Revelation 12" Dobsonian

[derekm]

Didn't expect anyone to dismantle a 'scope! I just thought that someone who had changed the springs may remember the size of screw.

I can't wait for the new toy to arrive!

[arad85]

Good Personally, I'd just buy from Bobs Knobs - they aren't too expensive and unless you have an odd spring supply workshop close by, it may be difficult to find the right ones.

I will be interested in your views compared to the Skywatcher... Would you mind posting a review to this thread when you've had a chance to put the 12" through it's paces?

Thanks

[derekm]

OK. Will do.

[derekm]

Revelation 12”. My first impressions and some comparisons with my old 8” Skywatcher.

The Rev has now arrived and is built, waitng for first light. .

All arrived without damage, but as stated by a previous poster, it is a pity that the OTA could not have been in a double walled box. Being the size of a coffin, it is quite vulnerable. The internal moulded polystyrene packing is good and the overall impression on opening the OTA box, is one of well engineered quality. There were however no instructions, but more of this later. In particular, the altitude bearings are extremely well conceived, are machined from solid aluminium to a very high standard and are fitted with ballraced thrust bearings. The ingenious but simple slider arrangement of the alt. bearings (in cast metal [aluminium?] rather than plastic) means that the tube can be balanced without extra weights. Once a suitable setting is decided upon with “normal” additions at the viewing end, it should, with the help of the adjustable friction in the alt. pivots, be possible to swop from light to heavy eyepieces without the tube running away. This will be partly due to the fact that the weight change up front will be only a small percentage of the total tube weight, unlike a smaller scope.

To compare with the 8” Skywatcher the Rev. replaced, the SW altitude bearings are a larger diameter and are simply moulded plastic wheels/trunnions, running on non rotating, round nylon bushes. There was a ring of friction material fixed/glued to these wheels/trunnions, which pressed against the side of the Dobsonian mount, with an adjustable thrust bearing for extra friction. This system results in a lot of stiction and jerky movements, even with the thrust bearing totally slack. I had to come up with a system of Velcroed weights to compensate for different eyepiece weights. The smaller diameter bearings of the Rev. are IMO a better solution as there is a greater effective leverage to overcome any stiction. So far, the Rev. is streets ahead.

Onto the bases now. The SW was all chipboard, the Rev. is mdf for the front panel and the rest chipboard. Before the assembly of both, I varnished the raw edges to avoid the entry of damp. The SW boards were covered in a fairly thick hard Melamine type of covering, whereas the Rev. has a paper thin plastic sheet bonded/glued to it. Frankly a poor solution which would not cost the manufacturer many pounds to rectify. Indeed the enlarged photo on the Telescope House website shows a white base a la SW, which looks as if it has the melamine type covering.

The azimuth bearing of the Rev. is a Lazy Susan roller bearing about 10” diameter and friction is controlled by another ballraced thrust bearing on the central bolt, which incorporates a moulded handwheel. The SW was “classic” Dob, with 3 Teflon pads and with the relatively light weight of the scope, the movement was not particularly fluid. I installed a Lazy Susan ball bearing from Axminster (Axminster Power Tool Centre : Power Tools to your door ) and used washers cut from plastic milk jugs and placed over the central bolt to add some friction. This required spanners, so one ex-cycle spanner was screwed permanently under the base engaging the bolt head and the adjusting spanner retained in the top, near the nut, with stick on teflon. The Rev. is still streets ahead.

The secondary spiders are very similar, probably ½ mm, (unlike a well known British Dob I have seen ,+/- 3mm from memory??) to minimise diffraction. The secondary mirror mounts are also quasi identical and of decent quality. Why cannot manufacturers fit thumb screws for adjustment? A set from Bob’s knobs costs far more than the manufacturers pay for the complete spider & secondary mount.

Down to the mirror cells and here the boot is possibly on the other foot. The SW 8” has, by nature of it’s modest size, a solid cast ring mirror cell. This is very rigid and maintains collimation with out the need of the locking screws. It still requires a screwdriver for adjustment though. Why no knobs? Strangely, there were no springs on the adjusting screws, but columns of small o-rings! Odd, but it worked fine, at least with the lightweight mirror. The Rev has an impressive looking 9 point mirror cell, supported by springs, however, as is already well documented, these springs are far too soft and also, the main “chassis” of the mirror cell is too flexible. Even with stronger springs fitted, collimation shifts 2 or 3 mm from 45 deg to the zenith, if unlocked. If the lock screws are tightened, there is still a slight shift of a mm or so. It may not be noticable optically, but it would be better if it did not happen; it is not rocket science. Because of it’s mass, the Rev. mirror has a small DC fan to aid with cooling, but this was not needed on the SW. “My” SW had a low expansion Pyrex mirror, but the Rev has, to the best of my knowledge, normal (plate?) glass. It’s a pity that the makers could not have gone the extra few yards. I appreciate that genuine Pyrex disks are moderately expensive, though not so expensive as to warrant the £60 premium that was on the SW in the UK when I bought it (from Germany!). Some posters have said that Guan Sheng use Pyrex, but I feel sure that if that is the case, they and their importers would be making a marketing point out of it.

When the OTA is stood on it’s tail, as it most likely will be whilst the base is loaded into the car, it actually stands on the collimating screws. This may or may not affect collimation, but it would be preferable if it did not happen. Either the screws need a lower profile, or the bottom ring of the OTA needs to be be somewhat deeper. A minor point maybe, but one that would have no cost to rectify. The SW has gained some points here!

My Rev. was far from collimated on arrival, in fact the secondary could only “see” about half of the primary! No problem for me, as this is my third Newtonian. I appreciate that this is not really a beginner’s scope, but because there were no instructions for building or collimation, someone upgrading from a refractor or cute little ETX could well be baffled by the lousy images it would have produced out of the box. In contrast, the Skywatcher’s booklet gave comprehensive building instructions (not really needed with common sense) and equally comprehensive and well illustrated collimation information. A black mark I’m afraid GSO.

When fully assembled and ready to observe, the Rev. has buttery smooth movements in both axes, with the friction of both finely adjustable. The SW on the contrary, though brought up to scratch in azimuth with the addition of the LS bearing, was always lacking in altitude. I tried silicone grease, spray-on curtian rail glide and even slackening the nylon bushes so that they could rotate, but there was always considerable stiction, making altitude annoying to control.

Now to the focussers. The SW had a conventional rack & pinion. After this was dissembled, cleaned, removing the “Chinese standard” treaclegrease, followed by a coating of our normal bearing grease on moving parts, movement was quite acceptably smooth. The Rev.though, is 21st century, with a beautifully machined Crayford type focusser and 1:1 and 10:1 reduction drive. There is no comparison, the Crayford being n times smoother and the 10:1 for fine adjustment simply puts it in a class apart. I believe that there are still “high end” scopes using rack & pinion. When will they see the light?

Overall, though both scopes are from China, the Rev. would seem to be some years ahead of the SW from a mechanical and engineering point of view, apart from the material covering the base and the mirror cell chassis and springs. Wake up GSO!

If we have clear skies tomorrow night, it will be out for 1st light and I will try to get some indication of mirror quality with a Ronchi grating. I’ll try to get a photo of the Ronchigram to post.

At a later date the mirrors will be tested properly on the bench and depending on the results, I may have the main re-figured and replace the secondary flat if necessary. Even if the mirrors do turn out to be less than brilliant (12th wave maybe, just maybe, but how smooth?) it’s a whole lot of scope for the price and it should be a wake-up call for other manufacturers.

[arad85]

Interesting - I'd agree with all of that and nice to see the Rev is ahead in most of the mechanical bits. I also get the collimation problems depending on where the 'scope is pointed so your description sounding like an inadequate mirror cell sounds to be the root cause. I wonder if there's anything that can be done simply to help it. The mirror is supposed to be the same as the Meade lightbridge which uses BK7 (as far as I can find out). Very interested in your assessment of mirror quality - this is my first newt, so I don't have anything to compare it against directly.

[derekm]

I suspect the only way around the collimation shift would be to make a new, stiffer mirror cell. The "fingers" of this cell are relatively long and narrow and are only about 3mm thick. Not really rigid, when we are talking about a movement of a fraction of 1/100 of a mm to make a change to the collimation. I'm assured by someone who knows far more about mirrors than I will ever know, that visually, it will not make any difference whatsoever, but I just like things to be "spot-on". ( I prefer my Omega to a Casio, but the Casio kepps better time!) If every aspect of a scope was made to the "she'll be right" standard, we'd be back to the level of the Eb*y scopes, ie. junk, but there are limits as to what can be produced for a certain price, even in Taiwan.

When I get the chance to make a layman's judgement on the mirror, any criticisms will need to be tempered with the understanding that a good 12" mirror would cost far more than we have paid for our scopes.

As an analogy a Ford is quite serviceable, but doesn't come near a Mercedes, which equally doesn't come near a Rolls. However, we have far better value than the Rolls.

After a clear day here in Scarborough, it's now raining. Hope it's clear tomorrow night!

[derekm]

It was clear, but couldn't get to the site for snow!!!

[themos]

I think I'd like to add a lazy Susan az bearing to my SW 300P, the teflon pads are just too sticky.

[derekm]

It's an easy job.

The bearings are cheap enough from Axminster, around a tenner I think. For my SW I made a circular former from a scrap of laminate flooring, which I screwed to the groundboard to retain the bearing in position and the bearing itself I fixed to the upper rotating board IIRC. A small pile of washers cut from milk jugs/bottles was added over the central bolt to add some friction. It will drop into place when you see the parts.

Go for it!

[themos]

Thanks, I am not sure about some of the terminology: what do you mean "circular former"? Any chance of taking some pics?

[derekm]

Sorry, can't take pics, the scope was sold to fund the Revelation!

The circular former is simply a circle of laminate flooring, approx 7 or 8" dia., sized to fit inside the ring of the bearing and to retain the latter in position. The laminate we put in our conservatory just happened to be the right thickness.

Buy the bearing and then it will all become self explanatory. These things are difficult to describe precisely in words.

[derekm]

FIRST LIGHT AT LAST.

Clear skies last night so managed to get out to one of "our" dark sites to try out the new toy. It appears to live up to it's name and is quite a Revelation! There was a half moon, strong enough to throw shadows, so not ideal observing conditions, at least for deep sky.

After checking and having to adjust collimation, probably due to both the rather flimsy mirror cell and a drastic change in temperature, we tried a few Messiers. Orion was brilliant, despite the moonlight. Andromeda was just an oval fuzz with a brightish centre, after we managed to find it. There was so much moon, we couldn't see the guide stars to hop to it! In the end, I swept left & right until it appeared. M81 & M82 were I think, as bright as they were through the 8" Skywatcher on a dark night. M51 Whirlpool, despite the moonlight, was brighter than I ever saw it through the 8". Could not see spiral arms, but the bright centres and the overall outlines were quite crisp. I can't wait to see the double cluster on a dark night; last night it was brilliant, despite the moon. M78 was quite clear and I think it would have been invisible in the 8" SW under last night's conditions. The moon itself, through a polarising filter was stunning.

There may have been a touch of astigmatism in the images, but as yet, I have not checked the mirror clamps in the mirror cell. Reading other threads, they are frequently too tight.

I replaced the eyepiece with a Ronchi grating and was pleasantly surprised to see a set of very straight lines, indicating a good figure. I tried to get a photo of the Ronchis, but either the camera would not pick up the image or I was not aiming correctly! In the fairly near future, I will have the mirror properly evaluated on a test bench by a man who knows, to find out definitively how good or bad it is.

With finders and eyepieces dewing up and a layer of frost on the scope and mount, we called it a night. Roll on a clear moonless night now!

These first visual impressions are very favourable and as a well made mass produced 'scope, it would seem to represent excellent value in our generally overpriced "rip-off Britain" marketplace.

[arad85]

Great stuff... and yes, I think it's a bargain too - just a shame they don't do a 16" I have to say, the only thing I was worried about was how the mirror seemed to compare to other well known Newtonians - having only owned SCTs before this one. I look forward to the mirror test

I gave up after 30 minutes last night - everything was too moonwashed from my location...

[derekm]

The moon was really too strong here too, but I just had to get out to play with it! I think it's maybe a good job they don't do a 16"; I got the long face for days with the 12" and it practically ended up in divorce, except that we're not married!!

One thing I have to say about SCTs is that everyone in our club, including those with SCTs commented on the image quality through my 8" SkyWatcher and frankly, I've never seen a WOW image through a Meade SCT. Our society has a 12" and honesltly, it's junk. The image shift on focussing with anything less than minimum magnification puts the target out of the field . A buddy has a Celestron 8" and IMO it's better all round, but even he preferred the SW images. He was impressed with the Rev last night, but I think that with a dark sky, it will blow his mind!

[parkway]

Hmmmm!!! Very interesting thread guys. I was looking at the Skyliner 12" but it's a little out of my budget for the time being and I couldn't quite bring myself to purchase the SW 10". (aka. aperture fever) Then I spotted this thread and the Revelation 12" Dob. Such a huge difference in price when comparing the Rev against the SW.. it somewhat concerns me about the quality of the mirror. I would love to hear Derek's report on the mirror.. The Rev just seems too cheap considering the general prices of Telescope House (when compared with FLO). Sooooo tempting though.. so tempting.

[arad85]

It's supposed to be the same mirror as used in the Meade lightbridges so..... (that's why I took a chance in the first instance - they are produced by the same factory!)

[John]

The Revelation 12 uses a GSO mirror I believe - the same as the Meade Lightbridge's. In terms of optical quality they are reputed to be quite consistant and about the same as the Skywatcher equivilents. The "achillies heel" of the 12" Revelations and Lightbridge's used to be that the collimation springs fitted to the mirror cell were too puny to properly support the mirrors weight making it difficult to hold good collimation. I've often wondered if this factor fueled rumours of inconsistent quality optics ?. It's quite possible to replace the inadequate springs though although hopefully by now the manufacturer has caught on and fits stronger ones as standard.

[FLO]

It's supposed to be the same mirror as used in the Meade lightbridges so..... (that's why I took a chance in the first instance - they are produced by the same factory!)

If the original Revelation Dobsonians are anything to go by (I owned one) the mirrors will be made at the GSO factory in Taiwan, the same factory that manufactures mirrors for Meade Lightbridge, and they will offer a similar performance to Skywatchers. But, the GSO mirror-cells are not as sturdy as Skywatchers (Bob's Knobs in the US offer an upgrade kit) nor will they have the protective silicon-dioxide mirror coatings that are applied to Skywatcher and Meade Lightbridge telescopes.

Regarding the protective coatings. I have no way of determining whether the latest Revelations have them or not but it would go some way to explaining the lower cost. If someone can clarify either way I will be very interested.... ?

Steve

[arad85]

If the original Revelation Dobsonians are anything to go by (I owned one) the mirrors will be made at the GSO factory in Taiwan, the same factory that manufactures mirrors for Meade Lightbridge, and they will offer a similar performance to Skywatchers. But, the GSO mirror-cells are not as sturdy as Skywatchers (Bob's Knobs in the US offer an upgrade kit) nor will they have the protective silicon-dioxide mirror coatings that are applied to Skywatcher and Meade Lightbridge telescopes.

Regarding the protective coatings. I have no way of determining whether the latest Revelations have them or not but it would go some way to explaining the lower cost. If someone can clarify either way I will be very interested.... ?

Steve

Hi Steve,

I can't believe adding a SiO2 layer to a 12" mirror in a factory with the equipment in China would cost more than a couple Chinese Yen. After all, it's one of the most abundant materials on earth... I think it's more likely down to the manufacturers (not the dealers ) making their markup rather than anything else

I'm sure Telescope House (the importer for those who don't know) would know though....

[FLO]

According to Synta:

"Most reflectors today usually have a coat of aluminium as the mirror surface and then an overcoat of silicon monoxide or silicon dioxide to protect it. Silicon dioxide produces a much more durable coat than silicon monoxide but requires specialized equipment to apply it and is therefore more expensive..."

[arad85]

Yes, but don't GSO put SiO2 onto the Meade 'scopes so they should have access to it as it's the equipment that costs, not the coating. I've mailed TH and will post here if they give me an answer.

[parkway]

Great idea to contact TH. It would make sense they were the exact same mirrors. I did a bit of googling and came across this:

Zhumell Z12

Apart from the OTA being black (which is lovely!) it's the same scope from what I can tell.. and reviews are very positive.. same old complaint about the primary springs not being adequate.. definitely looks like a GSO scope though!

One dealer had a FAQ for the scope and this featured on it:

Q 1. what type of glass is the main mirror? 2. what type of coating on the main mirror?

A: The Main mirror is made of a Pyrex glass. The glass is coated with aluminum coatings with a magnesium fluoride coat to provide bright images full of detail.

Exactly the same as Meade say about their LB's.

Digging a bit more on the Zhumell I found a Magazine review of the 10", which stated the following:

"The 10-inch aluminum oxide/silver oxide-coated mirror provides excellent contrast and definition and offers 1/12-wave wavefront accuracy, along with 92-94% reflectivity. Mirror Coatings - Aluminum plus Sio2 (Silicon Dioxide)"

I also found GSO had a website and found this info on their webpage:

"Our parabolic mirror size from 6" to 16", all of these mirrors were made by BK7 quality optical glass and all the mirror surface quality over 1/16 wave RMS at the least, typically better. All mirrors have an aluminum reflective coating, reflectivity is approximately 91%. The aluminum coating is protected by a properly applied protective coating (SiO2)."

Phew!

[FLO]

Yes, but don't GSO put SiO2 onto the Meade 'scopes so they should have access to it as it's the equipment that costs, not the coating.

Yes, they clearly are very capable, but they did not apply a silicon dioxide overcoat to the first generation of Revelation Dobsonians (that was the explanation offered when I commented on my mirror pitting after only two years). I can't help but wonder whether these new Revelations are the same.

People are asking why the Revelations are cheaper than Skywatcher. As a dealer for Skywatcher telescopes I am also curious. If it is simply that the supply chain is more efficient then so-be-it, kudos to whoever commissioned them, but if it is that they are built to a different spec then it is only right that people know so they can make an informed choice.

That is all

[FLO]

I also found GSO had a website and found this info on their webpage:

The aluminum coating is protected by a properly applied protective coating (SiO2)."

If that applies to the mirrors manufactured for other brands then all is well

1/12-wave wavefront accuracy .... mirror surface quality over 1/16 wave RMS at the least, typically better ....

Those are remarkeable figures.....

[John]

Those are remarkeable figures.....

Remarkable indeed ....... Orion Optics quote around £1,000 for a 12" mirror set to 1/10th PV wave (as opposed to RMS) and Hilux coated.

Who is fooling who I wonder

My Meade Lightbridge 12" was a nice scope ........ but not 1/12 PV wave I'd wager