Low Profile Focuser

[barx1963]

Am looking at seetting up by little Newtonian for imaging. Have confirmed that I cannot get focus with my DSLR, feel like about 1 more turn of foicus knob would do it.
Would a low profiule focuser do the trick, suich as the one Bintel sells under their own brand name.
And is there a way of determining if it can get in far enough to achieve focus?
Any thoughts anyone?
The scope is 130mm with 900mm FL if that helps.

[ausastronomer]

Hi,

It's unlikely this modification will give you enough additional in travel.

The easiest way to modify your scope for imaging purposes is to remove the primary mirror cell and shorten the tube by about 35mm. This should allow the scope to reach focus with your DSLR camera attached to the focuser, as is. When you plan to use the scope for visual observing you just insert a standard extension tube into the focuser if you have insufficient out travel to reach focus with any eyepiece. Try all your eyepieces before you buy the extension tube.

In some cases, depending on your scope, the existing secondary size and other parameters, the secondary mirror may vignette slightly. However, the Synta scopes as they come off the rack generally have an "oversized" secondary if anything and this shouldn't be a problem.

In all probability shortening the tube a tad shouldn't cost anything, except for an extension tube, if you need one.

Cheers,
John B

[mldee]

I have heard some folks say rather than cut the OTA tube, a simpler way is to replace the mirror hold-down screws with longer ones, then move it further up the tube and re-collimate after seeing if you can gain focus. I've never done this myself, but it seems a more elegant way than attacking the tube with a saw, as you can at least go back if you're not happy.

Would like to hear results if you proceed.

[ausastronomer]

That physically might allow you to reach focus but it will not do anywhere near as good a job. If the screws are too long and the mirror is supported too far out from the base of the cell, it is not properly supported and the entire mirror cell assembly will be more prone to vibration and wobble.

Who said anything about "attacking the tube with a saw". A "1/2 decent" handyman could dock the tube and no one would ever know that it had been shortened, unless you measured it.

If you plan on "attacking the tube with a saw", you probably don't possess the tools, or the skills, to do a decent job of it and I would be asking someone that does, to do the job.

Hint: An angle grinder with a thin cutoff blade, a dremel with a fine cutting wheel, or a fine tooth metal blade in a jigsaw, then cleaned up with a file, will all do a very neat job, "if you have the patience" to mark the tube out properly before cutting and go slowly and neatly. It is important to go slowly and neatly and let the tube cool so that you do not "burn" the steel. A dremel will be pretty slow going but can do a neat job if you have the patience.

Cheers,
John B

[dannat]

Malcolm - bintel are great to deal with - if you take the OTA in minus the focuser - they will let you try out their low profile one with your camera i imagine. The longer bolts at the mirror end i think is good if you only need a small distance like 5mm-10mm..once you get more than this you will have to cut the tube
Keep in mind for visual you will then need an extension tube to reach focus with your eyepieces, usually the 35mm one is sold.

Lastly your scope being 900/130 gives f7, which is getting to the slower end of newts for imaging - most use an f5 - the other thing a lot of 130-150mm OTA now come with a focuser which has a camera t-ring attachment, and a screw on focuser which naturally sits higher so no ext tube is required.