Always with The Questions

[rabbithutch]

Because I haven't yet purchased a scope (but leaning toward an 8" dob), there is a great deal that I don't know. For starters, I really don't have a feel for the size and weight of an 8" vs a 10", for example, though I've read the posts and product descriptions.

From what I've read, Orion and Skywatcher are the main suppliers of dobs. The posts seem to indicate that they are roughly equivalent in terms of optics (though if there are opinions to the contrary, I would like to hear them) and that accessories are basically interchangeable (again, educate me if I'm wrong).

That takes my thinking then to mounts.

Reading other threads here, I get bits and pieces of descriptions of the parts of the dob mount and understand little about them. It seems to me that the sheer size of the OTA (outer tube assembly, right?) and the added weight of mirrors, spiders, ep's, et al would make fixing the whole so that it doesn't shift in use a particular problem to overcome. Continuing with the inference, I surmise that high friction surfaces that can be adequately engaged are required. From sales pics, I see knobs for this. Then, I see adverts for Bob's Knobs which I think are aftermarket improvements sold to improve the friction holding capabilities.

Because the dob must move for azimuth and altitude, I thought that there must be differences in the way the friction devices engage - assuming that holding altitude is more difficult than holding azimuth. Close or not? I wondered, too, if the 10" size increase means that these features require different solutions than for an 8".

I wonder if I could make so bold as to ask those with Orion, Skywatcher, or other dobs to discuss the bits and pieces of their mounts that do the holding and maybe even post close up pics of the bits that do the job.

I would like to know the pros and cons of the way Orions vs Skywatcher vs any other makes perform wrt to these functions as they seem that they might make a critical difference in performance. Even though I'm unaware of a source for Skywatcher in the States, I'd like to know if it is superior to the Orion in this area or any other.

Thank you for your patience. I'm trying to pick the brains of those who are far more experienced than I. Maybe someday, I'll be able to help others.

[MikeP]

I can answer some of your questions ....

My most used scope is a 12" SkyWatcher FlexTube - Dobsonians - Skywatcher Skyliner 300P FlexTube Dobsonian. I'm 61 and not particularly big but when I separate the OTA from the mount, I can still carry them fairly easily.

If you look carefully at the picture, you can see a double thickness of board at the bottom - between them are a couple of thin steel plates and some bearings allowing the upper section to rotate in azimuth about the lower board which sits on small feet on the ground. I removed the feet and added three lockable castors so that I can roll the whole thing around - useful for viewing when objects disappear behind trees, but otherwise it is as you see in the picture.

Referring back the picture, you'll see a short black handle protruding (there is another on the other side). The OTA has two circular plastic discs attached to its sides and to put the OTA on the mount you simply lift it onto 4 (2 each side) plastic studs so that it is free to "pedulum". The black handles then screw into the centre of the plastic disks through the sides of the mount. This stops the OTA falling off and allows the OTA to be held at whichever altitude angle is required. One of the handles incorporates some bearings, so by tightening the handles carefully, the OTA will stay in place, yet can still be moved in altitude quite smoothly.

As you'd guess, the pivot point is not a million miles away from the centre of gravity, so as long as not too much weight is added to the upper assembly, it doesn't require the handles to be tightened too much. I have a right angle finder and a Rigel QuikFinder - Finders - Rigel QuikFinder Compact Reflex Sight on mine and I've also replaced the supplied focuser with a Moonlite. Even with a stonking big eyepiece there is no problem with balance. If anything it is still bottom heavy.

If you get a FlexTube or some form of truss tube, a shroud is a good investment to keep light out - over here it also helps stop the secondary from dewing up but that shouldn't be a problem in TX.

Hope that has answered some of your questions.

Mike

[NGC 1502]

Hi Rabithutch, I'll try to answer some of your questions as best I can.

8" and 10" scopes are not so very different. The OTA ( optical tube assembly )

of the two is usually about the same length 44/46" as they are very often the

same focal length, 1200mm. (sorry to mix imperial and metric) Typically an 8"

OTA is about 10" dia, a 10" about 12" dia, weight about 20 lbs and 26 lbs. Thats

the tube only of course. The 10 has about 50% approx greater light grasp.

Many of the "Asian Dobs" will come from the same factories, but have a different

name / paint job. Most are very good optically, especially given their competitive

pricing. I do realise that the costs involved are quite a hit for many.

Standard 1.25" and 2" fitting eyepieces will fit almost any scope in todays market.

Mounts : I assume you are thinking of Dob mounts ? Most are plastic coated

chipboard with low friction teflon bearings, some have a "lazy susan" bearing for

the azimuth ( side to side ) movement. Ones I've tried I liked.

Bob's Knobs are retro fit to make collimation ( aligning the mirrors ) easier.

Hope this is of some help to you, please come back to ask more if you need.

Best regards, Ed.

[earth titan]

Whichever you pick of these, you will end up with an excellent scope. Don't discount Meade, who make the lightbridges in all the sizes you have mentioned (sorry to have just added another into the mix for consideration) but if I had the money I would go for the Orion as the optics are generally considered better.

Bobs Knobs are retrofitted to simplify collimation. There are sets for the secondary and the primary and having fitted them to my scope, I can report they are easy to fit and save you needing tools in the dark.

[ollypenrice]

The reason Dobs work is down to balance. The bearings need to have static and moving friction as similar as possible so they don't start with a jerk as you push them, and the axes should have similar friction. When the friction is properly weighted all round and the balance is close, they will move sweetly. It is worth reading a book called The Dobsonian Telescope by David Kreig (of Obsession fame).

Olly

[rabbithutch]

Thank you, All!

From your replies, I take it that the mounts are about equal among the brands (Orion, Skywatcher, Meade). I noted the information about Orion optics. The reason that I did not mention Meade in my op is that I've read threads complaining about Meade having poorly made, plastic mounts on some of their scopes (but it was not referring to dobs).

Thanks for the tip on the book, Olly. I just ordered a copy.