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Pentax PF80 looooong distance recomendations (1 Viewer)


In case it helps anyone, I've tested the Powermate 2.5x with the PF80ED-A and it works just fine!

The chrome barrel doesn't go all the way in, but still I'm able to reach focus in all focal lengths using the Powermate with the smc PENTAX Zoom XL 8-24mm.


Well-known member

thanks for sharing your results. Could you comment on what is the usable maximum magnification of this combination - or is it good up to 160x (I would be impressed).



Well, light loss is noticeable, but I'm still surprised that it works!

When the zoom is at 8mm with the powermate it gives around 161x, and you can see eye-floaters, but it's not a surprise since the exit pupil in that case is less than 0.5mm. As it can be read in Televue's eye piece calculator:

An exit pupil greater than 7.5-mm may be too big for a telescope with central obstruction (most reflectors). Exit pupils of less than 0.4-mm are small enough to allow eye-floaters to interfere and also indicates an eyepiece with too high a power for useful viewing.

I find that when setting the zoom to higher values than 8mm (9, 10, 12) the scope is pretty usable, but I guess it depends on many factors (the individual unit, atmospheric conditions, eyesight, etc), so I can't recommend with 100% confidence to buy the powermate.

For instance, the other day I looked at the moon and was surprised to see how good it looked even at 161x!


Well-known member

thanks for the impressions - it seems you have got a very nice example - congratulations.
And yes, you're right - that does not means tha another example will perform the same - not even the same one with different seeing or another observer (like some person with exceptional eyesight who gets beyond the resolution limit of the 80mm objective at that magnification).



Well-known member
Regarding reading number plates at 5 miles.
This type of observation was easy with my 120mm refractor, but at 3 a.m. on good nights.
A clock tower clock about 5 ft diameter. But the clock was lit from behind.
The minute ticks were maybe an inch wide and 2 inches high or smaller.

My 6 inch Maksutov might have also done this on really steady days in September, but not in England.
Islands 5 mile distance were amazingly clear. There seemed to be no haze reduction at all at sea level. The air in Britain is too polluted for this.

The 70 inch aperture Big eye satellites were able to resolve better than 4 inches at 200 to 250 miles. Maybe using adaptive optics.
A 6 inch Maksutov type optics could lock on the specular reflection from a car door handle at 90 miles so I was told by someone involved with this type of optics.

Aircraft at 80,000 ft can photograph golf balls on the ground.

The magnification on the 120mm f/8.3 refractor was 250x. 330x was tried but no gain.
The 6 inch Maksutov was used at 95x, but I would need to increase this to 150x or 200x.

I have read a hotel name sign at 11 miles with a Japan Celestron 20x80 but cannot remember how large the letters were.

With car number plate letters some are difficult, some easy, depending on crowding.
The light levels are critical for long distance viewing

Horace Dall showed me photos he took with his 110mm aperture f/30 camera obscura objective corrected for 4 colours.
Some were of church gutters 17 miles away, which were clearly visible with detail.

Also photos of Mercury with an equivalent focal length of 1,100 metres taken with his 16 inch Dall Kirkham. He used his atmospheric dispersion corrector, which I think he invented.

Seeing the daytime scene in his darkened attic on the large round white table presented by his camera obscura in beautiful colour was an experience like no other. There was a table microscope to show very fine detail.

The views of the planet Mars at 400x with his own made 8 inch Maksutov through his specially selected and ground attic plate glass window, was another experience that had to be seen to be believed.

I think that an 80mm Pentax spotting scope may be a little small to read number plates at 5 miles.
Someone I think described seeing 0.22 inch holes at 1 mile with a 100 Pentax spotting scope.
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Well-known member
Actually for long distance work a classic 5 inch or 6 inch f/15 or f/16 refractor would be best.
A heavy duty mount is needed.
An observatory quality instrument if possible.
The main problem is temperature gradient and local Seeing conditions. Early morning and late evening are usually best, with the Sun behind you.

Sticking with Pentax, a good example of the 100mm f/12 would be a smaller option. This can take high magnification say 250x at least at 3 a.m. but in daylight maybe 200x in the best conditions. My one was very good indeed.
A Russian Tal 100mm f/10 has good reports.

For a present day option I would choose a Skywatcher 120mm or 150mm refractor with as long a focal length as possible. The simple doublets are the cheap option.
There are older 6 inch f/10 refractors also. D and G I think.
A 120mm or 150mm ED doublet or ED triplet Skywatcher would be good. These are £1,000 to £4,000 for the Optical tube assembly and more with a mount. A really heavy duty mount is needed.
I used an ex. gov tripod able to take an adult's weight easily.

For me 5 miles was the usual limit to give good views.

For 20/15 vision I think that 7 miles is the furthest one would have a chance at reading a car number plate in optimum conditions with a fine 120mm aperture refractor.
I used the refractors without any diagonal or prism. Views are upside down, which is not a problem for an astronomer.
A 1/10th wave or better star diagonal would be the next option giving a laterally reversed image. I would avoid prisms, although one could try a Televue or Williams.

Long focus refractors are not only better because of optical considerations, they also are less affected by tube currents and changing temperatures than short focus refractors. Maybe the shallower curves on the objectives also help.

Another choice is a 6inch or 7inch Maksutov left outside for an hour before observing to stabilise the temperature. Either Skywatcher, Russian or a Quantum 6inch or Questar 7 inch. Again a very heavy mount is needed.
My 5 inch SCT was not good as there are temperature effects. Same with the 8 inch, but on occasion they were good.
The 12.5 inch Dall Kirkham would make easy work of a number plate at 5 miles in optimum conditions.
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