10" F4.7 Newt vs 5" f11.8 Mak exposure times?

[Space Cowboy]

How much longer exposure times does the f11.8 127mm Mak need over the f4.7 254 mm Newt for deep sky imaging?  Being a simple farmer whilst driving the tractor this morning  I worked out around 5 times longer exposure needed for the Mak. Am I far out?

Using a canon 1000d.

[ronin]

Isn't it one divided by the other then squared ?

So I would have said 6.3 times. << Calculator.

Bit more then 5 but not much, however I am guessing/assuming that some old much unused brain cells have retained correct information. And that is a low probability.

[michael.h.f.wilkinson]

6.3 is correct. Bear in mind that the dark current also builds up by a factor of 6.3x on average (can be sorted up to a point by darks), which will give a 2.5x higher noise contribution from the dark current.

[Pete Presland]

what about the F ratio myth? i was beginning to believe the F ratio was not that important.

[michael.h.f.wilkinson]

what about the F ratio myth? i was beginning to believe the F ratio was not that important.

F ratio is important in (DSO) imaging, aperture is important in visual. As the brief calculation shows, you need 6.3x the exposure time to get the same number of photons per pixel, but the noise caused by dark current increases by a factor of 2.5 in the slower scope.

[Pete Presland]

so its not the signal that is improved, but the signal to noise ratio?

[michael.h.f.wilkinson]

so its not the signal that is improved, but the signal to noise ratio?

Yes. If you have enough time, a slower scope can gather the same signal per pixel as a faster scope. Due to factors like dark current and read-out noise, the results will contain more noise. Besides, tracking and alignment problems  multiply with increased sub lengths. The speed of a scope is the best measure of how much light will reach the sensor per pixel (or per unit of surface area). The faster you can get your photons, the better.

[Space Cowboy]

Thanks guys! Thats interesting because it means my Dob only needs to track accurately for 10 seconds to produce the same exposure as my mak does in 1 min and with 2.5x less noise!

[michael.h.f.wilkinson]

2.5x less noise due to dark current, the part of the noise due to photon noise remains the same.

[dph1nm]

The speed of a scope is the best measure of how much light will reach the sensor per pixel (or per unit of surface area).

Which is quite true, but should not be taken to imply that the faster scope will go deeper in the same time. That depends on whether read-noise make a significant contribution to your noise. If it doesn't then there will be no difference in the depth a 'fast' or 'slow' scope of the same aperture reach in the same exposure time.

NigelM

[michael.h.f.wilkinson]

Which is quite true, but should not be taken to imply that the faster scope will go deeper in the same time. That depends on whether read-noise make a significant contribution to your noise. If it doesn't then there will be no difference in the depth a 'fast' or 'slow' scope of the same aperture reach in the same exposure time.

NigelM

I assume that we are using the same camera on both scopes, for a fair comparison. The read noise grows with the square root of the number of subs. The dark current noise grows with the square root of the length of the subs as well. Besides, as the dark current fills up the electron wells in the photosites, the effective dynamic range is reduced. Thus, given the same camera, the faster scopes can give significant improvement s in dynamic range and S/N

[Space Cowboy]

Very interesting! I'm working on the Dob tracking as we speak so if I can get the beast tracking well for 30 sec subs that would be great!

[dph1nm]

I assume that we are using the same camera on both scopes, for a fair comparison. The read noise grows with the square root of the number of subs. The dark current noise grows with the square root of the length of the subs as well. Besides, as the dark current fills up the electron wells in the photosites, the effective dynamic range is reduced. Thus, given the same camera, the faster scopes can give significant improvement s in dynamic range and S/N

I am assuming the sub lengths are the same. Anyway, if you are sky-limited in a single sub (in both scopes) then read noise and dark current are not particularly relevant. Also, once the pixel size in the "fast" scope starts to exceed the local seeing then you become less deep for stellar images, due to the increased sky contribution.

NIgelM

[michael.h.f.wilkinson]

I am assuming the sub lengths are the same. Anyway, if you are sky-limited in a single sub (in both scopes) then read noise and dark current are not particularly relevant. Also, once the pixel size in the "fast" scope starts to exceed the local seeing then you become less deep for stellar images, due to the increased sky contribution.

NIgelM

Your statement is somewhat confusing, as you cannot keep exposure of the subs equal at the same sub length.  If the sub lengths are the same, the number of subs needed in the fast scope is larger, so the read-out noise contribution is higher (by the square root of the number of subs), also the dark current accumulated over that increased time is larger, As darks only correct for the average dark current per pixel, the noise contribution from dark current must increase (by the same factor). If you increase exposure time such that subs are exposed in the same way, the read-out noise problem is avoided. The dark current issue persists.

[Space Cowboy]

Should that not read "if the sub lengths are the same the number of sub's needed in the SLOW scope is larger"

[dph1nm]

This is my point. You do NOT need longer subs or more exposure time using the 'slow' scope providing you are not read-noise limited (or dark noise limited) in a single 'slow' sub. To put it another way, if the noise per pixel in a single sub is dominated by the background sky, then the same sub length and same total exposure time will get you to roughly the same depth, irrespective of whether your scope is 'fast' or 'slow'.

NigelM