90mm mak and .5 reducer

[doug mc]

Has anyone tried a .5 GSO 1.25 reducer, the one that screws into your eyepieces. I have a 90mm maksutov of Saxon origin and would like to use it at lower powers for terestial observing. The 32mm plossl gives 40x, it would be nice to go lower.

[Don Pensack]

It's a good idea. Most long focal length scopes can't get to low-enough powers for terrestrial observations.

It's a bad idea. As the magnification goes lower, the exit pupil grows larger. So, unfortunately does the size of the secondary shadow.

Let's take a hypothetical 90mm Maksutov with a 1350mm focal length and an f/ratio of f/15. A 32 Plossl would yield an exit pupil of 2.13mm, of which the secondary shadow would occupy the central 0.64mm. That works OK if your daytime pupil is at least 1.5mm (1.5-2mm is common).
But, cut the f/ratio to f/7.5 (a 50% reducer), and suddenly the exit pupil is 4.26mm and the secondary shadow occupies the central 1.28mm of that.
If you daytime pupil is 1.5-2mm, the shadow of the secondary will occupy most of the field and you will see the shadow covering the central 75-80% of the view, which would be unacceptable for daytime viewing.

The secondary shadow problem is why nearly all spotting scopes (daytime scopes) are refractors.

[Wavytone]

Sure you can insert focal reducers, or use huge eyepieces, but ultimately the field of view in a small Mak like yours is limited by the diameter of the hole in the back.

Nothing you do with eyepieces or reducers will change that fact.

If you want to go lower its better to buy a small 80mm refractor around f/7 that will give a really low power and still fil a wide field eyepiece.

Some years ago I bought a 102mm ed f/7 refractor to complement my f/15 Mak.

The lowest power on my Mak is 54X with a 2" 50mm eyepiece, the lowest useable power on the refractor is 102/6 mm = 17X, which corresponds to a 42mm eyepiece (a 2" vixen LVW 42, massive thing).

While it seemed like a good idea at the time as the focal length is 1/4 that of the Mak, these days the refractor doesn't get much use.

Ill suggest you really think carefully why you really want this; you might find it isn't much use.

[dannat (Daniel)]

The GSO reducer while ok for webcam work isn't high quality when used visually, I checked mine& the reducer intro aberrations

[doug mc]

Thanks guys, i will stay with the 32mm plossl for lowesr power.