Are All OTAs Created Equal?

[rabbithutch]

I've been shopping around for an 8" dob. It seems that they have been in the market for a number of years and each maker seems to have come out with new and improved models over time. Prices on older models seem to have fallen, when the new models came along.

Frankly, I can't tell much difference from one model to the next. Some seem to have boasted of their OTAs being made from steel. Others don't seem to mention the material theirs are made from.

All of which leads to my question, do the different makers use different materials for their OTAs? Have they changed materials over the years? What are the advantages and disadvantages of different materials for an OTA?

It seems to me that steel would have certain advantages for strength and durability yet might have disadvantages, too (heavier, more difficult to modify, . . .).

Would really appreciate all information about materials. Also would like to hear from dob owners as to what material their scopes OTAs are made from and how they like the material.

TIA

[ollypenrice]

Following on from the original ingenuity of John Dobson many earlier Dobs have cardboard tubes originally intended for moulding concrete columns. They are better than you might thnk but suffer from long cooldown times.

I don't think anyone is doing these any more.

Bigger dobs tend to be open truss for practicality and cooldown. As for steel, I can't see anything wrong with it though I think it should be painted black to radiate away heat despite absorbing more in the daytime. This is the thinking of the makers of the best Newt I have ever used, Ralf Ottow and Bertus Van Gemeren.

Olly

[Capricorn]

Suspect that you are asking about the newtonian scope and not the dobsonian mount. Everyone does.

Some will use lower quality components and materials then another. Steel may sound fine but ignoring the weight steel expands and contracts by significant amounts. Could I suppose mean collimating a newtonian every hour if outside in changing temperatures.

The mount may be decent plywood, cheaper MDF, or some plastic composite. The mount bearing is probably best if PTFE but I guess a lot will be a plastic substitute.

Then comes the bit that holds the secondary in place. Many factors involved here. Simple ease of use and stability is probably a big one and that may not depend greatly on materials, but durability will.

The focusser, lots of options, at a range of prices and qualities.

So I would say no, not all OTS's are created equally, even if they started off with the same mirror.

Same holds for refractors, is a 100mm F/7 refractor with FPL-53 equal to a 100mm F/7 with FPL-51. Both will be ED and APO.

[NGC 1502]

Personally I woudn't worry too much about tube materials. My previous scopes

had steel tubes, and had no issues. My current 10" has an aluminium tube.

( thats aluminum in US I think ! ) It was purchased used, and I only found out

it wasn't steel when I tried to attach a magnetic counterweight ! I suppose that

aluminium sounds better, and doesn't rust, but really it doesn't bother me either

way. Some early model Celestron "starhopper" Dobs had cardboard tubes. If a

used one came up, in good condition, the cardboard wouldn't put me off.

HTH, Ed.

[dweller25]

I had the basic 8" F/6 aluminum solid tubed Skywatcher/Orion dob and it was superb. I did flock the insides and put a cooling fan on but it cost very little and turned out some great planetary views.

They are easy to modify with with very basic tools and give the best bang for the buck going.

I have no issues with the tube material at all.

[russ]

Following on from the original ingenuity of John Dobson many earlier Dobs have cardboard tubes originally intended for moulding concrete columns. They are better than you might thnk but suffer from long cooldown times.

I don't think anyone is doing these any more.

Olly

My Meade Starfinder was a Sono tube. Wife said it looked like we had a giant toilet roll on the lawn.

[rabbithutch]

Thank you for the responses!

Yes, my question was intended to be about the OTA on Newtonian scopes. I only mentioned a dob because that is what I intend to get. I have obsessed over details regarding the mount and it's components but had not given much thought to the materials used for the optical tube assembly (OTA). It was only after reading the technical specs for one (don't remember which) stating that it was steel that I really gave it much thought. Aluminum (aluminium) seems a reasonable choice of material except I was unsure how it might take tapping for accessory mounting bolts (although, that probably is not much of an issue very often). I would think that cardboard and plexiglas tubes might present similar issues but, again, might not be significant.

Again, thank you for the responses.

-rh

[NGC 1502]

Hi rh, whatever tube material you have, any holes you may drill for

attaching accessories, I think it's best to use large washers inside

& outside the tube to reinforce the thin material. They are usually

called "repair washers" in UK ( fender washers in US ? )

It's best to remove the primary before drilling, you can get away with

leaving the secondary & spider in situ, by protecting the mirror with

something soft like a childs sock.

HTH, Ed.

[rabbithutch]

Thank you, Ed!

That is good to know. Yes, we call those large washers 'fender washers'.

Does this mean,then, that most things are attached with through bolts and not using holes in the OTA that are then tapped for threads?

Thank you, again, for the information.

-rh

[NGC 1502]

Hi rh. The tube wall, whatever it's made from, is way too thin to take a tapped

hole. Self tapping screws would work, but not using a tap to cut a thread.

Other options include -

Drill right through, use bolts plus washers.

Sticky pads ( the Telrad can be attached with sticky foam strips )

Magnets, for counterweights, you would need a steel tube, or with an aluminium

tube like mine, attach a steel washer to the tube first. But doing that means you

are restricted where to attach the counterweight. ( Or use a steel strip, to vary the

attachment point, or a sliding c/weight )

There are other options, but these spring to mind.

Best regards, Ed.

[rabbithutch]

Thank you, again, Ed.

As you can tell, I don't have hands-on access to scopes of the type I'm considering; but I'm trying to learn as much as I can because I will probably purchase a scope online. I don't generally like surprises - much less disappointments.

-rh