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Friday, March 5th, 2010
First light with the Vixen ED103S.
Optical Design: Apochromatic Refractor
Objective Diameter: 103mm
Focal Length: 795
Focal Ratio: f/7.7
Optics Type: Air Spaced Doublet
Glass Material: Extra Low Dispersion
Optical Tube Weight: 7.9 lbs
My Initial Take
I'm already loving this scope! It was so nice to not have to let my scope cool down, check collimation, or try to overlook blocky stars and diffraction spikes.
I really like the overall results of the R200SS, but mechanically I'm more satisfied working with a refractor I think.
And I have yet to really put it to the test, but in my test shot, the refractor doesn't necessarily get outperformed by the reflector.
For visual observation (using the flip mirror) the stars were stunning. This scope definitely gives that "diamonds on velvet" view. Very high contrast and although I'm no connoisseur of chromatic aberration I saw no hint of CA at all.
Flatness of Field
William Optics "Flat 2" 0.8x Reducer/Flattener
Initially, I tried the William Optics "FLAT-2" 0.8x Focal Reduer Field Flattener assuming it would be the right thing to start with. It turns out it must have over-corrected because it made the edges worse and the elongation starts much closer in. There is much more of the field needing cropped then I'm happy with losing. Perhaps I can tweak it, but I doubt it. It just so happens that this shot also has the most vignetting.
So next I tried prime focus and was pleasantly surprised at how much of the field was flat. Even still, I'd need to crop out perhaps 600 pixels or so on either side losing as much as 30% of the frame if I only wanted to present the best stars. Not ideal, but better then the results with the FLAT-2.
So I tried the MPCC.
Much to my surprise, it flattened the field quite nicely! It's still not perfect, but gives me essentially 100% of my frame. In fact, in my test shot - because I switched targets - there are several galaxies just off the very edge of the lower left corner. With the FLAT-2 and Prime I would have cropped those out because of the poor surrounding stars. But with the MPCC, the corners are keepers so I'd be able to confidently present those as part of what I actually acquired in my imaging effort. Woot! As it turns out, this corrector produces the the least vignetting.
The Field Comparison Images
These are 100% resolution and roughly 800 pixel wide sections of each corner of a single unstacked, unprocessed, uncropped image starting at the outermost edge. The presentation of the corner placements match their placement in the original (top left, top right, bottom left, bottom right). Each image is labeled for which corrector or prime was used.
So one of the things I'm trying to get rid of is diffraction spikes
and this new refractor actually has some... doh! There are 3 clips on
the cell that are just obstructive enough to make a 6 spike pattern on
the brightest stars. I'll be looking for a way to mask that to
eliminate those spikes.
See a picture of the cell in the gallery.
First Light Target - M51
So, the first light target was M51 specifically to compare it to the previous session's M51 taken with the R200SS. It's surprisingly comparable given that it's 4" of aperture and f/7.7 as opposed to the R200SS which is 8" of aperture and f/4. I'm honestly a bit confused about that. This shot is 2 hours of 10 minute subs where the R200SS shot is 3 hours of 5 minute subs.
M51 - The Whirlpool Galaxy Image
- View The Full Scale Version 1.7 MB
I framed this shot (via crop) specifically to show the galaxy cluster to the left of the bright star HIP66004. I've never noticed that cluster before and I can't seem to identify it yet. It's not in my Starry Night Pro Abell, Swicky or other catalogs so please post if you know what that cluster is.
Scope: Vixen ED103S @ f/7.7
Mount: Orion Sirius EQ-G with GoTo
Guiding: Meade DSI Pro and PHD Guiding
Guide Scope: William Optics ZenithStar 66 SD
Camera: Canon EOS 1000D (Modified)
Special Settings: None
Exposure: 2 hours (12 x 600s)
Processing Software: Acquired in Nebulosity with high dithering,
Stacked in Deep Sky Stacker (Kappa-Sigma Clipping for all frames),
Levels, Curves, Astronomy Tools, Gradient XTerminator and Noise Ninja
Support Files: 40 flats, 40 bias, one dark used as a bad pixel map
See Gallery below for image
Image Gallery For This Session
01-M51-2h-ED103S.jpgClick picture above for larger version
On 10/29/15 at 09:22am Javier Polancos wrote:
Hi. I Have got the same tube and haD the same time spikes in the photos. I made a plastic piece to hide the spacers.
Obiously the aperture now is 3mm borrower but is better than have the spikes.
On 03/14/10 at 05:14pm Miquel wrote:
The M51 shot is fenomenal and if I understand correctly. The shorter exposure and slower refractor, it is outweighting the faster reflector. I don't know it is just me, but I can see more detail in your refractor shot than in your reflector shot. This is great! I'm starting to feel happy about my F12 Mak. Just kidding!!! ;)
Seriously, nice shot Neil! I love it.
On 03/09/10 at 04:04pm Barry wrote:
Huh??? LOL I wondered why I seemed to have unlimited film in my DSLR.
On 03/09/10 at 10:49am Greg wrote:
Thanks for the info, Mark. I still haven't read through it all (and didn't understand everything I did read!). I had run across a simpler form of this "myth" before and thought that might be it.
In the case of Neil's experience (roughly equal focal lengths, but very different aperture and f-ratio) I certainly can't explain the results. That is, the "myth" arguments are around whether aperture or f-ratio are more directly related to SNR (and the side arguments about what kind of SNR and whether that is important to "information"/good pictures), but Neil got a good (perhaps better) image with both less aperture and a slower f-ratio. I think the experiment is not sufficiently controlled to form solid conclusions. Obviously, there are differences in the optical characteristics beyond light gathering power, etc. The newtonian has lower contrast (effectively higher background level) due to the obstruction. Both scopes have some residual aberrations, but of different types. And, truth be told, a slight difference in focus accuracy would make the whole comparison meaningless. We would need to see many samples made very carefully to draw good conclusions.
Or we could just say "Great image, Neil!" and "That scope looks awesome!" Until I have more time and inclination to pursue the matter, I think I'll go with this latter plan.
On 03/07/10 at 11:52pm Mark wrote:
Hi Greg -
I wont be able to do the argument justice, but basically it comes from film photography where the focal ratio and exposure time sets the signal to noise ratio of an image - intuitively shooting at f4 is "faster" than f6 for example, and the exposure time can be scaled depending on what f# you're shooting at.
With a DSLR or CCD and telescope the focal ratio does impact the signal to noise ratio, but the aperture of the telescope and the nature of the image array also comes into play. More aperture always means more light which seems simple enough but the interactions take some thought to work out in understanding the quality of an image you can take with various telescopes and cameras.
There are lots of discussions on the web about this - when I started imaging I tried to work out the math and found that others have done a better job than I could have so a couple of links are:
For Neil's equipment, there are 2 telescopes with the same focal length (800mm roughly) but different apertures - the Vixen refractor is 103mm and the reflector is 200mm. Ignoring the mirror reflectivity and number of optical surfaces, the reflector has 2X the aperture and thus 4X the light gathering power. It's a faster scope by definition - f4 vs. f7.7 which is really acknowledging that the aperture between the two scopes is different but the focal length is the same, so the focal ratio has to be faster for the reflector for this to be true.
The image scale of am imaging system is given by the focal length of the telescope and the pixel size of the imaging array. The image scale realaly means - what is the smallest piece of the sky I can image on my pixel array and still see any visual difference between individual pixels. If you;re image scale is 3 arc-seconds per pixel, anything that has detail smaller than this wont be imaged as you cant divide individual pixels into sub pixels and form a better image (though drizzle procession tries to do this with some mathematical operations).
The field of view for each imaging system is given by the image scale in arc-seconds per pixel multiplied by the pixel array size. With the focal length of the two scopes being the same the image scales will be the same (using the same camera), and if the same camera is used the field of view will be the same. The reflector has more aperture to achieve this, so it should have higher resolving power to show finer details, and because its faster, it should achieve the same image quality in less time than the refractor.
The reality is that this isn't as straight forwards as it seems due to noise factors in the amount of light hitting
the image array and the noise characteristics of the camera. Faster images using a bigger aperture will gather more light, but it will also gather more sky glow, and because the image time can be faster, the read noise of the camera might start to dominate the image. Taking longer images helps get over the camera noise, but then the sky glow might start to dominate the image, or tracking errors on the mount might start to have the stars trail, so in the end there is a balance between imaging time and image quality.
When I tried to work this out in my head I learned a lot about what can be considered "signals" what is "noise" and what do we really mean when we say "signal to noise ratio". In the end to me, I want to take visually pleasing images and trying to figure this out has helped me make improvements.
On 03/07/10 at 05:15pm Greg wrote:
Mark, what myth is that?
On 03/07/10 at 12:32pm Mark wrote:
Nifty! I like a good refractor as well, it's a nice and simple point and shoot experience. I've found that I'm a visual purist - generally I don't like diffraction spikes so the ED103S gives you a nice compliment to your R200SS. Maybe you could settle the focal length ratio myth with the two rigs - similar FL, different f# and field of view!
On 03/07/10 at 12:34am Barry wrote:
Gorgeous!!!!!!!!!!! Now, THAT is an astrophoto!
On 03/06/10 at 08:41pm Dave wrote:
congrats on new gear..!
funny how smaller scopes seem to work as well...scientifically odd.........less photons and higher f#,etc..(???)
We will see/hear how this works out for you...!
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