Focal Ratio equation

[IanL]

Is it the aperture or exposure length and total time or camera sensitivity? 

That's like asking what the dominant factor is in being tall, long legs or platform shoes?

Aperture, focal length, sensitivity (including the net effect of the optical elements plus filters) and read noise are fixed for a given set-up, so you have to increase number/length of subs to get a higher SNR.  As to which is the "dominant" factor, that rather depends on the specific equipment.

[ollypenrice]

Fast optics are faster at a given focal length. So if you want to image at a galaxy focal length, say  2 metres, then fast optics will be faster than slow ones at that focal length. In understanding the F ratio myth (which in practice usually catches people out when focal reducers enter the story) don't throw out the baby with the bathwater. The bigger the aperture of your 2 metre FL scope, the faster it will pull in all ranges of data. 

2 metres at F4 means 50cm of aperture... That's rather a lot! And that's why we don't see it on the forums.

I've done some chasing of faint stuff. It tends to be widefield so I use the Taks at F5, though I did the Praying Ghost in the TEC at F7. This just meant 22 hours of data. In my experience you need long sub exposures. I am absolutely convinced that an 11 hour run in 15 min luminance subs gave less of the outer halo of M31 than 3.5 hours in 30 minute subs. So for me the faint stuff needs long subs, and a lot of subs. This means it has to be CCD because DSLRs can't take long subs. Also the secret of catching the faint stuff lies in the luminance layer. Shooting L is more than 3x faster than shooting RGB (whether in a mono or OSC camera) so for faint stuff use a mono camera. You are not normally chasing colour in the faint stuff, though we did try to find colour in the IFN. And at long FL and for faint stuff you should not be using tiny pixels so you can bin them in a mono camera. Not in an OSC.

To look at it another way the FSQ/11 meg Kodak works at 3.5 arcseconds per pixel, far too course according to some (and it cannot reslove to the scope's limit, that's certain) but there must surely be more APODs generated by this pairing than by any other single setup. The coarse sampling makes it fast. Because it's fast it goes deep and find things we don't always see.

Olly

[lensman57]

Personally I think the term 'faster' should be banned! It implies something which is generally not true and causes no end of confusion. A small f-ratio is all about getting a wider area quicker. There is no guarantee it is going to get you as deep on a single object in a shorter time, or deeper in the same time - that is all down to whether read noise is a significant factor or not. If it isn't then f-ratio makes no real difference (for the same aperture).

NigelM

Hi Nigel,

I tend to agree with you. As for reducers I am gradually coming to think of them as add ons to increase the FOV , I am more convinced  that these have little  effect in reducing the exposure time.

A.G

[dph1nm]

Because it's fast it goes deep and find things we don't always see.

Now this is where I have the problem. Assuming read-noise is not an issue, then if, let us say,  an 8" f10 can detect a 20th mag star (or galaxy) at 3 sigma in a given exposure time, an 8" f4 will need the  same exposure time to reach that limit. In fact if your pixels become too large (compared to the seeing) then the smaller f-ratio will not go as deep, due to the extra sky inside each pixel (this only applies to stars).

NigelM

[ollypenrice]

Hi Nigel,

I tend to agree with you. As for reducers I am gradually coming to think of them as add ons to increase the FOV , I am more convinced  that these have little  effect in reducing the exposure time.

A.G

If you want the entire image provided by the reducer they do reduce exposure time. Your alternative is to do a mosaic, which takes a lot longer...  I think reducers are great provided that you understand what they do. They fill all the pixels faster and widen the field. Now is that what you want? If it is, then they are wonderful. You have to want the whole image and not just a bit of it.

Olly

[ollypenrice]

Now this is where I have the problem. Assuming read-noise is not an issue, then if, let us say,  an 8" f10 can detect a 20th mag star (or galaxy) at 3 sigma in a given exposure time, an 8" f4 will need the  same exposure time to reach that limit. In fact if your pixels become too large (compared to the seeing) then the smaller f-ratio will not go as deep, due to the extra sky inside each pixel (this only applies to stars).

NigelM

Most amateur imagers with small scopes are trying to get rid of stars!! They catch far too many and they are too big. This is certainly where I am. Point sources don't follow the F ratio rule but, in small scopes, are stars point sources? They cover a hell of a lot of pixels...

Olly

[dph1nm]

Most amateur imagers with small scopes are trying to get rid of stars!! They catch far too many and they are too big. This is certainly where I am. Point sources don't follow the F ratio rule but, in small scopes, are stars point sources? They cover a hell of a lot of pixels...

A good question. Once stars are resolved (by the detector) they behave like extended sources - double the focal length and a star will cover twice as many pixels. But at very short focal length, even with UK seeing, they may only occupy a single pixel.

NigelM

[orion69]

Last year I shot Pacman (NGC281) and at that time I did not have FF/FR so I used only FF.  I'll be imaging same object with same gear except I'll be using Riccardi 0.75x FF/FR, just waiting for a clear night. Still have original fits,  I think it will be nice test of the "myth". 

[ollypenrice]

Last year I shot Pacman (NGC281) and at that time I did not have FF/FR so I used only FF.  I'll be imaging same object with same gear except I'll be using Riccardi 0.75x FF/FR, just waiting for a clear night. Still have original fits,  I think it will be nice test of the "myth". 

Does the target fit entirely on the chip both with and without? Remember that the myth can only be disproved or confirmed when the target itself is presented at the same size.

A good question. Once stars are resolved (by the detector) they behave like extended sources - double the focal length and a star will cover twice as many pixels. But at very short focal length, even with UK seeing, they may only occupy a single pixel.

NigelM

Well, when you image the same field you clearly see that a larger telescope with longer focal length gives you much smaller stars. As ever it is nigh on impossible to conjure up perfect comparative data because there are always variables that you don't want in the data you do have. But here we have the same target, the Sagittarius Triplet, shot in an 85mm F3.9 scope at FL328mm followed by a 2 panel mosaic shot in a 140mm F7 scope at FL980mm.  You'll have to look only at the field stars in the first image because the three main nebulae have been enhanced with long FL data. However, the star reduction arising from the larger scope is obvious.

(Alas, the chips weren't the same either, the short FL being done in an Atik 4000 with 7.4 micron pixels and the other in an Atik 11000 with 9 micron pixels. Sampling in the case of the short FL image was 4.65 arcsecs per pixel while in the long FL image it was 1.8)

I suppose the question is, 'Does the reduction in star size come from the increase in aperture or the increase in focal length or both?' Your point suggests that an increase in FL will bring an increase in star size.

85/328 image. (Too red! Note to self... );

140/980 image

When you overlay a big scope image over a small scope image the reduced star size is obvious and needs to be managed in the blending. What we now need is a comparison of star size using ad not using a reducer in the same scope. Doh!!!   

Olly

[orion69]

Does the target fit entirely on the chip both with and without? Remember that the myth can only be disproved or confirmed when the target itself is presented at the same size.

Yes. it does. This is without FF/FR:

 

[ollypenrice]

Yes. it does. This is without FF/FR:

Nice! This will be an interesting comparison, then. Look out for the effect on star sizes, too. Good luck with the shoot.

Olly

PS I think you're going to find more object in the wider field. Sara did a deep one recently.

[dph1nm]

At longer focal length the stars cover more pixels but cover the same size in arcseconds (given the same seeing & scope resolution).  So if  you scale the pictures to cover the same area of sky they should look the same, unless, of course,  your short FL pixels are much bigger than the seeing/resolution, as these stars will then appear larger. I suspect that is what has happened in Olly's example shots.

NigelM