12" Dobsons with f1200 or f1250?

[PeteMo]

Hi All
Further to my first post on upgrading my Optex 8", I have been looking at 10" and 12" GSO dobs from Bintel and Andrews.

Considering the economics and light gathering potential, the 12" looks better :Dthan the 10". Unfortunately, the 12" with a 1500mm focal length is not very portable :( compared with the 10" with it's 1250mm focal length.

On GSO's site the 12" dob has a focal length of 1250mm, yet both the Bintel and Andrews GSO's are quoted as having a 1500mm focal length.

Does anyone know if it is possible to get the GSO 12" dobson with the 1250mm focal length as on the GSO home site please? :whistle: Otherwise I'll go for the 10", but am worried that I could be missing out on some serious aperture with the 12".

Thanks

Pete

[ving]

yes the 12" is f/5 which means its fl is 1500mm :)

[ving]

you could go a meade lightbridge truss. that is more portable :)

[janoskiss]

Pete, that will be a typo on the GSO website, which also says 12" f/5, so FL = 5*12" = 60" ~= 1500mm. The scope you have in mind would be an f/4 and would need a coma corrector. You could probably have one custom made if you wanted.

You should go along to a viewing night at a local club to get an idea for how much each telescope can show you. The difference between 10 and 12" is noticeable but fairly subtle. I will be replacing my 12" and 8" Dobs soon with a 10". The 10" is just a lot easier to handle than a 12" both on and off the field. Much more comfy for seated observing.

[casstony]

I couldn't find any decent alternatives to my recently purchased 10" f/5. Orion Optics make a nice looking 12" f/4 but there are no dealers in Oz and they do cost 1 or 2 more dollars. http://www.orionoptics.co.uk/acatalog/Dobsonians.html

[GrahamL]

hey pete :)

I went through your dilema 6 months back .. and went with the 10"
I have thought once or twice the 12" would show me more .

Having had a look at glencs nice 12" dob .. the portability issue you raise
is pretty evident .. the views are great .. but you cant help notice
this IS a big scope ..I couldn't imagine walking this baby down the hallway , down the back steps across the yard every night without the knees giving out ..not to mention bumping/nudging stuff along the way.
The truss option mentined is where I'll go if I decide to go bigger .

take care ns

[Scorpius]

I recently purchased a SW880 10" from NVT ($863 +$1 shipping) in Perth after being quoted $450 shipping for a GSO ex east. I had envisaged getting a 12" but also seeing that GSO were not available at the time (Bintel are GSOs) I decided to go for the SW 10" I was most impressed with the unit and saw a marked increase in efficiency over my homebrew 8"
Viewing the nebula in Orion and SMC (Tucanae) at a local club night under Perth's light ridden skies was quite impressive. I still wanted portability and the SW880 light bucket will fit in the boot of my car and the support on the back seat.

At the same time we also compared GSO SP EPs 25 & 10 with the SW EPs The Skywatcher EPs definitely had the edge especially in the peripheral vision. The focuser was a R&P and had a small amount of slop (See my message about this) I Later replaced it with a 2" Crayford 2 speed, which is excellent, from my older 8". A further test with an O2 filter on the EP brought the clarity up greatly. We also tried a regular 85A blue which also gave good improvement.

I received excellent service from NVT and their regular shipping prices are realistic enough to be outstanding. I find it hard to justify $450 (Bintel) or $250 (Andrews) when NVT can ship Oz Wide for $18 With similar or near Item cost prices.

The difference between 10" and 12" depends on how good your chiropractor is:whistle:

[janoskiss]

I might as well let the cat out of the bag. With my 12" GSO Dob about to be sold, I ordered the Saxon 10" Dob from the Telescope Shed. Afaik this is the same as the Skywatcher that Dave has got.

I was thinking of going for a 10" Lightbridge, but the Pyrex primary mirror (made in the US) clinched the deal in favour of the Saxon (not to mention $300 saved). I also like the mount design and finish of the Saxon/SW. According to Saxon's website their dobs now also come with Crayford focusers. The website often contains the wrong info so I'll wait and see.

[PeteMo]

Hi Scorpius

Another Perth local! Thanks for that info on postage. I find the cost surprising when you consider that you can buy an 8" dob for $450. It is also more than half the cost of a 10" Bintel/Andrews. What surprises me most is that Bintel and Andrews are supposedly long standing "Mail Order" specialists.

I have heard good things about SkyWatcher eyepieces, basically being advised that they are made at the same Guan Sheng factory as the Orion lenses. I know nothing about SkyWatcher scopes except that this seems to be the main brand flogged by Perth's BTOW in Malaga and Camera House in Midland.

My only concern about the Skywatcher scopes is that they are significantly more than Bintel/Andrews, yet don't come with the Crayford 2" focuser and mirror fan as standard. If allowing an extra $119 - $149 for a Crayford 2", that takes the true cost to over $1000 for the Skywatcher, in which case the Andrews 10" Delux wins at $649 + $250. Even the Standard GSO 12" from Andrews comes in close at $799+$250, so if postage is the decider the GSO 12" looks good.

I'd still be curious to learn about your SkyWatcher, Where abouts in Perth are NVT please?

[janoskiss]

The 10" SWs do come with a pyrex mirror though, which would add considerably to the cost. I know most people say that does not matter, but I am looking forward to better thermal stability from the pyrex glass for high power planetary viewing. With the 12" plate glass mirror I'd often miss the good seeing while waiting for the scope to cool. And the newer Saxon/SW dobs probably come with Crayfords anyway. I will let you know when I get mine.

[PeteMo]

Hi Steve

Thanks, I'll keep my eyes open for a review. What confuses me now is that according to one web site, the Tasco, SkyWatcher scopes are all low quality scopes, which seems to be supported by the rack & pinion focuser. How can we be sure what the mirror is made of and where it is made when so many manufacturer web sites contain errors?

cheers

[janoskiss]

Tasco, Skywatcher, Saxon, Orion etc are just importers that stick their name on a whole bunch of stuff. Some of it good, some bad, some so-so. The focuser is a bolt-on item and whether its one type or another says nothing about the quality of the optics. The GSO Dobs that come with R&Ps have exactly the same optics as those with crayfords.


I have discussed it with John Izzo from Telescope Shed and he assures me that the Saxon 10" Dobs he sells have US made pyrex mirrors.

[PeteMo]

Thanks Steve
that's good enough for me! I presently use Rack & Pinion so probably won't notice if there's no Crayford. Besides R&P has been the mainstay for about the last 150 years or so.

Think I'll be paying Telescope Shed a visit as I also learned that the SkyWatcher has a patented Tension Control handle which will be useful considering that I need to adjust the counter balance weight on my Optex every time I switch between my Hyperion and Plossl eyepieces.

Looks like my plans are going to change.

[PeteMo]

I've looked at SkyWatchers web site and note that the Focal length is 1200mm for their 10" Dobson. This is great as it means it has exactly the same magnifications as my Optex 8" but 57% more light.

[Scorpius]

So far I am very impressed with the SW880, I phoned Greg at NVT at the Myaree Business Center and it was delivered the following morning. Go to my web page for a direct link to NVT or look up their current advert in this months Sky & telescope.
It went on "display" the following day at a local club meeting at the Murdoch University Observatory and had some very good viewing, without exception everyone was impressed.
However I agree, Its about time they stopped making R&P focusers. Once you have used a 10-1 Crayford reduction gear you would never use an R&P again. If you want a "real" crayford and have $250 US go to this site :- http://www.focuser.com/cgi-bin/dman.cgi?page=productdetail&plugin=dstore.cgi&product=CR1

Once skies clear I will be trying it out with the EOS 350D and will publish some photos on my web and here. Been lousy seeing over here since I bought the beast, either light pollution or cloudy:(

[Scorpius]

PS didnt mention, by being a signed up member at NVT a further 3% discount is given on all items for sale. http://www.nvt.com.au/

[PeteMo]

Hi Scorpius
Thanks for the info. Sounds like the scope is great. I think the Saxon/SkyWathcer dobs are made by Synta, since they are noticeably different from the GSO offerings.

It will be some time early next year when I have the funds to get a bigger dob.

I spoke to BTWO two weeks ago and he also mentioned that they had problems with some optical issues with some of the GSO's but didn't go into detail. They were getting a load of gear in, mostly SkyWatcher scopes and eyepieces. I'll see what they flog the SkyWathcer 10" dobs for, as we live out at Henley Brook which is a short ride from Malaga.

There's no harm in joining NVT so I'll do that now!

The sky has been rubbish out our way too, so I've only been able to test my new Hyperion 8mm against a murky sky. I'll have to get the Baader adaptor ring so I can use my F9500 for pics.

[janoskiss]

I'm going to post more extensively on the 10" Saxon/SW Dob. I picked mine up today. very impressed. Build quality and attention to detail is commendable. Puts the GSOs to shame. For a newbie who does not want to tinker, fix up things, the Saxon or SW is a much better choice. It is a very well made scope. The mirror is just a whisker over 10-inches in diameter, between 255 and 256mm.

On the downside: R&P focuser only atm, and fitting a GSO Crayford might be a bit tricky. Saxon has ED80-style Crayfords available for these scopes. Bolt-on Orion Accufocuser is another possibility.

[astro_nutt]

Hi Steve
I brought my Saxon 10" dob a few years and the only mods I made for it was to change the finder for a right angle one (for comfort)..and built a motorised alt-azimuth mount...the optics are very good and collimation is easy with the large chromed knobs..I also fitted a fan from an old computer and wired it to a 9 volt battery..works very well indeed and am more that happy with it!!!
Cheers!

[Scorpius]

This is the mod I did to fit a GSO type Crayford with 70mm spaced holes on a sheet of 1/8" aluminium. I actually drilled four inner holes to mount it but you can use countersunk bolts from the inside of the plate and use the original 85mm spaced holes. I also put a thin rubber seal under the plate.

Alternatively if you want to spend an extra $100 check out the link in my message above for a focuser body that will fit inside the SW R&P mounting plate. They do a complete range of sizes. Alternatively do a Google search, there's a lot of stuff out there.

[PeteMo]

Thanks for all this info guys. I've decided to go for a SkyWatcher 10" dob some time in the new year.

Astronut - I'd be curious to learn how you motorised your dob mount, as I only know of the Orion XT Intelliscopes that have motors and Go To stuff.

Dave - My Optex only has a 1 1/4" focuser. The mounting screw holes are 58mm apart, centre to centre, and the hole cut out is 59mm. I therefore think I'd be struggling to add a 2" Crayford as these often come with mounting screw holes over 70mm apart, centre to centre.