Revelation 12" Dobsonian

[derekm]

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

Last autumn, the 12" Revs were around £700. Now they have dropped to £510. Would everyone have the same doubts if they were still at last years prices?

I strongly suspect that our UK retail prices bear little or no relation in normal western terms to the factory gate prices in China. They are likely to be based on waht the UK market can stand and at the moment we are in a recession.

Chinese factory workers IIRC earn around $1 an hour. Compare that with Europe or the US and that is the reason their scopes can be sold "cheaply" and also why UK made mirrors cost more than the price of a complete Chinese scope. The UK mirrors may be better, but \as with HI FI,the law of diminishing returns comes into play.

[arad85]

This from Telescope House (Ted). I have his permission to publish:

Hi,

As far as we are aware they do have the overcoat, I don't think any

modern higher quality scopes still just have standard aluminum coats

Regards, Ted Harrison

Not 100% I agree unequivocal, but.....

[John]

This from Telescope House (Ted). I have his permission to publish:

Not 100% I agree unequivocal, but.....

Very vague actually - gives you no guarantee whatsoever.

I've just had a phone conversation with TH on the Revelation 12" dob. I asked if the 1/12th wave specification was RMS or PV - the response was that they had no idea - they could only quote the specification supplied by the manuufacturer. I then asked if the mirror was overcoated and their response was that they assumed that they were but had no way of telling and the manufacturer was not specific about this.

[parkway]

As already stated, the dramatic price increases of higher quality mirrors as (for example) offered by Orion Optics UK clearly demonstrates they are quoting RMS. It would (although its very misleading) be very tempting by manufacturers to quote mirror wavefront's in RMS to make their optics sound better than they actually are. From what little I know, a 1/13.4 RMS equates to 1/4 P-V. (RMS / 3.35 = equivalent peak to valley)

It's a pretty safe bet that the mirror in the Rev (and all other GSO produced scopes) are 1/4 P-V or (slightly) better. (given they are quoting between 1/12 and 1/16 RMS)

All I want to know is does the mirror come with the SiO2 coatings! That's pretty much the only thing holding up my purchase. But from what I've found/read.. it's looking promising it has the coatings. Come next week I'll probably put an order in! Meanwhile, is it just the Primary springs that need replacing? May as well order them now to save the wait.

Cheers

[arad85]

Yes, primary springs need replacing and you could get a set of secondary knobs if you want to be able to tweak that too. The only other thing which appears to be an issue which causes a slight change of collimation (allegedly unnoticeable) is an under specified primary cell (just a tad too flimsy by the sounds of it).

[John]

All I want to know is does the mirror come with the SiO2 coatings! That's pretty much the only thing holding up my purchase. But from what I've found/read.. it's looking promising it has the coatings. Come next week I'll probably put an order in! Meanwhile, is it just the Primary springs that need replacing? May as well order them now to save the wait.

Cheers

Thats what I would want to be sure of as well and, as TH could not confirm that to my satisfaction (and they really should have known), I would not have placed an order. It's a nice scope but I don't want to run the risk of having to have the mirror sorted out within 24 months.

It's entirely up to you of course but I think there is a real chance that the Revelation 12 mirrors are not overcoated.

[derekm]

Thats what I would want to be sure of as well and, as TH could not confirm that to my satisfaction (and they really should have known), I would not have placed an order. It's a nice scope but I don't want to run the risk of having to have the mirror sorted out within 24 months.

It's entirely up to you of course but I think there is a real chance that the Revelation 12 mirrors are not overcoated.

With all due respect, if GSO, the makers of Revelation, Zhumell, Hardin and Lightbridge state on their website how their mirrors are coated (SiO2), why suspect that they aren't?

Re. the collimation springs, I got my replacement springs from a local old fashioned hardware store, for 50p each.

Even with the stronger springs and the lock screws locked, the collimation does shift slightly, approx 2mm shift of the peephole reflection in the Cheshire. Fabricating a new rear section of the mirror cell would not be not rocket science. Many things coming out of China have to be regarded if not as a kit of parts, then as a good value starting point in which ever field the goods are in.

The man who suggested that the collimation shift would not be noticeable visually is one of the best (the best?) mirror makers in the country and he will be doing a full test of the mirror when his time permits.

I think it is a pity that they use BK7 rather than Pyrex or it's clones, at least for their larger mirrors. It's coefficient of expansion is only slightly better than plate. Still, every few pounds added in China equates to a lot of pounds added here.

Depending on the results of the test, I may well have my mirror re-figured, but that does not detract from the fact (IMO) that the Rev 12" is generally a very well made scope and at it's present price, represents excellent value for money here in the UK.

[John]

With all due respect, if GSO, the makers of Revelation, Zhumell, Hardin and Lightbridge state on their website how their mirrors are coated (SiO2), why suspect that they aren't?

I really hope they are but time will tell I guess.

I agree with you entirely that most dobs are a "work in progress" when delivered - my Lightbridge 12" certainly was, but a really decent scope once the various mods were carried out

[derekm]

Chinese factory workers IIRC earn around $1 an hour. Compare that with Europe or the US and that is the reason their scopes can be sold "cheaply" and also why UK made mirrors cost more than the price of a complete Chinese scope. The UK mirrors may be better, but \as with HI FI,the law of diminishing returns comes into play.

Confirmation/update.

Source "A Bull in China" Jim Rogers, 2007

In 2004 China average wage was $0.64/hr.

Since then 31 provinces have raised minimum wage, but maximum still only $106/month.

[parkway]

Placed an order today with TH. Already received springs from Bob's Knob's. Will post a report once I get the scope and get first light. Forecast for rain all next week though. Pity.

UPDATE: Received scope today. Ordered yesterday at 10am and received today at 12pm. Brilliant service on behalf of Telescope House. Courier was Fed Ex.. gave me a bit of a scare the way the driver was handling the OTA out of the van. Upon closer inspection I noticed the box damaged.. my heart sunk. I had a look and everything appeared fine though. It's a real shame TH won't box the OTA in an outside box. Signed and set about building it. No instructions were included. My opinion is pretty much of the OP in terms of quality. The spider vane was hanging loose though and I did a bit of a flap when I noticed one of the thumbscrews was missing but luckily it was inside the box. Again, this scope is not for the light-hearted nor beginner. changing the primary springs to Bob's Knob's was straight forward and now it's time to collimate. So far, very very happy with the scope and cannot wait to get first-light with it. It makes my SW 8" look like a pea shooter! Will post back with first-light report.

Peter

[albedo0.39]

The guy who sold me my LB remarked how quickly the mirror deteriorated with time from new. It transpires that although Meade overcoat the mirrors, they are designed for less humid atmosphere's than the good old British Isles.

So.. he had the mirrors re-coated by Galvoptics before selling to me (kind person) and I can say that they look at real peach. Moreover, they are overcoated which although it potentially limits the overall reflectivity by a touch is a stable albedo from one year to the next. I've seen Galvoptics mirrors 5 years old that look stunning.

[derekm]

Oldham Optical (Oldham Optical Web Site ) uses Galvoptics for his silvering and coating, so they are definitely to be recommended. There's a lot of good information on his site too.

[nightspace]

OK.. On the advice of Steve from FLO, I ordered some primary springs for this scope. In the following two pictures you see the spring that comes with the scope on the left, Bobs spring on the right. Cost £10.60 for 3 springs delivered. CLEARLY a much heavier duty spring that should help hold collimation - once I've got it back again

Hi Arad 85

Do you have the specs for these stronger springs?

John

[John]

The Bob's Knobs springs certainly look far more robust than the standard items. I had that upgrade on my Lightbridge 12" (same mirror cell as the Revelation 12") and it held collimation pretty well. I stopped using the locking screws on the mirror cell - they seemed to cause flex to the cell when tightened (it's made of quite thin metal) and I found the scope held collimation pretty well without using them. I came to the conclusion that they were best reserved for when and if the scope was transported.

One thing I can't understand though - the weak standard collimation springs have been a well known and documented issue with the Lightbridge 12's and the Revelation 12's for years now - Tom Trusock reported the issue in his 2006 review and it's mentioned in virtually every thread on these scopes on forums worldwide. And yet they still use the same springs

Small, cheap componants but they have a big impact on the performance of the scope and, if not remedied, could leave a new owner ultimately dissapointed with their investment. On top of that, changing the springs is not an easy modification, especially if you are new to scopes.

They managed to add a 2-speed focusser and much nicer altitude bearings - but can't beef up the little springs that support the "1/12th wave" mirror. Baffling .....

[arad85]

Sorry, no - I don't have any specs for the stronger springs... As John mentions, they do hold collimation better than before, and I also don't use the locking screws as it flexes the mirror cell...

[derekm]

We finally got around to testing the mirrors, so here are the findings.

First, a very pleasant surprise; both mirrors can be considered excellent by any standards!

The tests on the primary were carried out in autocollimation.

There was no astigmatism in the mirror, checked at 333x mag..

On the Ronchi gratings, the Ronchi lines were straight, the surface was smooth and there was no turned edge. There was a slight depression in the centre of the mirror and later measurements indicate that it is well hidden behind the secondary flat. The general impression of the experienced eye doing the testing (not me!!) was that it was likely to be about 10th wave.

The Foucault test (knife edge) showed the central depresson to be 70mm dia. and max depth 1.27mm (50 thou) and this is completely silhouetted by the secondary, therefore not illuminated.

There was no zoning, the the cut-off by the knife being uniform and near instantaneous, except for the depressed (unseen) zone

We found slight longitudinal spherical abberation, the mirror being .1016mm (4 thou) under corrected.

The edge was good, showing a diffraction ring all around the circumference.

Running the Foucault figures through the ZEMAX ray tracing program showed the mirror to be just diffraction limited. Just, but diffraction limited nontheless.

The peak to valley was 1/13th wave (actually 1/12.97), the Strehl ratio .943, RMS .0384.

The secondary flat showed no astigmatism and tested 1 convex spherical fringe with a master flat and sodium light.

There is absolutely no need for re-figuring (a possibility I had considered), so this makes me a very happy bunny!

The Revelation 12" then, at £510 offers a useful aperture with excellent optical quality, for a sensible price.

It has to be considered unreservedly as great value for money and there are no apologies to be made for it's provenance, nor allowances to be made for it's optics, or it's general engineering.

It's a little puzzling why they use BAK7 (or B270 as on their website) for the mirrors as this is an optical glass, is likely to be more expensive than Pyrex or Suprax and has a far worse coefficient of expansion. A first surface mirror has no need for optical glass, or even a transparent substrate.

As a final word, it was a pleasant surprise to find that the optical tube was an easy slide fit over the outer rim of the mirror cell, facilitating removal and replacement of the cell. The Skywatcher cell was a very tight fit in the tube, leading to difficult and dangerous (for the mirror) situations when removing or replacing the cell. On the Rev. it was two or three minutes single handed, each way.

I have no regrets with my purchase. My only advice to prospective purchasers would be to measure your car boot carefully! The Rev. 12" is big!!

[arad85]

Derek - absolutely great news - thanks for this detailed report. I too can sleep safely at night (when there's clouds of course... )

[derekm]

Two corrections to my post 91, the test of the Rev 12", as I can't work out how to do an edit!!

1) Astigmatism was not checked at 333x. Because it was in autocollimation, it was 666x.

2) Where I stated that the mirror was just diffraction limited, this was using the figures given by autocollimation, in Zemax. This means that all and any errors are magnified 2x. Result, the mirror is well and truly diffraction limited in normal observing mode, which aids contrast on faint objects.

[badgerchap]

Great write up...

Me want one now .. ooh Xmas seams such a long way...

My 9 yr old daughters B'day's end of August... wonder if she'd like one

Guy.....

You're a cruel man, but I like you style! Not sure your daughter would agree though.... heehee

[bombassa]

Since the Revelation is made by GSO can I then assume that the Dob's branded 'GSO' will be the same?

[John]

Since the Revelation is made by GSO can I then assume that the Dob's branded 'GSO' will be the same?

I used to have a Meade Lightbridge 12" which was also made by GSO - optics and primary and secodary mirror cells were identical to the Revelations and suffered from the same issues - weak collimation springs and flex when the collimation locking nuts are used.

The solution on mine was to fit a set of upgraded springs and replacement collimation knobs (top and bottom) supplied by "Bobs Knobs" in the USA.

[nightvision]

Interesting thread. I have a Rev-12" dob, bought back in 2005 from Telescope House. It has seen a lot of use and abuse, no problems with the surfaces despite being stored in a shed and now in a garage. I agree the mirror springs and mount are too weak but impressive quality mirrors. Thanks to Derek for the detailed test and report.

Tony.

[John]

Interesting thread. I have a Rev-12" dob, bought back in 2005 from Telescope House. It has seen a lot of use and abuse, no problems with the surfaces despite being stored in a shed and now in a garage. I agree the mirror springs and mount are too weak but impressive quality mirrors. Thanks to Derek for the detailed test and report.

Tony.

Glad the mirrors are holding up Tony

There were rumours around a while back that the Revelation mirrors were not overcoated which would lead to a more rapid degredation of the aluminising.

If yours are still sound then that was clearly not the case

[AlanK]

likewise Tony, glad to hear your mirrors are holding up.

I got a Rev 12" (the version with the big spring tensioners) 4 or 5 years ago. I washed the mirror yesterday as it had been stored in my garage for around 3 years, and it seems to be quite tarnished. I also washed a 6" mirror which had had the same storage conditions and it came up almost spotless. I know the 6" was overcoated, but suspect that the Rev 12" is not.

see 'down the tube' comparison photos below.

[nightvision]

Hi Alan, that looks like mine did after long term storage in the garage (house move and other priorities meant no astro stuff for over 3 years). After washing it came out like your 6" so maybe I was just lucky with the type of deposits!

What's the light pollution like in the IOM? I'm getting frustrated with the poor seeing here, I couldn't see M81/82 last night despite no clouds, I get better views when there are scattered clouds and no mist.

Tony.