shortening a refractor tube for imaging?

I have an Explore Scientific 80mm apo, with which a Vixen NLVW 30mm eyepiece will not reach focus with the diagonal attached. It seems the tube should be shortened, so I seek advice on how much to shorten for convenient imaging.

The OTA has a 480mm focal length. The very well made dual speed focuser has a draw of 100mm. By holding the tube in the direction of the setting sun, I found that the focal point appears to be almost exactly at the end of the extended drawtube. This seems a mistake, as there is no useful optical device that would need a focus at the actual end of the drawtube.

Furthermore, with the 2" diagonal, all my eyepieces come to focus very near the in travel limit: a 42mm Plossl focuses at 30mm extension, while the older Vixen LVW types focus at 10mm. The NLVW, comes almost to focus at the inward limit of the focuser.

So, I could have it shortened. How much should it be shortened for optimal use as an astrophotography instrument?

Robert:

This is the way I look at it:

Typically for astrophotography one does not use a diagonal so you should be able to reach focus with a camera attached directly to the drawtube without shortening the tube.

Jon

Robert-- Let me clarify what you are saying....

>>>a Vixen NLVW 30mm eyepiece will not reach focus with the diagonal attached

This is a problem, I agree. I take it the focus tube is all the way in, and the eyepiece will still not focus. And I assume you are trying this at infinity. Did you notice if the end of the tube can be screwed off and allow you to attach the diagonal directly (without the two inch adapter)?

>>> the focal point appears to be almost exactly at the end of the extended drawtube.

I don't see this as a problem as long as when you put in the diagonal you can get the other eyepieces to focus--and from your next paragraph, it seems you can.

Before asking how much to cut off to make it ideal for imaging, you should probably remove the diagonal, put in the camera and all the things you plan to use with it (auxiliary focuser?? field flattener/corrector??, t-ring adapters, special diagonals for solar viewing, whatever) etc, and see if it will come to focus at infinity (not on a daytime object--make sure it is infinity.) Does it come to focus? Then don't cut anything off to make it good for imaging. As Jon pointed out, one does not use a diagonal for imaging (except for special purpose diagonals like Herschel wedges, Lunt, etc.). If it does not, then come on back and tell us what you have found out. I suspect it will come to focus if you have determined the focus is just past the end of the extended tube. (Cameras typically need 30-60 mm in from the front of the camera to the imaging plane.)

If you still want to cut off some to make your NLVW work, then it is up to you to decide how much.

You do want to keep the focuser tube as short (in extension from the main tube) as possible for imaging. The less you have hanging off back there the better.

Alex

If you only need a couple mm more back focus,
one fix is to insert 48mm filters into
the light path. Typically one filter will
gain about 0.7mm backfocus. Using clear (or UV)
filters with multi-coating will have very little
impact on brightness. You can thread them
on to the diagonal (preferred) or the eyepiece.

Another fix is to find a prism diagonal instead
of a mirror diagonal. That will gain about 0.7"
of back focus, but will also add some (probably
minor) spherical and chromatic aberration.

Robert Morein said:

I have an Explore Scientific 80mm apo, with which a Vixen NLVW 30mm eyepiece will not reach focus with the diagonal attached. It seems the tube should be shortened, so I seek advice on how much to shorten for convenient imaging.

The OTA has a 480mm focal length. The very well made dual speed focuser has a draw of 100mm. By holding the tube in the direction of the setting sun, I found that the focal point appears to be almost exactly at the end of the extended drawtube. This seems a mistake, as there is no useful optical device that would need a focus at the actual end of the drawtube.

Furthermore, with the 2" diagonal, all my eyepieces come to focus very near the in travel limit: a 42mm Plossl focuses at 30mm extension, while the older Vixen LVW types focus at 10mm. The NLVW, comes almost to focus at the inward limit of the focuser.

So, I could have it shortened. How much should it be shortened for optimal use as an astrophotography instrument?

I do not use a diagonal when I image with a WO66.

Even though a diagonal is almost always necesary for visual (except with binovoewers).

Take away the diagonal, add extensions....please don't cut. Unless you do not care about re-sale value.

just my $.02

Have you taken the subject up with Scotty? He is about the nicest person in the world, and would not want one of his scopes to be less than satisfactory.

Maybe he knows the problem and how to fix.

Alex