Imaging at f/5.9 vs f/4.8

[smr]

Hi,

Just wondering what the difference is for deep sky imaging between the two focal ratios ? Is there a big difference in terms of light gathering ? 

I haven't bought a focal reducer before and it would change my scope from 430mm to 344mm and 4.8 instead of 5.9.

In Stellarium when viewing between the two focal lengths there doesn't seem to be too much difference between the focal lengths, so is it a no brainer to buy a focal reducer and improve light gathering or should I save my money?

Thanks for any advice,

Joel

[geordie85]

Since your aperture isn't changing size, the light grasp remains the same. You will have a slightly larger fov though.

[smr]

5 hours ago, geordie85 said:

Since your aperture isn't changing size, the light grasp remains the same. You will have a slightly larger fov though.

So that is the only benefit ? A slightly larger FOV ? For some reason I was under the impression that more light would be gathered and I could shorten total integration time. 

[alacant]

9 minutes ago, smr said:

more light would be gathered

Hi

If you arrive at f4.8 by increasing the aperture, then yes. All you have done is reduce the focal length, so the amount of light collected is the same. The only benefit will be as outlined by @geordie85.

If you don't need the reducer to also flatten the field, I'd stay as you are.

Cheers

[smr]

On 12/02/2021 at 21:46, alacant said:

Hi

If you arrive at f4.8 by increasing the aperture, then yes. All you have done is reduce the focal length, so the amount of light collected is the same. The only benefit will be as outlined by @geordie85.

If you don't need the reducer to also flatten the field, I'd stay as you are.

Cheers

I don't quite understand it, I come from several years of terrestrial photography so I'm so used to diaphragm controlling light passage and apertures, so a wider aperture letting more light in, what is the point in defining an f stop with a focal reducer in astrophotography, if the aperture remains the same. 

If your focal length is reduced but the light transmittance to the sensor is unaltered it's f/5.9 430mm without the reducer, and f/5.9 344mm with the reducer. 

[david_taurus83]

Larger FOV yes, but your pixel scale will increase also. So you will have more photons (in theory) falling on individual pixels which will add to its charge. This is why people say it is 'faster but not in the same sense as a camera lens.

[alacant]

1 hour ago, smr said:

what is the point in defining an f stop with a focal reducer in astrophotography

None. It serves only to liken terrestrial photography with astrophotography, which is both confusing and largely useless.

1 hour ago, smr said:

it's f/5.9 430mm without the reducer, and f/5.9 344mm with the reducer.

The reducer makes it like a zoom lens. Maybe you have seen lenses such as 75-300mm f4.5-5.6? The OP has a telescope of 73mm diameter with or without a reducer. He is changing the f-ratio by altering the focal length rather than the aperture. The zoom lens is this example changes both. The OP's telescope, focal length only.

Non reduced: 430/73 = f5.9

Reduced: 344/73 = f4.7

Both collect the same amount of light. But it gets even worse. As @david_taurus83 explains, the image may be brighter at f4.7

Cheers

Edited February 14, 2021 by alacant

[vlaiv]

It is exactly like in terrestrial photography - in this case, F/4.8 will be faster than F/5.9

This is because pixel size remains the same.

There seems to be so much confusion on this topic. Don't know why is that.

When camera remains the same and pixel size remains the same - daytime photography rule applies - faster F/ratio is indeed faster - but only when paired with that particular camera.

In light dominated regime - rule from daytime photography also applies here - rule of F/stops and how much faster system will be.

What we can't extrapolate from above are following:

- F/4.8 scope is faster than F/5.9 scope. That does not hold in general - it does hold for same camera and if we keep pixel size the same, but if we start binning or using different pixel size or whatever - it is no longer true.

- F/stop rule can always be used to calculate how faster system is. Again - this only works in above case where we have the same camera and same pixel size and we operate in light dominated regime. Once we start imaging very faint targets - other noise sources start to dominate single exposure and that math no longer holds

Back to original question. Benefits of using reducer while keeping the same camera and same pixel size:

1. larger FOV

2. faster imaging (where speed of imaging is defined as time needed to reach target SNR)

 

[smr]

1 hour ago, vlaiv said:

It is exactly like in terrestrial photography - in this case, F/4.8 will be faster than F/5.9

This is because pixel size remains the same.

There seems to be so much confusion on this topic. Don't know why is that.

When camera remains the same and pixel size remains the same - daytime photography rule applies - faster F/ratio is indeed faster - but only when paired with that particular camera.

In light dominated regime - rule from daytime photography also applies here - rule of F/stops and how much faster system will be.

What we can't extrapolate from above are following:

- F/4.8 scope is faster than F/5.9 scope. That does not hold in general - it does hold for same camera and if we keep pixel size the same, but if we start binning or using different pixel size or whatever - it is no longer true.

- F/stop rule can always be used to calculate how faster system is. Again - this only works in above case where we have the same camera and same pixel size and we operate in light dominated regime. Once we start imaging very faint targets - other noise sources start to dominate single exposure and that math no longer holds

Back to original question. Benefits of using reducer while keeping the same camera and same pixel size:

1. larger FOV

2. faster imaging (where speed of imaging is defined as time needed to reach target SNR)

 

The pixel size would be the same, 3.76 pixel size of the 2600MC-Pro. It would just be that I would add the 0.8 reducer.

So in essence the total integration time can be shorter, less time to gather more photons/higher snr for a set amount of imaging time, from what you are saying, which is what I am after.

Ie. If I were to image a Nebula on one night without the reducer and I spent 10 hours on it.

I then image it with the reducer the next night for 6 hours, the image would have the same snr as 5.9 to 4.7 is 2/3  stop.

Edited February 14, 2021 by smr

[vlaiv]

44 minutes ago, smr said:

I then image it with the reducer the next night for 6 hours, the image would have the same snr as 5.9 to 4.7 is 2/3  stop.

I agree with all except this part.

That is the part that I stressed should not be extrapolated from daytime photography and F/speed of lens. It is true when system is operating in target light dominating regime - which means target is much brighter than all other things that hurt image. In astrophotography - that is almost never the case.

There are things between two nights that can easily turn the tables on F/ratio alone. For example - you are imaging very faint target and first night you have very transparent skies. On second night you loose that great transparency - then it might be even the case that 10 hours at F/4.7 will not match 10 hours from first night at F/5.9

If all things are equal - and only thing that has changed is F/ratio, well in that case - improvement will be even larger than F/ratio alone suggests. This is because of the read noise. You probably won't be able to see it - but yes, theoretically results will be even better than F/ratio alone suggests.

What you can conclude is that imaging at F/4.7 will bring you more FOV and will also bring in a bit more SNR for same imaging time than imaging at F/5.9. Exact numbers vary from situation to situation and from target to target, so that is not something that can be easily stated.

[smr]

47 minutes ago, vlaiv said:

What you can conclude is that imaging at F/4.7 will bring you more FOV and will also bring in a bit more SNR for same imaging time than imaging at F/5.9. Exact numbers vary from situation to situation and from target to target, so that is not something that can be easily stated.

This is the part that I was thinking about with total integration time., if using the reducer brings more SNR then total integration time doesn't have to be as much compared to without the reducer.

[ollypenrice]

Let's start with a target which will fit on the chip at either F ratio. So...

 - We lower the F ratio as in terrestrial photography by increasing the aperture. Now all pixels, including the target's, get more light and exposure time is reduced. You have captured more object photons. This is not controversial. But...

- Now we lower the F ratio by reducing the focal length, leaving the number of object photons exactly as before.  We cannot get something for nothing. What we can do, and have done, is concentrate the same number of object photons onto fewer pixels, causing them to reach the desired S/N ratio more quickly at the cost of a smaller image of the object.  If this is what you wanted, go ahead. You could also capture bin the image, or software bin it or resample it downwards, to get a very similar result. 

Let's now consider a larger target in which you actually want all the new parts of the target in the wider FOV with the reducer. And, yes, your new, wider field image will indeed reach the target  S/N ratio more quickly in accordance with the terrestrial F ratio calculation with which you're familiar.

What this boils down to is that there are really only two sound reasons for using a reducer. 1) to increase the FOV because you're interested in capturing all of it. 2) to turn useless oversampling in longer FL systems  into productive time-saving. As pixels get smaller this second reason will become ever more pertinent.

Olly

[Simon Pepper]

Hi all,

This content is really helpful and I have been following, however I also have a question around flatteners / reducers. I currently have the RedCat which I have been using for Nebula and wide field imaging, but I want to go a little deeper perhaps things like M33 and M101, Hercules etc and checking astronomy tools FOV I think I want something around the 500mm focal length (which is still forgiving for a beginner) I will pair this with either my DSLR or buy a ZWO 294 MC Pro. I am keen to stay with William Optics as the build quality and image quality is great, so I am leaning towards William Optics GT-81 IV (478mm) or William Optics Zenithstar 81 APO (559mm) there is quite a difference in price here and I assume this is down to the F ratio being far superior on the GT. My question is do these two scopes need / require a flatter to obtain a flat field across the image? I don't really want to be adding a reducer as that will increase the FOV, price and bring down the focal length which I want to avoid as I already have a wide field Redcat it seems silly spending loads of pennies if I am only going from 250mm to maybe 350 / 400m once a flattener is introduced on the above. Any recommendations on this or other scopes would be appreciated. Checking the OP specs I believe they are using William Optics Zenithstar 73 II APO 2019 which is another scope to for me to consider.

Thanks 

Edited February 15, 2021 by Simon Pepper

[ollypenrice]

3 hours ago, Simon Pepper said:

Hi all,

This content is really helpful and I have been following, however I also have a question around flatteners / reducers. I currently have the RedCat which I have been using for Nebula and wide field imaging, but I want to go a little deeper perhaps things like M33 and M101, Hercules etc and checking astronomy tools FOV I think I want something around the 500mm focal length (which is still forgiving for a beginner) I will pair this with either my DSLR or buy a ZWO 294 MC Pro. I am keen to stay with William Optics as the build quality and image quality is great, so I am leaning towards William Optics GT-81 IV (478mm) or William Optics Zenithstar 81 APO (559mm) there is quite a difference in price here and I assume this is down to the F ratio being far superior on the GT. My question is do these two scopes need / require a flatter to obtain a flat field across the image? I don't really want to be adding a reducer as that will increase the FOV, price and bring down the focal length which I want to avoid as I already have a wide field Redcat it seems silly spending loads of pennies if I am only going from 250mm to maybe 350 / 400m once a flattener is introduced on the above. Any recommendations on this or other scopes would be appreciated. Checking the OP specs I believe they are using William Optics Zenithstar 73 II APO 2019 which is another scope to for me to consider.

Thanks 

Just for info, in AP 'deeper' is generally used to mean 'fainter.' And another note: you won't want more focal length for M31. It is utterly enormous. Check out Gorann's recent rendition at 400mm in which he didn't quite fit it all in. When I did it at 530mm with a full frame camera I still needed to make two panels.

For any chip of APSc size or above you will need a rear lens element of some kind. This can be in the form of a Petzval or Petzval-like scope design in which the rear element(s) are in the main tube, or it will take the form of a flattener on the drawtube. Some manufacturers list flatterers which are not reducers, AKA 0x flatteners. You'd need to browse through your options with this in mind.

Olly

[The Lazy Astronomer]

9 hours ago, Simon Pepper said:

Hi all,

This content is really helpful and I have been following, however I also have a question around flatteners / reducers. I currently have the RedCat which I have been using for Nebula and wide field imaging, but I want to go a little deeper perhaps things like M33 and M101, Hercules etc and checking astronomy tools FOV I think I want something around the 500mm focal length (which is still forgiving for a beginner) I will pair this with either my DSLR or buy a ZWO 294 MC Pro. I am keen to stay with William Optics as the build quality and image quality is great, so I am leaning towards William Optics GT-81 IV (478mm) or William Optics Zenithstar 81 APO (559mm) there is quite a difference in price here and I assume this is down to the F ratio being far superior on the GT. My question is do these two scopes need / require a flatter to obtain a flat field across the image? I don't really want to be adding a reducer as that will increase the FOV, price and bring down the focal length which I want to avoid as I already have a wide field Redcat it seems silly spending loads of pennies if I am only going from 250mm to maybe 350 / 400m once a flattener is introduced on the above. Any recommendations on this or other scopes would be appreciated. Checking the OP specs I believe they are using William Optics Zenithstar 73 II APO 2019 which is another scope to for me to consider.

Thanks 

The GT81 is a triplet, the zenithstar is a doublet, so that accounts for the price difference (triplets are expensive).

I think, for larger sensors, basically every scope needs a flattener if it doesn't already create a flat field by design. Not sure if there is a non-reducing flattener either of those options you've listed above, but I'm sure someone will chime in with an answer to that. 

[alacant]

15 minutes ago, The Lazy Astronomer said:

non-reducing flattener

Anecdote and link.

Left by a visitor some time ago. It bears no distinguishing markings which would lead to easy identification. A nice surprise to find out what it was. Works great even with rubbish refractors e.g. a 72ed: corner to corner over aps-c @ 132mm spacing. Use the official spacings at your peril.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p10307_TS-Optics-REFRAKTOR-1-0x-Flattener-Bildfeldkorrektor---2--Anschluss.html

HTH

Edited February 15, 2021 by alacant

[smr]

Thanks for all the replies. 

[jimjam11]

On 11/02/2021 at 19:32, smr said:

Hi,

Just wondering what the difference is for deep sky imaging between the two focal ratios ? Is there a big difference in terms of light gathering ? 

I haven't bought a focal reducer before and it would change my scope from 430mm to 344mm and 4.8 instead of 5.9.

In Stellarium when viewing between the two focal lengths there doesn't seem to be too much difference between the focal lengths, so is it a no brainer to buy a focal reducer and improve light gathering or should I save my money?

Thanks for any advice,

Joel

It looks like you have the flat73 and are looking at getting the flat73r. I have both but the reducer isn’t perfect:

1. It doesn’t support full frame.

2. Star sizes are worse; my long term average fwhm is 3.9” vs 5” with the reducer.

3. Star shapes are significantly worse in the corners, spacing is very finicky (although easily adjusted).

4. I have never noticed an snr improvement, there are too many other variables at play.

 

Having said that, it does give a slightly larger fov which can be very useful! It also feels better made and the rotator is vastly superior.