A Direct Comparison Of 200mm f/4 and a 90mm f/7 Scopes

[Neil Heacock]

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: Indian Trail Spring (Bortle 1)
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.

[Russ Ruggles]

Interesting. Goes against everything I've read. Perhaps I need to stop reading articles written by folks looking to sell something. :)