Sampling rate and focal reducer queries

[choochoo_baloo]

Dear All,

 

I am a good way into Making very photon count. Now being fairly confident with the theory behind the different pieces of equipment, I wish to ask:

 

since sampling rate = (206.265*pixel size)/focal length  [in microns and mm respectively],       

am I correct in saying that sampling rate is proportional to resolution, or is a lower arc second per pixel value better and getting a sharper/higher resolution image? I cannot conceptually decide which way round is better. 

I gather from the book that one should aim for a sampling rate of between 1 - 3 " per pixel? So a similar question is which end of this range one should try to be closer to, for a higher resolution image?

 

 

 

Also could someone confirm the focal ratio to exposure time relationship for me please; I understand it to be:

 

(assuming f/new < f/old, for a faster 'scope modification) :              ((f/new)/(f/old))^2 

 

e.g.  for the Celestron C8 SCT;          (6.3/10)^2 = 40% of previous exposure time needed?

 

 

 

Thanks in advance.

[Leveye]

You're really taking the high road here. Coming from an artistic background i could point you in the right direction but would need to know just what type of pictures your looking to capture in the end. Are you doing research or  Astrophotography? PM me if you'd like.

[lensman57]

Dear All,

 

I am a good way into Making very photon count. Now being fairly confident with the theory behind the different pieces of equipment, I wish to ask:

 

since sampling rate = (206.265*pixel size)/focal length  [in microns and mm respectively],       

am I correct in saying that sampling rate is proportional to resolution, or is a lower arc second per pixel value better and getting a sharper/higher resolution image? I cannot conceptually decide which way round is better. 

I gather from the book that one should aim for a sampling rate of between 1 - 3 " per pixel? So a similar question is which end of this range one should try to be closer to, for a higher resolution image?

 

 

 

Also could someone confirm the focal ratio to exposure time relationship for me please; I understand it to be:

 

(assuming f/new < f/old, for a faster 'scope modification) :              ((f/new)/(f/old))^2 

 

e.g.  for the Celestron C8 SCT;          (6.3/10)^2 = 40% of previous exposure time needed?

 

 

 

Thanks in advance.

Hi Mathew,

I don't think that you need to get bugged down with sampling rate and all that, so long as you are not under sampling by a large margin you should be ok. This rate in practice is also very much seeing dependant, if seeing is not good all the theoretical resolution is just hot air. As for the other question what u have to understand is that an FF/FR will not increase the amount of photons reaching the sensor through 8" of aperture, it will give u a wider FOV for sure but it can not increase the photons. I know it is difficult to grasp it but if you had two 8" SCTs , one a native F8 and the other a native F4 then the later will be 4 times as fast so you could decrease the exposure accordingly but by employing a reducer the scope will not become faster as such it just gets a wider FOV.

Regards,

A.G

[DoctorD]

Hi Lensman57

Sorry, I disagree with your statement:

"I know it is difficult to grasp it but if you had two 8" SCTs , one a native F8 and the other a native F4 then the later will be 4 times as fast so you could decrease the exposure accordingly but by employing a reducer the scope will not become faster as such it just gets a wider FOV."

An 8" F8 scope with a 0.5 focal reducer will place the same number of photons on a given sensor as a native 8" F4 scope - the focal lengths will be the same and the FoV will be the same.

There may be some loss due to less than optimal transmission in the focal reducer but to all intents and purposes it will give the same image.

I agree that a focal reducer cannot increase the number of photon captured by a scope, however the focal length of the scope will determine how many of these fall on the chip.

Regards

Paul

[sharkmelley]

An 8" F8 scope with a 0.5 focal reducer will place the same number of photons on a given sensor as a native 8" F4 scope - the focal lengths will be the same and the FoV will be the same.

That's right.  Indeed manufacturers will sometimes sell a reducer quoting the focal ratio that will be achieved.  For the Meade F10 SCT you can buy a F6.3 or F3.3 reducer.  These indicate the focal ratio you can expect to achieve with the reducer attached.

Mark