Bintel optics any good ?

[doppler (Rick)]

Originally Posted by 2stroke

Rick its mean Periodic error http://www.cloudynights.com/item.php?item_id=2750 . The worm on mine was aluminum gear with a brass worm, they gears were badly turned and nothing like today's quality. You can get lucky with some parts of the worm gear and others are just bad, then the worm it self was pretty shotty limiting to around 20 secs. The mount has a number of things from the dc gear drive to the plastic bushes used for bearings which cause error. Some people have machined them out and added tapered roller bearings and even a new worm drive which then makes them very nice. For visual and planetery work though these things are the beast, lol gave up on the eq platform when i got mine because the thing could do a way better job

Just pulled the cover of my drive . The gears are all brass with brass bushes and screws to tighten backlash? The motor is 240v. I have searched cave optical and they were top of the range hand crafted scopes in the 60's and early 70's. I have had it tracking perfectly for 30 mins with only minor adjustments to a variable speed drive corector a guy in Adelaide made for me in the 80's. (and of course good drift alignment)

[ausastronomer (John Bambury)]

Originally Posted by brian nordstrom

Here is my 10 inch f10 newt with a 1 inch diagonal , thats her in 2 pieces .
the Dobbo base , next to Goldie , and the top 1/2 , now you want to see Jupiter in this baby !! .

Go buy an 10 inch f/5 , it will give great views up to about 200x , thats where my 5 inch f/13 is just comming on song ..

Brian .

Brian the F- Ratio of the telescope has nothing to do with the magnification it is capable of supporting. What determines that are the following parameters:-

1) The thermal stability of the optics
2) The quality of the optics
3) the aperture of the optics
4) The atmospheric conditions (seeing)

Given good thermal conditions and seeing, it's a pretty poor 10" telescope that will not go infinitely higher than 200X on anything. That's only 20X per inch of aperture. A good telescope irrespective of F-ratio should do at least 40X and generally 50X per inch of aperture under favourable conditions, irrespective of F-ratio. In the early days it was a lot easier to make a good F10 mirror than it was to make a good F5 mirror, consequently, some of the faster mirrors were of poor quality and would not hold up under high power, but that changed decades ago. Skilled opticians are now producing < F5 optics which are of outstanding quality and capable of excellent high power planetary performance. The design parameters of the telescope where it has a thin mirror which cools fast and a small < 20% central obstruction are the most important factors and far more important than the F-ratio.

I have two 10"/F5 'ish telescopes. One is a 10"/F5 tubed dobsonian with a GSO mirror (which is very good) and the other is a 10"/F5.3 SDM truss dobsonian with a Mark Suchting mirror. Once the scopes have cooled properly they will both pull 500X plus under favourable conditions, on their ear.

Cheers,
John B

[Satchmo]

Originally Posted by wulfgar

You mean fast optics will handle stuff like spherical aberration and zonal errors exactly the same?

Absolutely. 0.5 wave of Spherical Aberration on an unobstructed system yields a Strehl ratio of around 60% regardless of F#.
Another factor that may contribute to the reputation of slower mirrors is that a fast one takes a lot more figuring . If the optician does not have good skills the surface roughness of the optic will build up. A well made fast optic is technically capable of just as fine planeatry images albeit with a slightly larger secondary , although for visual use this is only a slight increase.

Another way to look at it is that if you are capable of nailing the parabolic correction to within 10% then the mirror may be better than 1/4 wave for one with a 2.5 wave required correction. However a 10% error on a large fast mirror requiring 20 waves correction will result in a 2 wave error.

In my view the reason some large fast mirrors fail spectacularly is a residual under-corrected edge, that is often hard to read with a foucault test. This residual can be a steeply sloped as the remnant of the original sphere, causing a very poor star test. Such an error would have to polished onto a slower mirror but is much more likely to remain on a fast one as a job incomplete.

[Satchmo]

Originally Posted by ausastronomer

I have two 10"/F5 'ish telescopes. One is a 10"/F5 tubed dobsonian with a GSO mirror (which is very good) and the other is a 10"/F5.3 SDM truss dobsonian with a Mark Suchting mirror. Once the scopes have cooled properly they will both pull 500X plus under favourable conditions, on their ear.

Cheers,
John B

Sounds like you got lucky with the GSO , John . The point is that an individually crafted mirror ( hopefully) alleviates any doubt about the mirror quality. Mass produced mirrors are a bit of a lucky dip- more so on the larger sizes.

There is also the long term advantage of having a well annealed piece of Pyrex rather than a coarse annealed glass similar in expansion coefficient to plate glass.

[wulfgar]

Originally Posted by Satchmo

Absolutely. 0.5 wave of Spherical Aberration on an unobstructed system yields a Strehl ratio of around 60% regardless of F#.
Another factor that may contribute to the reputation of slower mirrors is that a fast one takes a lot more figuring . If the optician does not have good skills the surface roughness of the optic will build up. A well made fast optic is technically capable of just as fine planeatry images albeit with a slightly larger secondary , although for visual use this is only a slight increase.

Another way to look at it is that if you are capable of nailing the parabolic correction to within 10% then the mirror may be better than 1/4 wave for one with a 2.5 wave required correction. However a 10% error on a large fast mirror requiring 20 waves correction will result in a 2 wave error.

In my view the reason some large fast mirrors fail spectacularly is a residual under-corrected edge, that is often hard to read with a foucault test. This residual can be a steeply sloped as the remnant of the original sphere, causing a very poor star test. Such an error would have to polished onto a slower mirror but is much more likely to remain on a fast one as a job incomplete.

Let's rephrase this. Are you saying that an F4 with a spherical mirror, will reveal no more final error than a F12 with a spherical mirror?

[ausastronomer (John Bambury)]

Originally Posted by wulfgar

Let's rephrase this. Are you saying that an F4 with a spherical mirror, will reveal no more final error than a F12 with a spherical mirror?

Hi Wulfgar,

No that's not what he is saying at all. Not even close in fact as no one has made mention at all of spherical mirrors, which only come with the smallest cheapest telescopes. Inter alia he is saying that a mirror with a 1/2 wave of spherical aberation error (eg undercorrection), has the same level of error, regardless of the F ratio of the mirror. Consequently it will have the same strehl ratio. Your earlier post alluded to the fact that the level of the error changed as the F-ratio of the mirror changed, which is clearly incorrect. The amount of parabolic correction required to properly parabolise and correct the mirror increases as the F-ratio of the mirror gets faster, that is all. If you get it right you get it right, the faster F-ratio just makes it a bit harder to get it right because more work and skill is required.

Cheers,
John B

[wulfgar]

Originally Posted by ausastronomer

Hi Wulfgar,

No that's not what he is saying at all. Not even close in fact as no one has made mention at all of spherical mirrors, which only come with the smallest cheapest telescopes. Inter alia he is saying that a mirror with a 1/2 wave of spherical aberation error (eg undercorrection), has the same level of error, regardless of the F ratio of the mirror. Consequently it will have the same strehl ratio. Your earlier post alluded to the fact that the level of the error changed as the F-ratio of the mirror changed, which is clearly incorrect. The amount of parabolic correction required to properly parabolise and correct the mirror increases as the F-ratio of the mirror gets faster, that is all. If you get it right you get it right, the faster F-ratio just makes it a bit harder to get it right because more work and skill is required.

Cheers,
John B

So if all that is true.

Why does a spherical mirror work for an F12 and is something of a disaster in an F4?
I recall at one stage Vixen's 6" F5 Newtonian was put out with a spherical mirror (20 or more years ago), the customers were not happy.

So far I've been getting tautologies that a final wave front error of 1/2 is no worse than a final wave front error of 1/2.

[ausastronomer (John Bambury)]

Originally Posted by Satchmo

Sounds like you got lucky with the GSO , John . The point is that an individually crafted mirror ( hopefully) alleviates any doubt about the mirror quality. Mass produced mirrors are a bit of a lucky dip- more so on the larger sizes.

Hi Mark,

Yes I did get very lucky with that 10"/F5 GSO mirror. What is sad is that I tried for ages to sell that scope with Argo Navis and a heap of other modifications for less than $1,000 and couldnt get a buyer. The Argo Navis, encoders and mounting hardware are worth nearly $1,000. David Collis-Bird actually said to me one night we were using the scope, "I don't know how the Chinese can make a mirror this good for this price". Star testing indicates that the mirror is probably a 1/7th to 1/8th wave mirror at the wavefront, it is a tiny fraction undercorrected which isn't such a bad thing. It has a good edge and is reasonably smooth. It doesn't quite have the smoothness level of my 3 premium mirrors, but its pretty good. It also doesn't cool as well as a thinner pyrex mirror, so it takes a bit longer to stabilise and deliver its best images.

As you correctly point out these mass produced mirrors are a bit of a lucky dip, but so are mirrors from a couple of supposed premium US mirror makers, that cost infinitely more money. I won't go there but you and I both know who we are talking about.

I have used quite a few of these mass produced mirrors and generally in the 6",8",10" and 12" sizes they are pretty decent, with a few being outstanding and a few lemons slipping through the cracks. I have found in the larger 14" and 16" sizes they are a lot more hit and miss. I have seen a few dud 16" GSO's and a couple of dud Synta 14" mirrors. That having been said Rick Petrie's 14" Synta mirror is excellent and as you know I have a good 14" mirror in my own stable to compare it to.

Cheers,
John B

[ausastronomer (John Bambury)]

Originally Posted by wulfgar

So if all that is true.

Why does a spherical mirror work for an F12 and is something of a disaster in an F4?
I recall at one stage Vixen's 6" F5 Newtonian was put out with a spherical mirror (20 or more years ago), the customers were not happy.

Wulfgar,

I think you are confusing the terms "spherical mirror" and "spherical aberration".

Here is a 2006 thread which ran on Cloudy Nights which explains the Rayleigh and other criteria for establishing at which aperture and F-Ratio a mirror needs to be parabolised.

In simple terms as the aperture increases the F ratio at which the mirror needs to be parabolised increases.

A 6"/F5 spherical mirror would be useless for anything except low power widefield views. A 6" spherical mirror needs to be about F12 to be diffraction limited or better.

Cheers,
John B

[wulfgar]

Ok John. But would you agree that same spherical figure, produces a different wave front error in a faster system.
Or does it produce it produce the same wave front error?

Spherical aberration is produced I believe because the image plain itself is flat, hence distance from the edge of the mirror is shorter that that from the center of the mirror. If the image plain itself was a sphere, then there would be no error, no matter how fast the optics.

Again, does the same turned edge produce produce a different wave front error in a faster system or the same wave front error?

Originally Posted by ausastronomer

Wulfgar,

I think you are confusing the terms "spherical mirror" and "spherical aberration".

Here is a 2006 thread which ran on Cloudy Nights which explains the Rayleigh and other criteria for establishing at which aperture and F-Ratio a mirror needs to be parabolised.

In simple terms as the aperture increases the F ratio at which the mirror needs to be parabolised increases.

A 6"/F5 spherical mirror would be useless for anything except low power widefield views. A 6" spherical mirror needs to be about F12 to be diffraction limited or better.

Cheers,
John B

[Satchmo]

Originally Posted by wulfgar

So if all that is true.

Why does a spherical mirror work for an F12 and is something of a disaster in an F4?
.

Err...now I see where you are coming from ...who makes a spherical mirror these days ?

A 6" F12 has acceptable level of spherical aberration. (probably 1/8 wave wavefront). However the manufacturing tolerances on the surface are no greater than an F4 ....

[Satchmo]

Originally Posted by wulfgar

Again, does the same turned edge produce produce a different wave front error in a faster system or the same wave front error?

Same.

[wulfgar]

Originally Posted by Satchmo

Same.

Well OK I accept that the result is much the same in either optic, if they depart their ideal figure by the same wavelength. But error doesn't tend to depart the ideal figure by the ideal amount. Error is error.

However I suspect just as with a spherical surface, the same zonal error in either optic, will produce different wavefront errors.

Well, I'm sure. But either the following opinion is correct or not.

http://poormanastro.tripod.com/page8.html

That umpteen millionth of an inch error in the figure of an f4 scope might produce an error of 1/2 wavelength and create a "soft" or slightly blurry image (even when at best focus), but that same umpteen millionth of an inch error in the figure of an f15 scope might produce an error in the figure of only 1/8 wave and, thus, still produce a tack sharp image. It is simply cheaper to grind a good f15 glass versus a good f4 glass. Faults such as chromatic aberration and spherical aberration are much easier to control in a slow glass. We will touch on aberrations in more detail a bit later.

[Satchmo]

Glass height on the surface and wavefront error are totally related. A 1/8 wave high physical bump on the surface of a mirror causes a 1/4 wave bump in the wavefront regardless of the F# of the optic.

There is a lot of confusion in popular literature between geometric ray tracing and diffractive optics. Getting too fixated on `rays' - thinking of the stellar focal plane as diffraction pattern is more helpful. A case that illustrates this is the comparison of different f ratios. The logic of the quoted article would suggest that a given physical surface error should form a tighter spot in a slower mirror which is not the case.

My discussion is confined to diffractive analysis of the situation which applies to finished mirrors- ones with low levels of aberration.

[Peter Ward]

Interesting to read the mixing of terms in this post....e.g. wavefront error, vs wave error measured on an optic's surface.

Strehl is a dreived number, after a measure from RMS smoothness, and is a number you can compare to what you'd expect from a perfect optic....

Hence, sure you can have a 6 inch F12 spherical mirror produce just peachy images, as at F12, a small section of sphere and parabola are almost identical....but if the optic is rough...it should show in RMS measurments, with an accordingly poor strehl and soft images too boot.

I'm of the opinion many confuse this with airy disk size (and intensity ) which can vary with obstructed vs non-obstructed systems, but by no means, makes a measured Strehl 0.99 optic less-perfect in either case.

In basic terms, all you want a telescope to:
Focus all the light it physically can collect, into the smallest spot it can.

Lots of things don't make the above true. From the wrong curve to the optics, to the atmosphere churning up the starlight.

(plus rough optics, unreflective coatings, inhomogeneous glass, poor alignmment, poor thermal compensation, mechanical distortion, chromatic error (lenses), etc. etc.)

..and if you want to take images...the mounting + atmosphere is rarely holding the image stationary enough even approximate a diffraction limit of a larger optic.

Just my 2 cents worth....

[Tandum (Robin)]

I'm a firm believer in you get what you pay for but .. an F12 Reflector? What century is this .. where do you buy one? The tube would be 2 to 3 meters long. And it has nothing to do with an F4 GSO newt which is where this started.

[wulfgar]

Perhaps I'm being over enthusiastic about current commercial optics.

Here's a scale on Mel Bartels web page. There is agreement that commercial optics are well up a notch on what they used o be.


http://www.bbastrodesigns.com/ratemirrors.html

Here is Bratislav's scale:
1. Can't find anything wrong with it, absolutely perfect: '<expletive>' Yet to see one after ~25 yrs
2. Defects visible only in extrafocal images, and only after extensive star testing in best seeing conditions ( << 1/10 wf): 'You lucky b@$#@rd!' Can count these on fingers of one hand
3. Extrafocal defects readily visible, but really minor ( < 1/10 wf): 'Excellent' Best examples of best commercial telescopes (Zeiss,AP,Tak etc) Best examples of homemade optics
4. Extrafocal defects fairly obvious, but in focus image still essentially perfect ( 1/10 - 1/6 wf): 'Very good' Majority of current 'best commercial telescopes'; best examples of mass produced scopes
5. Large defects visible on extrafocal images, in focus image suffers only slightly ( 1/6 - 1/4 wf): 'Good' selected examples of mass produced telescopes, most well made amateur optics; some examples of 'best commercial scopes' can still be found here
6. In focus image visibly suffers ( ~1/4 wf): 'Acceptable' good mass produced scope, most good large/fast mirrors I've seen
7. Image deterioration serious, clearly beyond 1/4 wavefront: 'Light bucket' majority of older generation mass produced scopes, special purpose telescopes (astrographs)
8. It's difficult to determine when scope is in focus at all ( 1/2 - 1 wf): 'If you're happy with it ...' unfortunately, not that difficult to find !
9. Usable only at very low magnification ( ~1 wf): 'I don't want to have anything to do with this one'
10. Absolutely useless: '<expletive>' unlike 1, I've seen these :-)
99% of all scopes I've seen fall into '4-10' bracket.

[Satchmo]

Originally Posted by Peter Ward

Interesting to read the mixing of terms in this post....e.g. wavefront error, vs wave error measured on an optic's surface.

I use the term `wavefront' error in the sense of optical path difference or OPD . A 1/2 wave bump on the mirror surface will create a 1 wave path difference at the focus.

[Peter Ward]

Originally Posted by Satchmo

I use the term `wavefront' error in the sense of optical path difference or OPD . A 1/2 wave bump on the mirror surface will create a 1 wave path difference at the focus.

Indeed, which is also why smoothness is critical...and also why
Refractors are more forgiving of surface errors...they don't double the OPD.

[wulfgar]

Originally Posted by Peter Ward

Indeed, which is also why smoothness is critical...and also why
Refractors are more forgiving of surface errors...they don't double the OPD.

Well I'm not that knowledgeable about optics, so forgive my ignorance. I was comparing my N6 with my N10 on Eta Orionis as a test subject.
The N10 is an okay optic and delivers the resolution on something like Jupiter that out does my N6 and Vixen fluorite.
But on the lower mag double star the N6 reveals crisp airy disks with relatively little scattered light. The 6" mirror is a Parks from 20 years ago when Parks had popular reputation for good mirrors. The bods at the local Astro Club commented that it was a good mirror and extraordinarily smooth.
By comparison the N10 is a more ordinary optic with many times the scattered light.