F-stop and central obstruction

[KTachyon]

I was looking at the specs for the RASA 8” and I was a little bit confused about what Celestron actually has in their sheet.

So, the RASA 8” is a 203mm aperture with a 93mm central obstruction (ignoring camera cables), which they only state it is 46% of the aperture diameter. But does that 46% even matter at all?

I mean, if we calculate the area of aperture and subtract the area of the obstruction it will be about 21% (a far better number for marketing purposes).

Doing the math, this means that the aperture without the obstruction would be equivalent to a 180mm non-obstructed aperture scope and, therefore, a F/2.2, not a F/2 scope.

Isn’t this correct?

[vlaiv]

Central obstruction is stated as percentage of aperture diameter rather than surface area by convention.

It is much more important to visual telescope use at high power than anything else (modulation transfer function). In case of RASA, being photographic instrument - it is rather unimportant parameter. It might be helpful to know central obstruction when doing some calculations, but as you pointed out, even moderate change in central obstruction by diameter turns into rather small change in effective aperture. Just for comparison, weather conditions on a given night will have larger impact then central obstruction for light gathering. Transparency difference of only 0.1 AOD (for example between 0.2 and 0.3) will be equal to loss of 10% of light - that is how much light loss you get from 31% central obstruction (by diameter) in comparison to unobstructed scope.

Change in F/stop also depends on your use of F/stop. F/ratio is not changed, so whenever you use F/ratio to do something (like define ratio focal length and aperture relation or define angle of beam or whatever) - it stays F/2.

F/stop on the other hand has rather limited use with telescopes - it sort of represents "speed" of instrument, but it really has no meaning until you pair it with certain sensor for recording. It is usually counter productive to think about speed of telescope alone in terms of time needed to reach certain SNR.

Btw, same logic that you applied with central obstruction to get F/2.2 instead of F/2 can be applied to other aspects of instrument. In case of RASA - you should account for transmission of corrector plate, and corrector in front of sensor (about 99.5% for each air/glass surface - two for corrector plate, and about 6 for corrector if it is 3 element piece) and reflectivity of primary mirror that is about 96-97%.

[joe aguiar]

I look at it like theres always compromises in the type of scope. I kinda like the sct, nothing beats its af far as being compackness BUt sure it will have CO so contrast goes down a bit BUt then again theres a huge difference in a 8sct vs a 8" refractor. After the 6: refractor size its only a obsevertory class scope.

as far this yes "Doing the math, this means that the aperture without the obstruction would be equivalent to a 180mm non-obstructed aperture scope " but contrast will be bit better in a scope with no central O since all the light is too a point where any scope with a CO some light gets put into the airy disk.

I say have a scope for each occasion 😛

joejaguar

[andrew s]

F number can be used in different ways. As discussed above for exposure speed but also the maximum steepness of the light rays for matching accessories like filters, correctors and spectroscope and for calculating effective focal length for image scale.

Regards Andrew.