ok i give up

[the bishop]

Hi,

its been a hard day and

I’m sure there is a real good reason for it,

but just now I can’t for the life of me think why it is?

And that is...

Why are DOB OTA’s longer than the EQ mounted OTA’s???

Can anyone put me out of my misery before I forget that I posted this!!

Old age don’t come alone?

Hi,

I’m sure there is a real good reason........... :grin:

[Steve Ward]

The shorter tubes have faster optics more suited to imaging and are more manageable on EQ mounts.

[jimmyjamjoejoe]

The shorter tubes have faster optics more suited to imaging and are more manageable on EQ mounts.

Fairly sure its something along those lines. On a dob mount, tube length isn't really an issue until you get to 12" plus, where an f6 would put the eyepiece at quite a height. On the smaller dobs, they can use the easier to produce slower optics with no problems.

[swamp thing]

By using slower optics on the small dobs they can save money too. Slower optics are easier/cheaper to produce, this keeps the Dob prices lower.

The cheaper they can make their dobs, the more they'll sell.

[John]

With the Skywatcher scopes it's only the 8" size where there is a difference (dob = F/6, EQ = F5). The 6" is available in F/5 or F/8 EQ mounted but just F/8 dob mounted (a 6" F/5 dob would be a table top scope and Orion (USA) do have one of those also with Synta optics). With the 10" and 12" the focal lengths are the same, 1200mm and 1500mm respectively, for both the dob and EQ scopes.

Personally I wish the dob mounted 10" was a little longer - F/4.7 is pretty demanding on collimation and eyepieces for what is nowadays often a first scope for people. An F/6 10" dob would put the eyepiece at a nice height too.

[swamp thing]

Personally I wish the dob mounted 10" was a little longer - F/4.7 is pretty demanding on collimation and eyepieces for what is nowadays often a first scope for people. An F/6 10" dob would put the eyepiece at a nice height too.

Totally agree John, an F/6 10" Dob is probably the very best visual scope one could buy. Why its not a standard commercial size is beyond me.

[AndyWB]

Okay - so I'm determined to spend at least 18months with my little first scope - but I couldn't help but notice that Orion Optics Dobs seem to be available in 2 lengths... I presume that's why?

[Moonshane]

OOUK will broadly make any focal ratio you need.

I am visual only and have had (all OOUK):

6" f5

6" f6

12" f5.3

6" f11

12" f4

16" f4

I still have the last three and can confirm that the 12" f4 is one of the most manageable scopes with a large(ish) aperture I have ever used. You are better with a coma corrector though at f4. Scopes get really big at 16"+

[John]

I seem to recall reading somewhere that the tube lengths of imported scopes were determined, at least to some extent, by the needs of efficient shipping rather than the desire to produce the optimum specification for the astronomer. The need to get as many crates of a consistent size in the space available, that sort of thing. I don't know if there is anything in that though

[Moonshane]

you may well be right John. The shipper would certainly want to cram in as much as possible.

[Stargazer_00]

a 10" with 1500ml focal length would be the best in my opinion, I agree.

that said though a decent coma corrector sorts out many issues, so I'm told. The coma in the 10" doesn't really both me much. I'm looking at getting a 14" in a few months and that'll be slightly faster still.

Can Orion Optics make a scope to the F ratio / diameter you choose? Could you ask for a 10.75" F7.2? for instance.

[Moonshane]

As far as I am aware yes. it's just a case of getting the curve of the profile the right depth and shape. the fast the mirror the deeper the hollow.

[Hzadbhat]

An idea from an engineering point of view...

Whilst being able to pack as much as possible in a sea container is obviously an issue, this is (within logistics) easy to get around just by adjusting the thickness of the foam inserts in packaging.

The main issue is most big companies get their products made by just one or two manufacturers. Those same one or two manufacturers have set their machinery to do a specific task.

Changing out the heads and re programming for the new one is a lot of time. Time equals money.

Therefore, each manufacturer will onlt do a small number of mirror shapes. One factory in china is responsible for most of the 8" primaries made commercially. They will spend a week or three making hundreds, if not thousands of the exact same part. These go into the warehouse and the next shape is started. But here's the trick. Like an auto parts manufacturer, they only make a very small number of products and they simply will not do custom.

If you want to chech this, ask your local retailer how much for, say a 10" f4.7. Then find somewhere that can make one for you, put your cuppa down, (no one wants to spill their cuppa) and be horrified at the difference in price.

By doing it this way, companies lower their costs and, hopefully) the manufacturer can sell the item to us, the consumers, cheaper.

This is almost universally standard for manufacture world wide.

I found this out as I am soon going to build a 32". I have free access to the right machinery, but am still blown away by the costs involved.

I recall reading someones post here a while ago that mentioned all those long days after school hand grinding and polishing. Sounded like he spend hundreds of hours working by hand (from memory it was a 6"). Multiply that by whatever your wage per hour is, and you will start to see why custom costs so much more.

[LeeB]

Comfort....its the reason i built my 8.5inch f7.5 ish dob , focal length 1613mm it puts the eyepiece at a really comfy height, its also superb on planets and very forgiving of both eyepieces and colli shift. But on the eq i use a 10"oo f4.8 ,as the 8.5 inch tube would be a nightmare with its length on an eq6 in any wind. (being an older 1974 thick mirror it also weighs a ton.

[the bishop]

hey thanks everyone

enjoyed this thread

its amazing just where some posts end up taking you

in this case from the stars in south wales to container ships from china via WA AUS

very insightful chaps

gra