Something I've been thinking about for a few weeks is finding a lens that excels at capturing stationary stars. I'm going to paste some advice from another thread that I think represents the type of information I need to be searching for.
JimBuchanan wrote:
Steven, evenness of illumination is not the primary concern, shooting stars. Just comparing the Rokkor MC 50mm 1.4 versus the 58/1.2, the 50/1.4 is sharper across the field, at every aperture than the 58/1.2. You want to shot stars, you want a well corrected optic, and the 50/1.4 or any 50/1.4 will be better than a 1.2 optic, at any aperture. A 2.0 optic is, generally speaking, better corrected than a 1.4 optic. Yes, some companies have challenged that notion like Leica.
Use the slower optic at its optimum f/stop, at a faster ISO for better star fields.
I want the perfect lens. Fast, sharp across the field, well corrected, and with minimal light fall off (perhaps in that order of preference). Focal length really isn't a concern, so if it's the 24L, 85L, Rokkor 58/1.2, Rokkor 50/1.4, Pentax, Zeiss, Leica, whatever, I don't care. However, ideally I would like to keep the cost down as much as possible. So what's my lens? I'm thinking f/1.4 or faster; f/2 might be pushing it in terms of higher ISO, but I'm open to suggestions.
This might be a good case for comparing resolution test results. The old photodo used a weighted MTF parameter, based on results from more than one aperture setting and averaged across the frame. photodo also included MTF curves for some lenses, but not others. You can probably find MTF or other resolution curves for most newer lenses that would be worth considering - the trick will be comparing theoretical with measured data, for different ways of quantifying resolution. OTOH, the really good ones are really good, so you won't go too far wrong. Remember, "there is no perfect", ...I mean "spoon".
Here's the top nine from the old photodo site, with MTF(weighted) > 4.5:
Steven, I would assume, ideally, the lens would be APO to cut back on any color fringing. This could prove an issue with some lenses - especially wide open. I don't shoot stars, but I've seen some star shots where even very modest CA ( in normal photography terms) obscured some stars/planets - giving almost a double image effect - the celestial object and it's CA 'rider'; so I'll toss in the Leica 90/2 AA. No photodo rating.
The Contax and Pentax 50mm lenses seem like good starting points because they're relatively inexpensive compared to the Leica 90/2 AA, but I'm not averse to going all out the first time and getting the right lens for me. What's the average price for a Leica? And should the Summicron 50/2 be in the mix?
Okay then, the Leica 90/2 AA is out. It'll simply have to be a lens I lust after. That's too bad. Maybe I'll start with the Pentax SMC-F 50/1,4 and then the Summicron 50/2.
I have a non-APO summicron 90 on it's way along with the 35/2 summicron. Stars aren't usually my thing (lotsa light pollution here) but I'll give both a crack in that regard when my adapters arrive
Thrice, I would be most interested in any sample pictures you would be kind enough to share on the matter. I'm unsure when I'll be able to procure this "star shooter" lens, and even less sure about when I'll have time to go out and experiment with it myself, so any insights I can gain in the interim will be well received.
No matter how fast and sharp a lens you get, if you want perfect pointlike stars, you'll still be severely limited by trading off exposure time against sky movement and noise. You might consider setting aside ~$1000 from the cost of the lens to get a good quality tripod tracking mount, a motorized tripod head that rotates to keep the camera pointed at the same stars. Then you can use lots of slower but tack-sharp lenses (you're no longer limited to lenses that are super sharp at f2 or wider), shoot at ISO 100 to minimize noise, and take really long exposures.
mpmendenhall wrote:
You might consider setting aside ~$1000 from the cost of the lens to get a good quality tripod tracking mount, a motorized tripod head that rotates to keep the camera pointed at the same stars.
Good advice, though a $1000 mount is overkill for the typical "starscape" focal lengths. A $250 EQ-3 or similar lightweight german equatorial would serve fine for anything up to telephotos. For ultrawides, even a "barn door" improvised tracking mount would suffice.
The way to avoid the equitorial mount, and use a simple tripod, is to take a 10 or 20 or 100 for that matter frame series at limited shutter speeds versus one long exposure. Then feed the files into a "stacking" program, which produces a composite with much greater signal to noise ratio. Yahoo groups has a DSLR astro group with all the answers.
G. Thomas wrote:
I've been thinking about trying my hand at some astro. I'd like to know more about these barn doors.
The simplest barn door mount is a piece of wood of specific length with a polar hinge on one end, a tripod screw in the middleish, and a screw on the other end of known pitch to turn slowly by hand at a specific rate in order to match sidereal time. More advanced units incorporate a stepper motor so you don't have to sit there cranking the thing manually.
This information is fantastic, guys. Keep it coming! Although I don't want to increase the weight I travel with, I might be interested in this tracker pictured above. Where can I get one?
olyacme wrote:
The simplest barn door mount is a piece of wood of specific length with a polar hinge on one end, a tripod screw in the middleish, and a screw on the other end of known pitch to turn slowly by hand at a specific rate in order to match sidereal time. More advanced units incorporate a stepper motor so you don't have to sit there cranking the thing manually.
I don't know how much of a help this is, but Celestron makes a relatively cheap dual-axis motor drive for tracking stars:
