spectro-aras.com

Olivier Thizy wrote:Rainer,

The input collimator is F/5. If you use a faster scope, you will loose light inside the spectrograph as light will go outside the collimator diameter.

But you can always use a slower scope such as F/6.3 or even F/10 scope. The disadvantage of doing so is that you will loose more light at the slit entrance. The exact loss would depend on your telescope diameter and your actual seeing. But it would certainly be smaller than using a faster than F/5 scope.

Cordialement,
Olivier Thizy
Vous ne verrez plus des étoiles comme avant !
http://www.shelyak.com/en/

Hi Olivier,

So can I then say that my f/8 AstroTech 8" telescope with the reducer down to f/5 is the best solution ?

Rainer,

it would depend on seeing but with a 8", I would personally leave it at F/8 with a 23µm slit and save the light loss within the focal reducer.

Cordialement,
Olivier Thizy
Vous ne verrez plus des étoiles comme avant !
http://www.shelyak.com/en/

Olivier Thizy wrote:Rainer,

it would depend on seeing but with a 8", I would personally leave it at F/8 with a 23µm slit and save the light loss within the focal reducer.

Cordialement,
Olivier Thizy
Vous ne verrez plus des étoiles comme avant !
http://www.shelyak.com/en/

Aha,

So I spent for nothing about US $ 500,00 for pushing down my f/8 to f/5 (and the reducer price is not included there ! because I had it already for something else) and now all that discussion to have a fast scope is a Soap Bubble. This I asked many times and always got the answer

.... Yes it is better to have f/5 ...

Rainer,


I do not think the answer is as straight as I mentionned above.

First, you do not want to go faster than F/5 so it is better to be at F/6 rather than F/4 for exemple.

Now, how do you compare F/5 and F/8... F/8 will end up with a lower beam within the LISA so you will shine less grooves on the grating, ending up in loosing some resolution as the resolving power of the spectrograph is actually based on the surface enlighten by your light beam. If I'm not mistaken, if you go faster than F/8, you regain resolution but when you reach F/5, you start loosing signal internally.

Also, at F/8, your star size will appear larger due to seeing. It depends of course on your telescope diamater (larger diameter is and more sensitive to the seeing it will be) and your actual seeing. But in a nutshell you will loose more light at the edge of the slit for the same slit size at F/8 rather than at F/5 for the same telescope diameter.

In summary, for two telescopes with the same number of optical miror/lenses but one at F/5 and the other one at F/8, best is to use F/5.

Now, you are adding up more lenses using a focal reducer to bring your F/8 to F/5. This is what you have to take into account in the light loss equation. Going F/5 this way reduces the signal level but keep the resolution of your F/5. So it is somehow a trade off you have to make and I would recommend to try both configuration on the same night to compare the results.


Cordialement,
Olivier Thizy
Vous ne verrez plus des étoiles comme avant !
http://www.shelyak.com/en/

AHA

Will do some tests in December when I have time ...