f ratio

[Stub Mandrel]

 f-ratio relates to the amount of light collected when all else is equal. The fact is, however, that in astrophotography the times when all else is equal are few and far between, so rare that probably the only way to legitimately compare f-ratios would be to use an aperture mask to stop down a scope whilst not changing anything else.

[ollypenrice]

18 minutes ago, Stub Mandrel said:

 f-ratio relates to the amount of light collected when all else is equal. The fact is, however, that in astrophotography the times when all else is equal are few and far between, so rare that probably the only way to legitimately compare f-ratios would be to use an aperture mask to stop down a scope whilst not changing anything else.

I'd have thought that the key thing you mustn't change is focal length. As long as you pin focal length and vary only the aperture you can keep the F ratio exposure rule intact. Exposure time will go as the square of the F ratio. It's when you vary focal length, or focal length and aperture, that you paddle off a cliff!

Olly

[RichLD]

@ollypenrice I recall seeing an excellent diagram you had regarding this - I've had a quick search on SGL but can't find it. Have I imagined it? It's definitely possible that I have...

Anyhow, if it is indeed real, would you mind either pointing me in its general direction or reposting please?

No rush, as I'm sure you're a busy man!

All the best

Rich

[ollypenrice]

1 minute ago, RichLD said:

@ollypenrice I recall seeing an excellent diagram you had regarding this - I've had a quick search on SGL but can't find it. Have I imagined it? It's definitely possible that I have...

Anyhow, if it is indeed real, would you mind either pointing me in its general direction or reposting please?

No rush, as I'm sure you're a busy man!

All the best

Rich

Busy watching the Tour de France and avoiding the weeding. That's busy!

The middle setup is the baseline. On the left we add aperture, on the right we subtract focal length. These are not equivalent actions even though they both produce F3.5.

As Craig Stark points out, an imager unable to expose for long enough to get the faint parts of M33 above the noise floor at native will benefit from the reducer because it will put the available photons onto fewer pixels.

Olly

[RichLD]

Many thanks sir! The weeding can wait! 

[The Admiral]

30 minutes ago, ollypenrice said:

Busy watching the Tour de France and avoiding the weeding. That's busy!

The middle setup is the baseline. On the left we add aperture, on the right we subtract focal length. These are not equivalent actions even though they both produce F3.5.

As Craig Stark points out, an imager unable to expose for long enough to get the faint parts of M33 above the noise floor at native will benefit from the reducer because it will put the available photons onto fewer pixels.

Olly

Sorry Olly, but surely there is no difference between comparing telescopes of 2 different focal lengths, and comparing a single telescope with and without reducer? You seem to be saying that there is a myth with one and not the other, or have I missed the point? In both cases, either shortening the fl or by using a reducer, you are increasing the number of photons per pixel according to the change in f-ratio. Therefore to achieve the same SNR, the exposure time for the shorter fl can be reduced in proportion to the f-ratios. No?

Ian

[ollypenrice]

38 minutes ago, The Admiral said:

Sorry Olly, but surely there is no difference between comparing telescopes of 2 different focal lengths, and comparing a single telescope with and without reducer? You seem to be saying that there is a myth with one and not the other, or have I missed the point? In both cases, either shortening the fl or by using a reducer, you are increasing the number of photons per pixel according to the change in f-ratio. Therefore to achieve the same SNR, the exposure time for the shorter fl can be reduced in proportion to the f-ratios. No?

Ian

You're right that there is no difference between a 200mm native F3.5 and a 200mm F5 reduced to F3.5. I just use a reducer in the example because that's usually how the subject comes up.

Your second point is discussed carefully here. http://www.stanmooreastro.com/f_ratio_myth.htm

The reducer will, in my view, allow you to make a smaller image faster. But so will the native scope! We all know that to present an image at full size requires far more data than to settle for a 50% screen presentation. The full size image needs more incoming information because it will try to project more outgoing information. The reducer, on targets which fit the native chip, brings in no new information. It may, though, take the signal above the noise floor in an insensitive system.

Olly

[The Admiral]

Thanks for that Olly, sorry to have dragged you away from the cycling!

Fair comment about the aperture; clearly the more photons the better, but it seems to me that that is only the case if they can be put to good use. A reduced image size isn't always bad.

Ian

[ollypenrice]

24 minutes ago, The Admiral said:

Thanks for that Olly, sorry to have dragged you away from the cycling!

Fair comment about the aperture; clearly the more photons the better, but it seems to me that that is only the case if they can be put to good use. A reduced image size isn't always bad.

Ian

Indeed a reduced image isn't always bad. It's better than a full size image which doesn't hold up. If your photons have taken you above the noise floor you will be able to put them to good use, though.

Olly

[StuartJPP]

How sciency people explain it:

 

Focal reducer:

A focal reducer works opposite to the way a Barlow functions.  It is a positive lens and has the effect of causing the incoming beam of light to converge faster, shortening the focal length.  This also decreases the focal ratio of the instrument.  The result is a wider field of view and faster photographic speed.  A focal reducer is essentially a positive lens as in a refractor (although three to four elements are normally used for the best correction).  The shorter the focal length of the focal reducer, the greater the reduction factor when placed into the optical path of a telescope.  In addition to the focal length of the reducer being important, the distance from the reducer lens to the focal plane is critical to determining the reduction factor.  Placing the focal plane closer to the reducer results in a longer overall focal length, while moving the focal plane away from the reducer decreases overall focal length.  As an example, a standard 0.63x focal reducer has a focal length of about 250mm and reduces an f/10 telescope to f/6.3.  This is true if the distance from the reducer to the focal plane is about 100mm.  If this distance is decreased, the focal ratio will be larger than f/6.3.  Increasing this distance will make the telescope faster than f/6.3 (although the image quality suffers).
(From here: https://starizona.com/acb/basics/equip_optics101_optical.aspx)

 

 

How I understand it: