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Wednesday, June 1st, 2011
A Direct Comparison Of 200mm f/4 and a 90mm f/7 Scopes
 
I've had a few conversations recently about the difference between my Vixen R200SS and my Vixen ED103S.
 
The R200SS
Aperture: 200mm (8")
F Ratio: f/4
Design: Newtonian
 
The ED103S
Aperture: 103mm (masked down to 90mm due to spacers in the light path causing unwanted diffraction spikes)
F Ratio: f/7.7 (but f/8.8 after the masking and f/7 after introducing the reducer)
Design: Apochromatic Refractor
 
I've used both of these scopes for over a year each and shot many of the same targets with them. Some are from my house in a Bortle 5 suburban sky and some from a Bortle 1 dark sky location.
 
In the comparison below you'll find a crop of M13 and the Veil. Here are the specs on the images for both scopes:
 
M13 (for both scopes)
Location: My house
Sub Exposure: 5 minutes
Total Duration: 90 minutes for the R200SS and 120 minutes for the ED103.
 
Veil (for both scopes)
Location:
Sub Exposure: 5 minutes
Total Duration: 90 minutes for the R200SS and 120 minutes for the ED103.
 
As a side note, the typical difference between 1.5 hours and 2 hours of shooting through any scope is negligible. In order to see significant increase in detail, you'd need to exponentially increase your exposure time, so don't think that adding 30 minutes to the ED103 shot is going to make a significant difference. I could have just as easily stacked only 90 minutes and we wouldn't really see much if any difference - my purpose in the additional time is just to get more subs for increasing signal to noise as opposed to more signal. The point here is that I'm not doing anything to compensate for the smaller aperture or the longer f ratio of the refractor.
 
In both images, the ED103 is on the left and the R200SS is on the right.
 
The bottom line of the point here is that all of the math and the conversation says that the 200mm aperture should outperform the 90mm aperture and, for sure, f/4 focal ratio should outperform the f/7 focal ratio - even more significantly since we are both doubling the aperture and cutting the f ratio in half. The R200SS should *significantly* outperform the ED103S... but it doesn't.
 
Every faint star, every background galaxy, every wispy bit of detail is in both shots. They are both picking up the same magnitude of objects and displaying them at roughly the same brightness.
 
So my conclusion is that at these pretty small objective sizes (100 and 200 millimeters) it makes little to no difference between them and additionally the speed of the f ratio between f/4 and f/8 also makes no difference.
 
Now, I have a friend who has a C14 with a Hyperstar in it... THAT makes a difference!
 
So where is the breaking point? I don't know. Maybe it's tripling or quadrupling the aperture or f ratio. But at these smaller aperture it doesn't seem to be at doubling it and after this experience I don't think that reducing an f/7.5 refractor to f/6 is going to make a speed difference at all - even though that's what we're told.
 
Perhaps if you have a 200mm f/4 and a 200mm f/12 you will see a difference. Or if you have a 100mm f/8 and a 400mm f/8 you will see a difference. But comparing a 200mm f/4 to a 90mm f/7 makes no difference at all it seems.
 
Granted, to make a more fair comparison, I'd do them on the same night, or even side by side at the same time with 2 cameras, but I've shot many of the same targets from the same locations with both of them using my same acquisition and processing workflow and the results are always the same: Same amount of detail and background stuff, sharper images with the refractor. For me, 100% of the time, the smaller, slower refractor wins.


Image Gallery For This Session
M13 Compare Veil Compare
   

M13-Compare.jpg
Click picture above for larger version
If the picture seems too bright or too dark, try adjusting the brightness below.
Brightness:
|
  



Comments:
On 06/05/11 at 04:55pm Simon wrote:
I have a feeling I was the instigator of this thread, or at least the timing of it. I came across Neil's journal entries while searching for information on both the Vixen R200SS and the ED103S. Neil kindly helped out with the questions I asked and much more besides. Initially I was fairly convinced the 8" reflector would be the better option simply because it's faster with a larger aperture. I live in the UK where clear nights are at a premium, so obtaining the maximum number of usable subs is a distinct advantage.

Although I've been bouncing back and forth between the two scopes a little, the refractor does now seem to be the most logical choice. It's lighter and therefore easier to mount with a focuser which should provide enough back focus for a CCD camera with filter wheel in the future. I've still to see these scopes "in the flesh" to make sure I'm happy with the overall mechanical quality, but I think it's almost certain I'll end up with the ED103S. Finding a second hand Stellarvue SVR90T for sale at half the price of the ED103S has certainly made the decision more difficult, but ideally I wanted something with a little more focal length than 630mm and a bit more aperture than 90mm. Stellarvue scopes are as rare as hen's teeth here in England and by all accounts the customer support is legendary so I hope I'm making the right decision!

On 06/04/11 at 01:43am Miquel wrote:
Great comparison Neil. I enjoy the analysis of the pictures and the scopes.

On 06/01/11 at 02:54pm Neil Heacock wrote:
I'm sure I've got screenshots around, but my guiding is pretty much always .3 RMS or so. Guiding was not typically any better or worse with either setup.

I recently made some changes which made a significant difference, but I chose not to use a recent image for the comparison. The M13 and Veil shots with the ED103S are both from 2010.

On 06/01/11 at 02:44pm Greg wrote:
Your conclusion seems absolutely correct. The term "detail" is problematic because both the depth of the exposure and the sharpness contribute to detail - sometimes in ways that are difficult to distinguish. In the M13 image it is apparent that the diffraction from the spider and/or secondary is degrading image quality. And, of course, fast optics are always more problematic. I'm sure that when Steve says "It's a good mirror" he means it, but he also means "for an f/4". I don't know how much better an f/4 mirror needs to be compared to an f/6 to produce the same image quality, but it's certainly a significant amount.

Another consideration is whether guiding quality enters into it. The 200mm reflector is likely to have more difficulty in guiding as it has both a greater moment arm and greater surface area for wind. Do you have any data to compare guiding with these 2 scopes?




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