Excellent video. As I mostly image with a short focal length refractor, dithering and drizzling are useful tools I use on a regular basis. And then of course, there's blurXterminator...😀
Good topics but outside the pale of this short video. And I wanted to stick with Starizona's conclusion from their article on Nyquist's work: "Consider that very few of the impressive images in magazines and on the web, and very few of the images on the Starizona Guide to CCD Imaging site was taken with a system producing a 2"/pixel resolution! Some were taken at 4.2"/pixel, while others are at 0.5"/pixel. Normally the most important factor is field of view. If you are imaging a large object you need a short focal length telescope. For small targets, a long focal length scope is necessary. Getting the object to fit in the field is a far more important consideration than arcseconds per pixel!" You can read the full article in the link in the description of the video.
Another thought-provoking video - thank you! I have small refractors at f/6 (60mm and 72mm) and a newtonian at f/5 (130pds) and a C9.25 classic at f/10 (235mm). I've long suspected that the f ratio emphasis has been somewhat overdone. The number of photons coming from a small region of the sky doesn't change when you change aspects of your telescope. Following my gut somewhat I've been working to get the target filling up as much of my field of view (which also means sensor real estate) as possible, while trying to avoid doing mosaics where possible. Especially for targets with larger angular or apparent size. It's nice to get a better understanding of this thanks to this video of yours. Now I have solid reasons behind my choice of telescopes. Oh yeah, for readers wondering if I have just the 1 mount and I have to choose each night what telescope to use - no! Each has it's own mount, with the C9.25 on a very hypertuned CGX and the 60mm on a tuned modified AZ GTi. And that also means 4 different cameras... Madness, but fun!
Perfect video !! Actually my dream combo would be RC 12-16 inch on native F/8 coupled with modern CMOS sensor with >6 um pixel size. Well we all know that we can easily get the RC scope...
Is the dawes limit still relevant these days? If i get a cam with tiny pixels i can easily drop below dawes limit (and average seeing) very easily, but would my images benefit?
That's a good question, and very relevant. In my experiments, I have found that many old assumptions don't quite apply like they used to. And there is the simple fact that most any decent modern telescope is capable of resolving below the atmospheric limits on clarity.
So we shouldn't use a focal reducer unless we need it to fit the DSO in the frame? What does this mean if I can use a smaller pixel camera to get effectively more focal length. Which would be better?
I think Starizona's conclusion is you should prioritize going about whatever strategy allows you to best fit your subject to the sensor. Tools like reducers give options that you have to weigh against your goals. For example: In some situations you may find the faster F ratio that comes with using the reducer outweighs the higher focal length you'd get without it.
You mentioned in your video a misconception of aperture and f/ratio vs speed of a scope. To get the higher focal lengths you are talking about, is going to take a lot of aperture. More aperture equals more light gathering capacity, for instance I have an 82mm f/5.6 refractor and a 12” SCT at f/10, going by yours and many others you say the same premise would say my SCT is much slower at f/10 than my refractor at f/5.6 but that just isn’t true, my SCT has almost 14x the surface area to gather light than my refractor. Hence more light gathering capacity equals brighter image and a “faster scope” if you double 5.6 you get f11.2 which will require 4x more exposure, but my SCT has 14x, then double it again to f 22.4 then you need 16x more exposure but that’s at a given aperture, so going by that my SCT would have to be around f16 to give the same exposure setting of my refractor. Given the fact that my SCT has 14x more light gathering capability, it is actually faster at f10 than my refractor at f5.6. The math tells it all. This whole slow scope, fast scope stuff needs to come to an end, there is no such thing in astrophotography these days, that went the way of film when you couldn’t shoot numerous sub frames to get an image and had to manually guide for an hour or more to get one shot. Who, these days shoots, for deep sky astrophotography, less than say 3-5 min sub frames, and some shoot 10min subs, but nobody shoots like in the days of film, one exposure for an hour or two. Unheard of today.
I will probably explore a related idea when I cover the role of FL, aperture and pixel size. One might say the additional light gathering capacity enables the higher FL. It also enables higher resolution, and that is one of the biggest advantages of wide FL. But the F ratio is a constant: FL divided by aperture. A gigantic hypothetical lens of F20 will yield the same exposure time as a tiny hypothetical lens of F20. While this may not seem to make logical sense, bear in mind light is a wave, and wave physics do not follow the common sense principals that typically apply to material physics. That's why so many of us are always chasing F ratio, which can come in the form of shortening FL or expanding aperture.
