How best to mount a dovetail bar?

Hi.

Potential dumb question alert.

I have a 4.5" newt that I want to put on an alt-az mount that uses a vixen dovetail system.
Also, I have an 8" dovetail bar with slots cut into it.

Clearly I have to drill the OTA, somewhere, but where (vis a vis balance etc)?
and do I have to tap the holes, or do I simply put a nut on the inside of the OTA and put a couple screws threw the dovetail bar.

In short, what's the right process? Thanks.
Tom.

Tom Lacovik said:

Hi.

Potential dumb question alert.

I have a 4.5" newt that I want to put on an alt-az mount that uses a vixen dovetail system.
Also, I have an 8" dovetail bar with slots cut into it.

Clearly I have to drill the OTA, somewhere, but where (vis a vis balance etc)?
and do I have to tap the holes, or do I simply put a nut on the inside of the OTA and put a couple screws threw the dovetail bar.

In short, what's the right process? Thanks.
Tom.

Tom:

In my opinion/experience, the best way to do this is to use tube rings rather than trying to mount the dovetail directly to the OTA. No holes to drill, and you don't have to worry about properly positioning the dovetail because you can adjust both the balance and the eyepiece position (rotation of the tube) any time you want to.

Rings also provide better support for the thin wall OTA, it spreads the load around the entire ring. If you do drill, it is best to have a matched curved surface on the dovetail and some sort of inner backing plate... Easier just to use rings.

Orion typically has the best deal on tube rings, these are under $20 and are the ones that fit the Starblast and short tube 4.5.

http://www.telescope.com/control/product/~category_id=mount_accessories/~pcategory=accessories/~product_id=07373

(ATWB doesn't seem to carry that particular diameter)

Also, I had a pair from a Meade 4501 OTA but I think I sent them to someone in a box of parts I was giving away, the fellow Astromarter may still have em...

Jon

I agree with Jon, that you want to get some rings. (I think they will cost more than he says, but they are the way to go.)

However, should you want to do it without, try the following:

Determine the center of balance. The easiest way to do this is to completely "populate" the tube. That is, put in an eyepiece, finder, Tel-rad, and all that goes on the tube when you are using it. Then place a dowel (or broomstick, pipe, whatever) on the ground. Place the loaded tube on top, and shove back and forth till you get it to balance on the dowel. The point it touches the dowel should be the center of the dovetail. This still give you some leeway should things change later in terms of what you load on the tube.

As for how to mount it, yes-- you need to drill some holes in the dovetail. Whether you tap them or not depends on you. But you will need to countersink if you do not tap them. A through bolt with the head countersunk below the level of the dovetail surface will work with the nuts on the inside. (Or, if you have tapped, you can put the bolt from the inside screwed into the dovetail. Remember to have bolts short enough so they do not stick past the dovetail. I would assume you would want a large washer on each bolt inside the tube also to spread the load. Then blacken the nuts/bolts/washers inside.

Again--rings are better in the long run.

Alex

I started out with a 4.5" reflector that came with the dovetail bolted to the tube. There was no problem with the bolts or nuts on the inside of the tube, but the fixed position of the focuser made looking through the eyepiece a pain at times. I bought tube rings quickly and they were much easier to use as you can adjust the focuser positon. Putting the eyepiece in a place that is comfortable to look through is very important and will definately increase your enjoyment of your telescope. Next get an adjustable viewing chair and you will be set.