Thoughts on new toy decisions?

[ravnostic]

While I’d love to get one of these:

http://www.bhphotovideo.com/c/product/732108-USA/Canon_4411B002_EF_300mm_f_2_8L_IS.html that’s still **slightly** out of my budget.

I’m looking to spend a grand. I’m considering a long and fast prime lens, or the $800 f/2 converter for my telescope. Thoughts?

Know of a place to get a good (and reputable) deal?

[sleeping]

What do you want it for? I mean, if you’re willing to go all manual you could probably get a used 300/2.8 (or maybe even a 400/3.5) AIS Nikkor and an F-EOS adapter for < 1000 without too much trouble. New, I'm pretty sure you're looking at slower or shorter.

http://www.keh.com/camera/Nikon-Manual-Focus-Fixed-Focal-Length-Lenses/1/sku-NK06009045243N?r=FE

[ravnostic]

For fast glass, I’d almost certainly have to go used (though Tamron has some stuff in my price range, and I’ve loved my 70-300mm–beats out my Canon one every time.) Piggybacking on the scope is totally doable, though, so fast glass seems a better purchase since I could use it elsewhere beyond astrophotography.

But…I really, really want that f/2 telescope rig…the shots taken with it are phenomenal, and it’s 20x faster than using the 6.3.

Decisions, decisions. If I could swing a 400mm, that would probably sway me as with the crop factor it’s more like 640mm.

[sleeping]

But…I really, really want that f/2 telescope rig…the shots taken with it are phenomenal, and it’s 20x faster than using the 6.3.

But that’s because the focal length is also ~1/3 of what it was before, right? I can’t see it making the aperture bigger…

[ravnostic]

It does, sleeping, because the f/2 is a function of the focal length, shortened by (appox 2/3). Over the 11″ diam. mirror, that makes a huge difference. The effective FOV goes from about 1/2 an arc minute to 2.9 arc minutes. But that’s not a bad thing–I can’t even get andromeda in it’s entirety with my current rig, and the moon is even a challenge (as is the sun, same difference…) when full. Granted, subjects would not be as large–but they’d be a heck of a lot brighter because I’m capturing all the light in that FOV bucket.

[Plamadude30k]

A 1/2 arcminute FoV is incredibly tiny. The moon is about 32 arcminutes (~0.5 degrees) in diameter on average, and Andromeda is about 3 degrees.

[ravnostic]

Oops–scratch 1/2 arc minute, replace with 1/2 degree, and carry on! (Hey, it’s a night shift–maybe I forgot my midnight coffee?)

[fluffybunny]

Hey rav,

I went through this decision process some time ago. I wound up getting a big achromat (120 mm f5) for < $300. You probably can mount it on that behemoth of yours too (might have to remove the dew shield on the 120mm). I guess it boils down to if you can tolerate chromatic aberration, but I'd be willing to guess there is a software solution for that (I know there is for my 10-22 f3.5 Canon lens).

[ravnostic]

Scopes are generally designed to limit achrom. ab. as a rule–though short of light band filters, nothing is ever perfect (and probably not then either). My understanding is camera lenses, with all the different elements, are serving the same purpose, no? fluffybunny

The thing is, I’ve often thought of longer glass when I’m shooting a bird from across a river, and I might be moving to a high rise in a year or so, so there’s always Hitchcock’s “Rear Window” angle. 😉

But other than that (and it isn’t much), I don’t know what I’d use it for. And yet except for maybe 1/2 dozen nights of astrophotography per year, I don’t know what I’d use the other for. It’s just the shots I’d want to use it for, are the ones I’d hope to be to die for.

//This whole glass thing. It’s complicated. I have time for this decision.

I don’t have much time for booking a hotel, so I emailed the Aunt to see if she’s a go, and I’ll be making bookings this weekend (for the solar eclipse gig). I’ll go up prior to and stake out locations, and maybe rent some glass//—///–__+++\

Jebus. WTF am I talking of spending on stuff I might not use often enough when I can just rent something for a weekend! It’s around a night’s stay in a 3.25 star hotel around these parts.

//Thread discontinued. Carry on.

[chupathingie]

That f2 rig would certainly tempt me, but there are some decent deals on some new glass out there (Tamron’s 200-500mm f5.6-f6.3 comes to mind) or you could go the extender route to reach out a little farther with your existing glass… so far I’m impressed with my 2x, been shooting deer all morning with it mated to a 70-200mm f2.8L (jesus, that thing’s like toting around a piece of artillery).

re: Achromat/APO and camera lenses… Lenses have all those elements only partly for color correction, the rest of the reasons fall into field flattening and keeping those optical qualities up to snuff as the lens is zoomed in and out (if a zoom, of course). Telescopes usually are concerned only with color correction; with field flattening/coma correction being the next-in-line flaw that folks purchase add-ons for. A good scope designed for astrophotography use will already present a flat field and an image circle of at least 40-50mm. Field flattening is not much of an issue for long focal ratios…for instance your field is already pretty flat at the scope’s native f-ratio, and somewhat more curved with the FR in place; although my money’s on your FR having been designed to minimize field curvature out of the box.

[ravnostic]

Extenders are cheap and mated to my 70-300mm Tamron might be the way to go (for now), thanks chupa. Damn you’re just full of lens knowledge! I wish I knew 1/2 what you do.

[chupathingie]

heh… I’m just chock-full of nearly useless trivia that enables me to take truly mediocre works of photographic point-and-clickitude. 😉

[ravnostic]

Chupa, Orionid,

I’ve taken the dead xTi into the resurrectionists…I’ll get the mirror fixed, but while they’re in there…

Should I convert to IR only (and if so can I stack with the reg. RGB astro shots?), or go for the IR filterless, but still keep the RGB pass?

I like the idea of having the IR only for the potential outside the astrophotography…but, can I stack an all-IR astro shot with an RGB astro shot? And insofar as the sky is concerned–IR seems limited, since our atmosphere kinda blocks things all together…but–that’s what the interwebz say should be done for good astro shots.

I haz confuzed. Input would be helpful–from anyone who knows more than me, which is most everyone here.

[chupathingie]

OK, removing the hi-pass filter will in fact improve the HA sensitivity of your sensor by 2+ stops. Personally, I’d leave the sensor filterless if you have the option since the added IR sensitivity can be corrected in post for normal photography; at least you’ll be able to dual-purpose the camera. Even tho the atmosphere blocks a lot, digital cameras all come with an IR block to prevent them seeing through clothing (resulting from a lawsuit against Sony years ago, if I’ve not been misinformed). If you wish to capture HA only (which really is a big deal, allowing shooting even with a full moon), use an IR pass filter in the imaging train of the scope or screwed into the filter threads of your lens.

**important** Make sure that whoever does the mod replaces the IR block filter with a clear replacement of the same thickness and refractive index. This isn’t any help for astrophotography, but insures that your autofocus will still function correctly for terrestrial photography. Autofocus is calibrated for having a filter in place, and a naked sensor will result in very soft autofocus.

[sleeping]

the added IR sensitivity can be corrected in post for normal photography;

I’m fairly certain you’d need an IR blocking (Hot Mirror) filter if you’re going to use an unfiltered camera for anything resembling normal photography in daylight (and possibly stack it with a UV filter too). The results you’d get it daylight otherwise will look like this:

Digital Infrared Filter Comparison Photos – Full Spectrum Clear Filter Image

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