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Old 09-08-2013, 06:07 PM
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LewisM
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f/5 vs f/4

Deciding on a new Newt for imaging. Is f/4 going to make a dramatic difference over an f/5 system? I am well used to slower systems, being a refractor imager.

So the decision comes down to simply f/4 vs f/5 - both 8", both Chinese. The difference is in the cost
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Old 09-08-2013, 06:11 PM
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rmuhlack (Richard)
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Quote:
Originally Posted by LewisM View Post
Deciding on a new Newt for imaging. Is f/4 going to make a dramatic difference over an f/5 system? I am well used to slower systems, being a refractor imager.

So the decision comes down to simply f/4 vs f/5 - both 8", both Chinese. The difference is in the cost
collimation tolerances are certainly easier to manage at f5. coma should be fully corrected with your CCDs with a MPCC or RCC also.
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Old 10-08-2013, 06:42 PM
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main difference is time. the f5 will take 25/16 times as long as an f4 to a given result, all else being equal.

As Richard points out, downside is tightness of tolerances at f4 - getting an f4 scope to work really well is much harder than getting the same from an f5.
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Old 10-08-2013, 07:24 PM
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main difference is time. the f5 will take 25/16 times as long as an f4 to a given result, all else being equal. ...
Only for nebulae (extended objects)!
For stars, it will be the same.
Actually a bit worse... because the central obstruction (secondary) must be slightly wider for f4 system
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Old 10-08-2013, 09:11 PM
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Only for nebulae (extended objects)!
For stars, it will be the same.
Actually a bit worse... because the central obstruction (secondary) must be slightly wider for f4 system
Hi Bojan. I think that the effect of FNo on star images depends on sampling. If stars are heavily undersampled, you are right and there is no difference. If sampling near Nyquist or oversampled, star angular size will be the same for both optics and determined by atmospheric seeing with the 8 inch aperture, so the energy of a star in the longer fl system will be spread out over a larger area (and more pixels). stars will thus be bigger and dimmer in the higher FNo system, just like an extended object (galaxy, nebula etc). If you want to be convinced that star brightness changes with focal length, try imaging with a Barlow in the optics - this will change only the focal length (not the aperture), but you will certainly see a major drop in star brightness.

The other effect of FNo for the same aperture and camera is that the higher FNo system will be able to resolve finer detail in some circumstances - again depending on sampling and atmospheric seeing.

Lewis, if this is to be used with your SX25, the f4 will be much faster, but the f5 will give noticeably more detail in good seeing. Either would be suitable, but I reckon I would go with the f5 to get better detail when it's available.

Last edited by Shiraz; 10-08-2013 at 11:35 PM.
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Old 11-08-2013, 09:01 AM
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Get both. You know you want to.
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Old 11-08-2013, 10:27 AM
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I'd take Ray's advice as he's gone right into it and the system he put together is working really well.

Greg.
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Old 11-08-2013, 10:28 AM
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.... try imaging with a Barlow in the optics - this will change only the focal length (not the aperture), but you will certainly see a major drop in star brightness. ...
I have tried that homework in the past, with my MTO, which after necessary modifications (removing focus stop) behaves as a system with variable barlow (it is possible to have a mag factor around .5 ~ 2 depending on sensor distance), and there was no difference observed (between F4 and F5) that I should've been concerned with too much. Of course, when star image (Airy's disk et al) is larger that pixel size, then you are right in theory, but additional Barlow is not a right way to determine this fact because there is additional element(s) in the optical train (not in the case of my MTO that I described earlier).

Also, you have to consider the fact that star images on CCD (CMOS) sensors are saturated very quickly - so their size will not be the measure of resolution of details..

So, for the non-saturated stars, the quick calculation shows that the airy's disk diameter for Theta (angle of first minimum) is:

Sin(Theta) =~ 1.22*lamda/Aperture_diameter
or
Theta =~ 1.22 *lambda*(Fno/FL)

The physical size (radius) of the Airy's first minima on the sensor is:

h = FL * Tan(Theta) =~ FL * 1.22*lambda * (Fno/FL)
h = 1.22 * lambda* Fno

For 8", F4 (800mm FL)and yellow light (Sodium line, 589nm) this radius is:
2.8um, or the diameter of the first minima is 5.4um

For 8", F5 (1000mm FL) this diameter is 7.2um.

The size of the pixel on my Canon 400D is 5.7um.
So, the theoretical image size for both F4 and F5 8" mirror is comparable with the pixel size, meaning slightly better resolution with F5, but only just.. not significant at all.

I think Lewis should consider price and FOV as the most significant/important parameter for his decision (I would..)
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Old 11-08-2013, 01:38 PM
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Decided on an f/5.3 system instead.

Thanks for the responses. Will post it when I pick it up!
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