Bintel optics any good ?

[doppler]

I am looking at upgrading to a 12" f4 bintel ota and was wondering if the optics were as good as they claim.

[Tandum]

You get what you pay for. It's a rebadged Guang Sheng from china.
Andrews (http://www.andrewscom.com.au/) has the same scope for $100 less and free shipping just now.

[alistairsam]

Hi
The optics are from what I've heard, quite good, but its the other bits that could pose some issues
James had done a nice review on this scope here http://deepspaceplace.com/at12in.php
It's still good value for the price, if you can correct the other issues

[wulfgar]

A 4" secondary, that's scary!

The original 3.1" secondary provides an acceptable field for visual use. A popular fast system with the imaging crowd, its characteristics might be a little less happy for the visual observer.

Here's another thread on the GSO 12" F4 Newt

http://www.iceinspace.com.au/forum/showpost.php?p=921871&postcount=1

[Tandum]

The current model has a 3.5" secondary.

[Moon]

Rick
That's a very broad question, but generally speaking yes the optics are as good or better than claimed (usually "diffraction limited"), bearing in mind the claims are not the same as those being made by the premium brands (eg Strehl Ratio of .96 or better from Mr Royce).

Does this answer your question?

James

[wulfgar]

Thanks, my mistake. Although what I wonder what the real working measurement would be?

I've heard claims GSO mirrors are very well corrected....1/20 wave? I'd put this down to the advantages of the latest computer aided technology.

[LewisM]

I use a Vxien 8" R200SS astrograph for visual and astrophotographic purposes interchangeably, but maybe that has something to do with the extreme accuracy of their optics?

[wulfgar]

I have a suspicion Vixen source optics from a multitude of makers and perhaps only re-figure at best. There's nothing wrong with a fast Newtonian, other than coma which can be corrected. Or the larger obstruction than reduces contrast and limits them as regard planets and double stars.
But I like planets and double stars, so my 6" F8 Newt has only a 1 inch secondary. That 6" scope certainly splits double stars way better than a 4" Vixen fluorite. But because there is still an obstruction the images are "softer" than in the fluorite.
It is a fact that exceptionally fine optics can make up for some of the loss of contrast by obstruction.

[doppler]

Thank all for the info. I am currently using an old equtorial pier mount with a 8" f6 primary (it could easily carry a 16' newtonian ota if I could find one) I have recently refurbished it with a coultar optical mirror set 1/8 wave and the images look pin sharp. It seems to me that fast optics are pre-requisite to avoid digital noise. My ideal choice would be a 12" f6 newtonian ota but I would need a bigger car or a dome. Large non dob newtonians seem to be hard to find.

[vaztr]

A 16' OTA!! This I want too see!!! HAHAHAHAHAHA

[doppler]

Meade made them in the 1980's 16" f4.5 before dob's kicked of. I nearly bought one but was put of by the cardboard (sono?)tube.

[rmuhlack]

Indeed. Providing a 100% fully illuminated field of the required size for imaging (usually greater than a 1" diameter) will necessitate a certain secondary size. Not because of optic quality but simply because of physics.

The calculator that James links to in his review of the AT12IN calculates the fully illuminated field size for a given set of telescope dimensions (mirror size, focal length, distance from secondary to the focal plane). "quality of the optics" does come into the equation. He determined that the secondary was too small to provide 100% illumination even at the centre of the focal plane, so he upgraded. I am thinking of doing a similar upgrade to my GSO 200mm f4 (AT8IN) for the same reason.

Anyway, the point im trying to make is that a newt "optimised for photography" will require a larger secondary than would otherwise be necessary for visual use. A larger secondary means lower contrast for visual. Precision optics (Vixen, Orion etc) may help to make that contrast reduction less obvious, but it cant take it away.

[wulfgar]

Well to me precision reflector optics is something of the order of AP's Mak.

http://www.astro-physics.com/index.htm?products/telescopes/10mak/10f146mak

For a high contrast Newtonian, something like a 15% obstruction by Dia is recommended. My 6" newt has an 18% obstruction.

My 10" newt about an 21% obstruction. AP's Mak has a has 23% obstruction, which is starting to get kind largish. But I wouldn't expect my 10" to deliver more contrast than an AP Mak. In fact I'd expect the Mak to do better because of relative optical perfection in everything else.

But then a 6" newt with an extreme FL or F/12 an very average optics and a tiny obstruction can compete with the best. You could buy a 5" Tak tube for $6000 and find cheap long FL newt costing $300 could give it a run for the money.

Going back to the original issue, on the face of it the 3.5" Diag should give a 1 inch fully illuminated field. But my guess that is the nominal size, and after the bevel and secondary holder he gets only a 3.1" real Diag. This only gives 0.5" full illumination.

I suspect the nominal 4" gives him a real working 3.5".

[rmuhlack]

point taken in respect to 'precision optics'. No question about the quality of the gear you refer to there. my comments related more to the previous post that implied 'optical quality' could somehow overcome the physical limitations of a scope optimised for visual or imaging and instead allow the said scope to excel at both...

Back to the GSO secondary size, I have found with my 200mm f4 that the position of focal plane is actually further away from the secondary than it really needs to be, and at that distance my secondary (according to the OWL Diagonal calculator) doesn't actually give 100% illumination even in the centre of the field. The secondary is simply too small. The solution is either a different slightly longer optical tube to bring the focal plane closer to the secondary, or a larger secondary (or just accept that the scope is performing less efficiently than it needs to). If that was the case with James' 300mm f4 too, then I guess that would necessitate the secondary mirror upgrade. ;)

[Moon]

Richard,

Yes you have it right. For both the 200mm and 300mm, the default image plane is a long way out from the tube, thus it needs a bigger secondary. I fixed it on the 300mm with a bigger secondary (the focal plane was in the perfect spot for my camera/corrector combination).
On my 200mm version, I moved the focuser + secondary down the tube a bit to fix the problem.

James

[2stroke]

Yep there dam big its called the starfinder 16" image below. I had a starfinder eq mount myself and they are built that tough its crazy, they make the eq6 look like a kids toy in terms of build. The only issue is the PE and that leaves them only good for visual. They also made a dob called the starfiinder but just add eq to your search.

[brian nordstrom]

:) Hi Wulfgar , I have a 5 inch f/13 reflector ( 1710mm ) I built in NZ many years ago and she has a 3/4 inch diagonal and on the moon and the planets the only scope that comes close and its my Takahashi Mewlon 210 ( 2410mm ) , a long focal length reflector's are very special .
Here is a photo of my 5 inch ( Goldie )
Brian.

[brian nordstrom]

:D OOPs , here is the 3/4 inch diagonal , sorry .
Brian.

[wulfgar]

It's long known standard. An F12 Newt with average optics can take on the best. That long focal length isn't as sensitive to optical faults as short focal lengths are. That's something fast Newtonian fans need to be aware of......................your F4 optics have to be good to work well. Any figure errors get multiplied by the fast optics. For that matter a similar rule applies to fast refractors.
Originally I used a 3/4 inch Royal secondary and a recessed eyepiece collar. But I got tired of Hex keying the eyepieces into the collar to change them, so I shifted the main mirror up an inch and put in the one inch secondary.

The downside of course is the step ladder required for Newtonian focal lengths over 50".

[wulfgar]

That's just typical of commercial scopes. They're a compromise between visual and imaging. I'm not sure why they have such long focusing units. Possibly because old style photography needed the racking space for adapters.

[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 !! ;) .
Dont knock the quality of a mirror Wulfgar , I made and figured both of these , along with their mountings ,
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 ..
Dont go here with my M210 ,,,
Brian .

[brian nordstrom]

:D OOPs , heres my 10 inch , in its 2 pieces , 2500mm f/l
Brian.

[doppler]

I have an almost identical mount made by cave optical in 1970. One question though, what is "the PE" an abreviation of? My mount only has a single axis drive but I have been using it for astrophotograpy for 25 years. Hand guiding with a drive corrector and illuminated recticle eyepiece is not too hard if the scope is drift aligned. I have marks on the concrete slab where the legs go and only have to slightly tweek the alignment. This has been great for imaging comets, eclipses, transits and rock solid for piggybacking telephoto lenses. Never did much deep space prime focus stuff though, mainly due to the hour long exposures needed for 35mm film. I am learning digital......finally.

[brian nordstrom]

:mad2: And the 2 week wait for 1 out of 24 exposurers to be delivered ( for us without dark rooms ) :shrug: .
I hear you , love this digital age .
Brian

[wulfgar]

In the early 70's (now I better not use funny money) b/w roll developed was 12 liters of petrol. The same thing in color was 30 liters of petrol.

[brian nordstrom]

:thumbsup: I moaned when it hit 50c a gallon . ( MK 1 Zephyr ) and the fuel was pink.
Built my 1st 6 inch Newt by then .
Brian

[doppler]

And you had to keep a diary to note down frame number, exposure time, focal ratio etc. But it was always a thrill when you got the photos/slides back from the lab and had managed to capture some decent images.

I caught this fireball while imaging comet Hyakutake.

[Satchmo]

Wulfgar - Eyepieces, focus and collimation are certainly more forgiving at F12 . A 1/8 wave error on the surface of an F12 mirror causes exactly the same ( 1/4 ) wavefront error as at the focus of an F4 , which has exactly the same effect on the Airy Pattern. What is more time consuming is sculpting a deeper parabola, but the final surface error tolerances are exactly the same.

"Any figure errors get multiplied by the fast optics. For that matter a similar rule applies to fast refractors."

Your post implies that say a 1/2 wavefront F12 mirror will give a better image than a 1/2 wavefront F4 mirror. The Airy Pattern in both cases will be identical .

[Shiraz]

Rick. Have read that Bintel use GSO bits. If it helps, my 12 inch f5 GSO star tests to show about lambda/7 (wavefront) but it has a little TDE and some minor roughness. Nett result is very good - certainly not perfect, but still nice to look through up to about 500x with my eyes if the seeing allows.
attached image from the scope is a bit seeing affected, but may help.

As others have pointed out, the mechanical construction of these scopes leaves a bit to be desired, but it is good enough as a starting point.

[doppler]

My main dilema is about compromise. I would like larger scope for visual use , but I also like to dabble with astrophotograpy. The 12" f4 has the same tube length as my 8" f6 and fits on the back seat of any small car, but from what I have been reading is not as good visually as an f5. If I could find a 12" f5 ota that would be great, but these seem to be available in dob form. I guess because of the xtra heavy duty mount needed to carry it. Would it be possible to convert a dob tube, using tube rings to fit an equtorial pier mount?

[Shiraz]

I have a ex-dob 12 f5 on an EQ6 for solar system imaging, but any 12 inch would need a much heavier mount for DSO imaging. what mount do you have?

have you considered a 10inch f5 OTA, which would probably fit in your car, be a big step up visually and be OK for DSO imaging (just) on an EQ6 or (better) a G11?

I am a bit wary about recommending any lower end f4, based on my experience with a 200/f4 - it was stressful, not because of the optics, but because the standard GSO mechanical components are pushed a little past their comfort zone by the alignment tolerances required at this FR.

[doppler]

I have an old motor drive equtorial peir mount... it was designed to carry up to 16" f4.5 / f6 ota's. There is pic of a similar mount on an earlier post in this thread. I would like a big jump in appeture. I am thinking that a bigger scope might fit in the passenger side with the seat down.

I should add that the motor drive runs of 240v mains (I have a variable speed drive corrector that lets it run of a 12v car battery) It also has a fully adjustable clutch plate on the drive to adjust the load.

[Shiraz]

sorry Rick - I didn't pay enough attention to earlier posts.

Your mount presumably does not have an external guide input, so will probably not be accurate enough for long period DSO imaging. An f5 would be best for visual and solar system imaging if you can make it fit in the car. If you get a Dob you could leave the side plates on the OTA and use it either EQ or Dob mounted. Will need to get some rings made up though - I couldn't find any that were readily available.

Also would recommend a motor focuser if you go in for solar system imaging. Thermal expansion is enough to compromise the fine focus needed and it is a pain (almost impossible) trying to refocus manually.

[doppler]

Andrews has GSO 303mm internal diameter tube rings for $49 a pair, but I think a 12" scopes tube rings would have to be at least 400mm?

[wulfgar]

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

[Tandum]

I think those 303 rings are for a 10". I put a ruler on this 12" GSO f4 and it's over 360mm.
I don't know of anyone in Australia who sells rings for a 12" tube.
I think some people have bought them from Orion UK but they where pretty expensive as I remember.

[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 :)

[Shiraz]

rolled from 40x10 bar and using fittings cut from a smaller commercial set.

[2stroke]

Those rings are mint Ray :) You should have them anodized.

[doppler]

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]

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]

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]

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]

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]

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]

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]

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]

Wulfgar,

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

Here is a 2006 thread which ran on Cloudy Nights (http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/1045540/page/85/view/collapsed/sb/6/o/all/fpart/1) 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?

[Satchmo]

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]

Same.

[wulfgar]

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

[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]

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

[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.

[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.

[wulfgar]

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.

[doppler]

It is always interesting the way conversations evolve in a thread. A lot of good info, and as I would probably do more visual astronomy I think a longer focal length will be the best choice for me. I particulary like the idea of getting a 12" f5 dob and fabricating some rings so that I can mount it to my equtorial mount or just use it as a dob.

A 12" f12 would be huge! I found this pic of a 12" f7 and mind you the model is apparently 6' tall to make the scope look less daunting. ps the mount used is similar to mine (same manufacturer).

[wulfgar]

I have seen this telescope described as F9. But F7 is the usual jumping off point for large planetary Newtonian's.

http://tech.groups.yahoo.com/group/suncityastronomy/message/1529

jeff Beish takes it a step further with a 16" F7.


http://www.alpo-astronomy.org/jbeish/Tippy_Jeff_16.jpg

http://www.alpo-astronomy.org/jbeish/RADIO_DISH.htm