Posts Made By: Joe Larkin

There is no simple answer. I personally think that collimation isn't that big a deal. But I have a 10" f4.5 scope with fiberglass tube. It seems to hold collimation well. I usually just tweak the primary while observing a star at high power.

Of course, a big truss tube scope is a different animal. On those collimation may be pretty far off every time it is put together. Or it may just need tweaking of the primary.

So there is no one answer. If your scope needs to have the secondary collimated every time it is set up, it doesn't matter if it is f4.5 or f6. It will still be annoying every time. If you just need to tweak the primary, it will be fast and simple.

If the mechanical parts of the scope are such that collimation is very tricky, focal ratio might matter more, but you still will need to get used to the process at any focal ratio. I think the mechanical aspects are far more important than the difference between f5 and f6.

All MO, of course, and worth every penny you paid for this opinion.

Well used amateur made 16+" truss dob with John Hall mirror.

A bit of a fixer upper, picked it because it was local and very reasonably priced.

I'm modifying it to lighten it and make it easier to (dis)assemble and transport.

Attached is a picture. The new scope to the left of me.

I got a chance to see it and use it last night at Sentinel Arizona.

It is even more impressive in person. The real Surrier truss and long focal ratio really added to the unique look. It is almost impossing at night when sitting next to it.

The scope provided high contrast images. It looks very open but it better baffled than it appears.

Unfortunately, I had more time to look at it than through it as intermittent clouds made observing less fruitfull.

Joe Larkin

This must be a dumb question.

Where is the number?

I see a "message_id" in the URL, but that is much greater than 50000. I just don't see anything near 50000.

My experience is in overhauling a 16" truss dob that was about 20 years old. It was pretty typical for the time.

It had a large mirror box, small rings and an upper cage that was a bit heavy. It also had about 20 pounds of counterweights.

I think that some extra weight in the bottom won't hurt. The upper cage seems to be the place where weight really causes trouble.

With a light mirror I think that a light cell will mean you need to have the balance point rather far forward. This would result in a high rocker box and a large mirror box and greater overall weight. Or you will end up with a lot of counterweights.

Draw up (even by hand on paper) a fairly detailed plan of the tube assembly. Put in some measurements and weights. Then use a spreadsheet to calculate the balance point. I suspect that the balance point will be far forward of where you expect if you assume a very light mirror and cell.

Then vary the weights a bit to determine how the balance point moves around with changes in weight.

I think heroic efforts are justified at times to lighten the upper end. I think that too much focus on the lower end weight will just cause trouble.

This all assumes a typical "Kriege style" design. If you are building a single ring upper end, then you may want a very light cell.

Let us know how this all turns out.

Thanks for the info Paul.

I've heard a lot recently that just about any claim on home insurance can lead to cancelation or difficulty in renewing.

If true, does this mean that these additions to homeowner's policies will lead to likely loss of home coverage if they are actually used? Should these be used only if the alternative is a very big loss since it might not be worth the money/hassle of obtaining a new policy once a person is "tainted" by making a claim?

Don't do it. Grinding the back could very likely destroy the front figure.

A simple piece of glass is more complicated than it seems.

1) The thing about a lighter cell is that it may not make the scope or mirror box lighter.

The weight of a truss dob is dominated by one of the lightest pieces -- the upper cage.

Lightening the mirror box doesn't help if you need to add counterweights to make the scope balance or if you need a much taller rocker box to reach the balance point.

You need to consider the whole system. Create a spreadsheet and calculate weights and torques. You really need to balance to torques, not weights.

Once you understand this, then ask yourself if you want to make the cell lighter.

Of course, if you are going beyond the fairly conservative K&B design, then you will probably want to save weight everywhere. But with the "standard" K&B, the mirror box weight may not be the limiting factor.

I couldn't find this product on the web. Even googling for: "ebony marble" laminate didn't get good hits. So I am sort of speculating.

The important thing about ebony star is that it is a "pebbled" finish. It isn't smooth. That is what makes it work well. But for a not so big dob, almost anything should be okay, so I would suspect it would be fine.

IMO, using real teflon is essential, tuning the friction by moving the pads is essential, while using ebony star is nice for moderate size and weight dobs.

For larger dobs, there are better materials.

One scope for both those mirrors will be optimal for neither. And it will likely be more complex and expensive too.

I would recommend making a simple sonotube dob with the 8" scope. Use cheap plywood and make it simple and cheap. An 8" is small enough that you don't need heroic measures to make it light, rigid and usable. Look at the inexpensive Chinese/Taiwanese small dobs for inspiration.

I bet when all is said and done, two scopes will be as cheap as one "superscope". And you will be observing quicker too. And you will end up with two scopes for different purposes.

All IMHO, of course.

Joe Larkin