Posts Made By: Robert Morein

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?

In contrast to RC and CAT designs, refractors appear to have a problem with very long imaging trains, incorporating:

a. A/O assembly
b filter wheel
c camera
d OAG or VanSlyke slider

The sticking point is the requirement for the field flattener to be placed at a very precise distance from the image plane. It can't simply be placed at the front of the imaging train, and these flattener cells typically have threading special to the telescope.

I've been looking at the very newest Petzval derived refractor designs, hoping that one would present a solution. Petzval designs have a flatter field to begin with, possibly negating the need for a flattener. However, in consultation with one importer, the field for his design is acceptably flat up to about 14mm, when a flattener is not used. This precludes the use of the new, large sensors.

I would be very interested to hear of solutions I have overlooked.