the tube.
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John C
Urban Observatory
A&M/Astreya 76mm F/6 APO
TMB/LOMO 80mm F/7.5 APO
Tak FSQ-106N F/5 APO
Tak FS-128 F/8 APO
Meade 178ED F/9 "APO"
Meade ETX-125AT
C14
Teeter 20" F/3.8 truss Newt w/ServoCat
CI-700, NJP, GPDX/SS2KPC, SP/SS2kPC
ST-10XME, ST-2000XM
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Anonymous
Unregistered
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By the way, "wave error" isn't a phrase. "Wave" is a unit of length, short for "wavelength." So "1/4 wave error" means the amount of error is 1/4 wavelength.
Also there seems to be some confusion in the above posts - what Jarad described is wavefront error, and what jrcrilly described is surface error. If a mirror is perfect except for a small area that is 1/8-wave too deep, then the P-V surface error is 1/8 wave. But the light that reflects off this spot needs to travel 1/8 wave more to reach the surface, then 1/8 wave more to get out of the hole again. So the P-V wavefront error is 1/4 wave.
Wavefront error of 1/4 wave is considered diffraction limited, although you can see improvement up to about 1/8 wave, I believe. The description in the link says "1/8 wave mirror" so I'd take that to mean a 1/8 P-V surface error, or 1/4 wave P-V wavefront error.
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Anonymous
Unregistered
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How do I find what my total wavefront error will be?
If my secondary has a wavefront error of 1/20th and my primary has a wavefront error of 1/10th what is my total wavefront error?
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BluewaterObserva
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Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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Don't sweat your secondary, it is so close to the focal point, it would be very hard to factor that in mathmatically.
As you can visualize, a 1% error at say 1 foot distance, is going to be very little error. That same 1% error at 4 feet is going to be a lot!!!!
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4260
Loc: Atlanta, GA
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I think you are supposed to divide the secondary error by sqrt(2) because of the 45 degree angle, and add it to the primary error. But since your primary is already the bigger of the two, it's not going to make much difference. The bottom is line is your total will be slightly more than the 1/10th from the primary, but the primary error dominates (in other words, don't bother upgrading the secondary further....).
Jarad
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Anonymous
Unregistered
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Well Im just curious since at "1" its optically perfect so the closer you get to "1" the better. I didnt see anything in the royce website that talked about distances. I was just curious for my own sake what my total error was with my two mirrors was. Im also curious on knowning how to figure this out to find what my old wave front was to see how much better together my optics have made.
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BluewaterObserva
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Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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No, it is just that waver error on a secondary unless just horrid, is not going to be detectable, even with star test rings really. It does have to do with the optical geometry of the secondary being so close to the focal point as I not so well tried to explain why.
The secondary is a flat, the is no figure'n involved at all, it is an optical flat, the surface roughness is what causes wave errors, and all even china born secondary mirrors are smooth enough to not be detectable at the small distances invloved from the secondary mirror surface to the focal point.
Of course I say that and somebody is going to come up with a newt design that uses a really oversized secondary and tube, which would make the distance and there for the chances of detecting it greater.
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4260
Loc: Atlanta, GA
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Uh, actually, if we are talking wave error, at "0" is perfect, not at "1"....
Jarad
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Anonymous
Unregistered
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Jarad,
Quoted from: http://www.rfroyce.com/standards.htm
"Strehl ratio is that it expresses the powerful aspects of the RMS method in a manner that is intuitive and memorable: 1 is perfection, .8 is okay. 9. is good, .95 is extremely good. This kind of familiar and easy to remember expression of quality is essential to creating unimpeachable standards for optics"
This is where I came up "1".
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4260
Loc: Atlanta, GA
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Oh, okay. Yes, that is correct for Strehl, we have just been talking about error measured in fractions of waves before (where less is better).
Jarad
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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I was confused too.. :-) I mean when did we get on Strehl ratio???? lol
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