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Why is "Fast" better for imaging?


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3 hours ago, Stub Mandrel said:

Just to queer your pitch

If you are addressing me Neil, then it's simply the case that a reducer will reduce the f-ratio by focusing a larger portion of the native image circle onto the camera.

A given lens cell or mirror assembly will produce a certain size image circle. It's a fixed parameter governed by the curvature of the lenses or mirrors. For the majority of us that run undersized camera chips, we miss out on all the photons falling outside the chip. A reducer will poke deeper into the light cone and catch some of those photons and will bring a larger FOV to focus at the expense of resolution (arcsec/pixels).

So it will indeed expose faster - f-ratio DOES go down, but only because it now collects photons from a larger portion of the sky.

With a given camera, pixel size fixed, you will ideally use focal length to match the camera to your seeing. Image scale is dictated only by pixel size (bin 1) on the chip and focal length. Once you have your match you can look at what happens if you were to open up the aperture for that same focal length by looking at higher end OTAs. Money spent rises faster than f-ratio drops, so better is not necessarily the word to look for.

As f-ratio drops through 5 and 4, focus will require some serious attention adding to the overall cost, and pushing 3 then some filters may well hit their design limit.

I have reduced all my OTAs, it sure made them faster but I do so to best match the OTAs to my cameras at my location. (Aiming for about 2 arcsec per pixel). There's certainly nothing wrong with barlowing or reducing but if you think of a reducer as a focal length reducer rather than an f-ratio reducer it tends to make more sense.

This isn't all aimed at you Neil, it's just an 'issue' that I think every astrophotographer should know very well.

/Jessun

 

 

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No, I'm just saying that photographers DO face some of these issues. Actually my teleconverter increases the f-number. i do have a wide angle converter as well, so I suppose that's a sort of focal reducer (although it goes in front of the lens) not convinced it decreases the f-ratio in practice.

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In photography (which I don't really know a jot about) you have the luxury if swapping lenses or stepping a lens down or up.

In astrophotography (planetary aside, which again I don't know a jot about) the aperture comes at a tremendous price and we're stuck with a given image circle. No magic helps here. We CAN buy a bigger CCD/CMOS to catch more of that circle at the back of the scope OR we can reduce the scope to shrink the image circle onto a smaller chip.

I wish we could swap lenses like on a DSLR which each have their own properties - f-ratio and such - but it's financially prohibitive.

/Jessun

 

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18 minutes ago, Jessun said:

I wish we could swap lenses like on a DSLR which each have their own properties - f-ratio and such - but it's financially prohibitive.

 

It is these days, luckily my instincts stopped me flogging all my old M42 lenses on ebay: 28, 58,135,400 and 500mm, plus 2/3x converter. Some of them are quite good too!

manual lenses, all ideal for astro

>smug mode<

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Well, the pixel pitch and focal length will give you your plate scale, but it's focal length and sensor size that gives you your field of view (Just like photography).

Incidentally, focal reducers have made an appearance in conventional photography to allow "Full-Frame" lenses to give their original field of view on crop-frame sensors. Google "speedbooster" They also give about a stop extra speed, hence the name.

And yes, as focal ratio goes down and the light-cone becomes steeper, focusing becomes much more critical, something I found with my new 80mm f/4.4, and NB filters can reach their design limits.

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