General astronomy discussion

Re: Why is there no low f/?.? optics?

SDB777 — Tue, 30 Jun 2009 18:24:52 GMT

Thanks for the replies.

What would the cost be? Cost, cost, and cost is a great 'beginning' of an answer, but it's not really an answer. After all, cost is only a factor, some here spend 10's of thousands on camera's....I've seen the photo's in the older posts(and they are amazing!). Computers dedicated to running CCD imaging and photo processing, small buildings with the only purpose of housing a scope, searching for just the right piece of property to get the best view of the night sky, and the list goes on..... Now all of those things aren't very 'cheap', and I wouldn't expect a high quality OTA to be 'cheap' either.....

Really....the thickness of an atom across 190mm-210mm's? Guess I was thinking in the wrong 'world', by trying to compare camera lenses.... After all Sigma(my favorite camera optics company) is producing 30mm f/1.4 and 50mm f/1.4(reasonable pricing), and up to 500mm f/2.8(getting right up there in cost)....I know, it's not quite the same.

I've done quite a large amount of 'fixed' tripod wide field imaging, and the light gathering of the little Sigma 50mm f/2.8 is quite fun, but it takes a lot of images just to get something halfway worth looking at. And I was thinking about my soon to be had Orion Mak-Newt 190mm with a f/5.3 taking twice as long to do the same thing(if I were to go for wide field image). This maybe be an apple to orange comparison, but if you reduced the f-stop on the little Sigma 50mm to f/5.6 I would need twice the amount of time to get the same amount of data to the sensor in the camera(correct?)

Say the typical(which doesn't really exist) exposure for astrophotography is 18x300sec(plus darks, flats, and bias frames) on a standard f/8 optic? Couldn't the same thing be done with a f/1.4 with 18x37.5sec(plus darks, bais, and flats)? The difference would be 78-3/4 minutes! Think of all the other imaging that could be done, all the clouds and wind that could be defeated, and the extra sleep before work!! Everyone likes some extra sleep....

Custom builders?

Scott (maybe they're holding them at Area51) B

Re: Why is there no low f/?.? optics?

CFB — Sun, 28 Jun 2009 16:05:40 GMT

I often use the EF200mm f/2.8 canon lens with my Digital Rebel XTi for astrophotography. It's fast, has a wide field of view (equivalent to about a 70mm telescope), the optical field is flat and can be consistently focused. Check out http://www.wvi.com/~rberry/imageprocessing/aip4win_dslr/aip_dslr.htm for more information.

CFB

Re: Why is there no low f/?.? optics?

chipdatajeffB — Sun, 28 Jun 2009 14:09:47 GMT

There are a number of builders working on F3 truss-tube dob optics, so watch for those. A big advantage is that a 20" F3 dob wouldn't require a stepladder for observing ...

Astrographs can be quite fast. The HyperStar camera-adapted SCTs have an effective f-ratio of F2. They're not cheap.

When you're talking about astronomical mirrors or lenses, a surface variation of roughly the thickness of an atom can make a difference. The larger the surface area, the harder it is to make it accurately. For the same diameter, the surface area increases as the curve gets steeper, and a faster F-ratio requires a steeper curve.

Re: Why is there no low f/?.? optics?

johnm — Sun, 28 Jun 2009 13:40:22 GMT

Cost- cost- cost! Plus the exteme difficulty of making telescope optics that fast and correcting all the aberations.

Why is there no low f/?.? optics?

SDB777 — Sun, 28 Jun 2009 13:35:48 GMT

Maybe the demand isn't that great, but wouldn't astrographers jump on some good quality fast optics? Say something in the range of f/1.4 to f/2.8?? Maybe a 190mm or 210mm-ish.....

It would be ideal for those prices to be under the $5K mark if any of ya'll optics companies are looking

Are there 'custom builders' out there that I just can't find? Anyone know of someone?

Scott (can't find one anywhere) B