RC8 vs RC12 acquisition times

Hi Igor,

@SemiPro is correct that Focal Ratio drives exposure time and Focal Length effects image scale. Example Celestron SCT at F/10 vs a Celestron SCT at F/7 (with reducer).  F/10 squared vs F/7 squared is 100 vs 49. So that means that the F/10 takes a hair over twice as long as the F/7 configuration with the reducer. I hope that makes sense.

CS Tim

Hi Igor,

I have both an RC8 and an RC12. When using them at their native f/8 focal ratio, they need similar exposure times and they are slow. But they both become pretty luminous when adding the 0.67x reducer (I have the TS-Optics and the Astro-Physics models and they work the same).

I use them always with the reducer, but if I want to use the native focal length, what I do is shoot a whole long session without the reducer and then another one with it on for the extra luminosity, to round up the data gathering. Then register the image to the higher focal length. I’m no expert and I don’t know if this is a useless thing to do, though. Can anyone please confirm me about this practice?

The RC8 with the reducer is what I use to substitute my 200/1000 f/5 Newtonian. I find it much better in terms of optical and build quality and the focal length and ratio is equivalent. Also, spikes are superb and the field is flat enough for me.

Just a word of caution: the RC12 is extremely heavy. You only want to have one if you’re going to have it still on a mount. Ask help when moving it and putting it up on the mount. Then collimate it, keep it covered and forget about it.

I Have an RC10 and use it at F/8 often. I do have several reducers that range from .8x to .5x and plan to evaluate them this summer. I recently built a home observatory and this OTA will now have a permanent home. This OTA is getting to large for me to have on the "Portable" list. I am older and that OTA isn't getting lighter… With that said, after a proper evaluation of my reducers, I will image with a reducer. Most likely a .8. With the equation earlier.  F/8^2=64 vs (.8*F/8^2)= 41.  41/64 is approximately 2/3.   So if you install a .8 that will reduce your exposure time by 1/3 to capture the same info.  

if you install your .67x reducer it will reduce your F/8 to a F5.4.   That is 64 vs 29.  That will give you the same info in 4.5 hours at F/5.4 that a 10 hour run gives you at F/8…

Moving to a RC12 is the same F Ratio as the RC8 and RC10 so the math and results are the same. 

CS Tim

Thank you guys.
It's now clearer that I won't get that much besides resolution by spending ~€9,000 on new equipment (with a new mount)

Igor Korenika:
Is it worth switching to RC12 (or RC10) from RC8 in order to shorten the exposure time?
I live in a place where there aren't many clear nights.
Today I have to shoot for at least 10 hours with my RC8 to get acceptable results.
It would be ideal if I could reduce the time by a factor of 3.

Igor,
That's a good question and it's one that comes up all the time.  In order to understand the answer, it is first important to understand that the radiometry of imaging is different for point sources like stars and extended objects, which are any objects with an extent larger than the point spread function of the telescope.  The irradiance of point sources is proportional to the square of the ratio of the diameter of the entrance pupil to the focal ratio; whereas, the irradiance of an extended source is proportional only to the inverse square of the focal ratio only.

Next, understand that total signal from your sensor (in photoelectrons) is given by the irradiance in the focal plane (in W/m^2) times the size of the pixel (m^2) times the responsivity of the sensor (e-/W-s) times the exposure time (s).  You also have to remember to take into account the optical throughput of the optics, which takes into account the size of the secondary and the reflectivity of the components.  You can find the effect of reflectivity by counting the number of surfaces in the telescope (N) and assume that each surface has a reflectivity of 0.98.  The total throughput will be given by 0.98^N.  For example, a RC with only two surfaces, a filter wheel, and a glass cover over the sensor, will have a throughput of 88.6%.  If you use a CDK with 8 surfaces, the throughput will be 85.1%.

So, to answer your question for an extended source, you have to take into account the F/# of each system, the size of the pixels that you use, the sensitivity of your sensor, along with the throughput of the telescope.

I did this comparison to look at what would happen when I moved from my 14" F/10 SCT to a 20" F/6.7 CDK (for extended sources).  If you assume equal exposures, the signal ratio calculation is show in the following slide along with the results.  (I don't think that I took the window in the camera into account here).  

With this information, you can plug in your own numbers to see how your two scopes compare.  You may find that adding a reducer and properly take into account all of the surfaces in the reducer, it may not be as advantageous as you think.  The other problem with most reducers is that they do not produce very good field correction and for that reason (along with a few others), I generally try to discourage folks from using reducers.

- John

John Hayes:
That's a good question and it's one that comes up all the time....

Wow, I will make this calculation. Thank You.

Great question.  I have just ordered an RC12 to replace my RC8 [plus a new mount to point and track this heavyweight].

I am interested in two things:  SNR per spatial (arcsec) resolution element and field-of-view.   Any f8 system will give same SNR per physical (micron) resolution  element but the arcsec/micron scale changes, as does the field-of-view, for longer focal lengths [driven by aperture for fixed f-ratio]. 

 I am not a fan of the description of f-anything as slow or fast [other than the description of the optical beam].  Rather they should be called  wide or narrow-field systems, as in much of professional astronomy.  

I am then driven by achieving a desired image scale per resolution element [ideally 1/2 to 1/3 of seeing disk] and a field-of-view close to the size my favourite objects [up to 30arcmin] while maximising  SNR in time available.  In this case aperture does matter.  Yes, I may have to bin - with a consequent read noise hit, but it is rare - even in Bortle 2 - to be anything other than sky noise limited.  


Having said that my reasons to upgrade from RC8 to RC12 were as follows

1) There is no effective field reducer/flattener than gives good imaging over a larger APS-C or full-frame sensor size with the RC8.  At least not one I could find.  I think because any such corrector would need to be substantially larger than the 2inch focusser on the RC8. [You may think I am crazy trying to use an FF sensor on an RC and I wouldn't disagree.]
2) The 0.67x reducer will give focal reduction, but exacerbates the non-flatness of the RC field.  I gave up on the  0.67x reducer very quickly because of image quality more than about 6mm off-axis.  
3) Even the 1x field-flattener I now use with the RC8 and FF sensor gives poorer images in the field corners, coupled with vignetting.  This is now correctable with BXT, but still results in SNR loss. 
4) The RC12 system I have bought -comes with a 0.8x reducer giving good imags to the field edge [or so the spot diagrams claim]. 
5) Thus I will have an f6.4 system wth good imaging over a sensor area about 1.5 - 2x as large as before. 
6) For observing, I used bin2 with the ZWO6200MM on my RC8, and will do the same with the RC12.   This format not only better samples my seeing disk, but also still give me better than 4K images- adequate for my purposes. 
7) Working at f.6.4 over a larger corrected and less vignetted field should also help with off-axis guiding, reducing tracking rms.

So, I think there are a number of real gains to be made by going to a larger RC, particularly if one is interested in full-frame imaging.   Whether it is worth the additional expense - well I will let you know if it works out for me.  


CS Brian