Whats the best all round telescope currently?

[gregbradley]

Originally Posted by Alchemy

RE ......AP RHA

Given F3.8 is the ratio of FL to width, which in refractors does give some comparison, but given the central obstruction and the interest in light gathering capability..... What is the central obstruction diameter and therefore the percentage variation in collecting ability in comparison to a refractor of equal dimensions and FL .....

Just trying to get an idea of its photon gathering capability, might be similar to F5 in a refractor.

I've done those types of calculations before. Because the radius is squared and multiplied by Pi 3.142 you get a rapid increase in area when you go up in aperture.

So say compare the RHA at 305mm and 150mm secondary at F3.8:

http://www.astro-physics.com/index.h...ducts/products

to FSQ106ED F5

You get 55,397 mm2 area in the RHA after subtracting the secondary area compared to
FSQ106 8825mm2.

When you consider F stops represent a doubling of light you get F stops:

F2.8 F4 F5.6

F3.8 to F5 then looks to be around 1 F stop = doubling of light.

So 55,397 of the RHA divided by 8825 of the FSQ = 6.2 times.

Now with the F stops I think you double this = 12.4 times the light gathering ability of an FSQ106.

Then factor in various CCDs and their different QE's although most are between 50% and 77% unless you use one shot colour and that goes down to 25%. The Sony 694, 814 and the new 12mp sensors are all 77% QE with 5 electrons or better read ( 3 electrons with the 12mp as smaller pixels means lower read noise).

A 180mm APO is 25,420 mm2 so that is about 45% of the RHA but again F7 versus F3.8 is now closer to 2 stops.

So the maths is very significant. You can't beat aperture and you can't beat fast F ratios when time is limited.

I must say I am impressed by the quality of many of the 8300, 694, 814 sensored camera images taken on small high quality APOs. There are many of these images on this site.

A small CCD on a small aperture with good optics is still an excellent setup and a way to get detail on smaller object.

Greg.

[Alchemy (Clive)]

Originally Posted by gregbradley

You get 55,397 mm2 area in the RHA after subtracting the secondary area

A 180mm APO is 25,420 mm2 so that is about 45% of the RHA

Greg.

Hard to argue raw numbers, plus 1159 mm focal length still gives you good imaging for most nights.
Must say I definitely like The idea of the reflective surface on the rear of the glass, although the effects of the glass needed to be factored into the optical train ( how thick is rear element ) , the idea of a surface that never needs recoating is brilliant, plus never needing collimating also brilliant. Hard to find a problem with it so far. At a shade over 30 kg it will need a solid mount, plus might take a while for thermal equilibrium, although specs say it barely shifts due to temperature.

Is it the perfect scope for imaging ...... Not if you're on a mountain top in the desert, but for everyday use, it's got a lot going for it.

Look forward to the results.

[Stefan Buda]

Originally Posted by clive milne

F4.38 or there abouts.

Spot on Clive! Half a stop extra for 50% central obstruction.