FAQ: prime focus film astrophotography primer
Posted 04 March 2004 - 09:17 PM
1) your camera
2) a way to attach it to your 'scope
3) a way to focus it
5) a way to guide your telescope (for longer exposures)
6) a way to polar align your telescope
1) The camera
If you've got an SLR, you can take astrophotos with it. Period.
If you've got a point-and-shoot, or a rangefinder, it will be much more difficult to get going. Go get a used SLR!
It MUST have a "B" setting (I think all do). That's it. Mirror lock-up is nice, mechanical "B" (otherwise you'll be going through batteries like M&Ms) is nice, replaceable focus screen is nice, all that stuff is nice, but all you really need is a camera to hold the film and a long term shutter. So, if you have ANY SLR, you're all set. If you don't, the following is a great discussion of which ones to look for from Jerry Lodriguss' website:
2) Attaching it to your scope
You need 2 things, a "T-ring" specific to your camera, and a nosepiece that screws into the "T-ring" and allows you to attach the camera to your eyepiece drawtube.
Get T-rings here:
Drawtubes come in 1.25" and 2" flavors - if you have a 2", get a 2" nosepiece to reduce vignetting, otherwise, well, you have to use a 1.25" nosepiece. Alpine Astro has very nice nosepieces from Baader in Germany (they call them "T2" system components, but they're just regular "T" threads):
If you have a refractor or SCT/MCT, then you'll be probably able to bring your scope to focus right away. If you have a Newt, you might not have enough in-focus to get to focus. You have four choices, easy to hard:
- Use a 2x barlow to get the focal plane "out" closer to the end of the drawtube - the downside is that you've increased magnification through your telescope
- Replace the focuser with a "low profile" focuser intended to allow you to get your camera closer to the focal point
- Move the main mirror forward (usually an inch will do it) in the OTA get the focal point "out"
- Buy a new scope!!!
3) Focusing your camera
You might think that you can just look through the screen and focus your camera, but there are a couple of things working against you.
- The typical "microprism" focusing area goes black at the slow focal ratios typical of telescopes (after all, your telescope is acting like a REALLY long telephoto lens)
- The rest of the focusing screen will probably be very dim
- Your eye "accomodates" meaning it's pretty flexible at making something that's slightly out of focus look IN focus
For initial shots of the moon, etc., you can just eyeball it and you'll probably get close, but if you get serious and want REPEATABLE focus, you need an auxilliary method that bypasses your screen and eliminates the effects of eye accomodation.
I use a Ronchi focuser for my Nikon cameras:
Again, Jerry has a great write-up about focusing:
Kodak Elite Chrome 200 slide film or Fuji Provia 400F slide film. Your lab can't screw up astro slides (other than cutting in the wrong place -- make sure you take a couple of regular daytime shots at the beginning and end of the roll) and since the image is positive, you can visually evaluate your slides without scanners, etc.
You might think faster (ISO800 and up) is better but a lot of faster films have poor reciprocity failure characteristics, so that the Kodak and Fuji end up being effectively as fast because they don’t lose sensitivity over long exposures as much as other films.
5) A way to guide your scope
With the previous 4 steps you’re all set to take pictures of the moon. You don’t have to polar align, or even be equatorially mounted, since the exposures will be short (fractions of a second). You WILL have to deal with “mirror slap” vibrating your telescope. If you have mirror lock-up, use it. Use a cable shutter release. If you still have vibration problems, try the “hat trick”:
For longer (10s of seconds and more) exposures, you need a way to guide your scope.
Since you’re just starting out, an auxilliary guide scope with reticle eyepiece is the easiest way to go. Any scope that can be rigidly mounted next to your imaging scope, of at least half the focal length of your main scope, can be used. Orion sells a relatively inexpensive guide scope setup:
Astro-Physics also makes a more expensive guidescope:
I like the following reticle eyepiece:
There's an excellent discussion of HOW to manually guide (and the tolerances required) in Robert Reeves excellent book "Wide-Field Astrophotography"
6) Polar drift alignment
If you want to take long exposures, you need to be accurately polar aligned to avoid field rotation. Drift alignment is the most accurate way to align to the pole. Here’s my favorite webpage on how to do it:
If you have an STV (then you aren't a beginner!), you can use it to polar align:
Posted 06 March 2004 - 08:53 PM
That is a very nice primer - short and useful.
I would like to stress that point 3, focusing your camera, is critical. I would advise every beginner astrophotographer to invest in a quality ronchi or knife edge focusing system. There is nothing more frustrating that taking that perfect, 30 minute guided shot only to find out the stars are out of focus. I use the KE focuser and finding the correct focusing position is *completely unambiguous*. You can be almost blind to use it. The one I use is from Borg (supplied by Hutech):
It's a superb knife edge focuser.
More advance astrophotographers using prime focus setups should also consider getting a field flattener/reducer. Sharpens stars out to the edge while reducing the exposure time.
Posted 11 November 2004 - 03:56 PM
Thanks guys for the links and instruction here....awesome!!
Posted 12 February 2005 - 10:17 AM
what is its advantage over other mounting options such as a t adapter?
Posted 12 February 2005 - 11:16 AM
Tele-extenders for cameras have additional features such as diaphram coupling so that metering still works properly.
Posted 12 February 2005 - 11:50 PM
Posted 13 February 2005 - 01:28 AM
Posted 18 February 2005 - 07:24 PM
ah i found it in an old meade catalog...
seems you put the eyepiece in the visual back.screw the extender on then the t-ring....hmm not sure how 1/2 my eye pieces will fit but i got an old celestron 26 to fit...
Posted 05 March 2005 - 12:24 AM
Posted 14 March 2005 - 09:53 PM
Posted 31 March 2005 - 10:44 AM
Where can one expect to find the focus on a fix mirror Mak like a Mk67? I can't find it and suspect the focus to be inside the focuser as with the Newts.
Posted 31 March 2005 - 01:21 PM
1) use a piece of waxpaper without an eyepiece, point at a bright star, Sirius is handy at this time of the year. Hold the waxpaper behind the drawtube and move back and forth until you have a tight dot - there's your focal point. This assumes that the focus point is outside the drawtube, of course.
2) Note where the field stop is on an eyepiece by guesstimating where it is along the side of the barrel - measure from the top of the eyepiece. The field stop is usually located at the focal point of the eyepiece so that the edges of the stop appear crisp in the eyepiece. Stick the eyepiece in the drawtube and focus. Find the position of the fieldstop from the top of the eyepiece. There's the focal point of your scope.
There's a good chance the focal point is outside the drawtube, because Maks are designed, unlike a newt, to be used with a diagonal, which means the focal point has to be well outside the diagonal. Usually you can use 1.5x the size of the diagonal as the additional path length. If you have a 2" drawtube, then it's designed to accomodate the additional 3" of path length introduced by the diagonal.
Posted 31 March 2005 - 02:41 PM
For 1.25" extension tubes, I've found that a Barlow with the lens removed works nicely. I've been lucky and haven't needed a 2" extension tube yet, but I imagine that you could do the same with a 2" Barlow, too.
Posted 28 July 2005 - 02:33 PM
Thanks for this efficient primer
i found some sheets yet, specially in CloudyNights threads , but as a primer, what about exposure ?
Posted 28 July 2005 - 03:58 PM
Deep Sky Film exposure is not an easy calculation as it really depends on several things.
First: What are you trying to capture? If you want inner detail you need a shorter exposure than if you want faint outer nebulosity. Also the larger the angular size, the light will be spread out and require a longer exposure. The Triangulum galaxy (M33) requires a much longer exposure than the apparent magnitude would make you think.
Second: How bright your sky is will be the limiting factor in exposure length. This can best be determined by trial and error. On a good night, go out with your most used film and shoot the sky (no drive required) at several locations. Shoot in the areas you are most likely to shoot later on. Expose for 10 min, 20 min, 30, 45 and 1 hour. Look at your pics and see at what point the background light (fog) is the same brightness as the faintest stars. When stars become washed out - that's your upper limit. Any more exposure is useless.
Be sure to try different f/stops if using a telephoto lens piggyback.
Third: The film you use will also come into the equation. D not assume that a roll of 200 ASA is less sensitive than 400, as in conventional photography. Reciprocity failure takes care of that thinking! Some films do much better than others, and some are totally useless for long exposures. Be sure they can record the Ha spectrum. The only way to know is to either experiment yourself or look at what others have done with the same film and scope setup.. just remember the SECOND rule above.
I use Fujichrome 400F Provira and Kodak E200 exclusively, and they both get about the same saturation even though one is 200 ASA and the other 400. With my 8" f/4 system I can capture a lot of detail in 30 minutes. I can shoot for 3 hours if I need to before sky fog becomes a problem - but I live over a mile up (6,300 feet) with very little light pollution and almost no humidity.
The only thing I can say is to experiment.
And when first trying astrophotography - expose for longer than you think you need. You almost always wish you had exposed longer. You can easily turn down the saturation in Photoshop, but if you never recorded some detail in the first place you can't get it later!
Posted 29 July 2005 - 06:59 AM
1. That's a good calculator and probably very accurate for bright objects like planets, moon etc.. Faint extendd objects will still require some experimentation although you will have a starting point.
2. The program is Windows only.... and some of us would rather slit our wrists first.
Posted 29 July 2005 - 12:52 PM
sure i have to experiment !!
but some basics are usefull...
as i'm mainly interested with deep field, i must confess Peter's experimental method seems good
for a Mac OS/X version, maybe ask the prog creator ?
Posted 29 July 2005 - 01:08 PM
Posted 29 July 2005 - 02:28 PM
i'm quite sick about PC and all its problems : just reinstalled ALL my config, ran fine for less than 2 weeks :O
unable to send any email... Outlook Express corrupted !
i uninstalled it (impossible to remove Internet Explorer !) and i'm now using Opera as net-browser & emailer
i was on Mac years ago, but prices were too high for me... now, there are attractive products such as MacMini, but prices are fixed, we can't discuss, not real competition
but i'm too tired loosing time and data, always a finger on the 'save' menu... so i'll go back to Mac soon LOL
Posted 29 July 2005 - 03:03 PM
Posted 21 November 2005 - 09:02 AM
Posted 22 November 2005 - 12:01 AM
Posted 21 December 2005 - 03:49 PM
2. The program is Windows only.... and some of us would rather slit our wrists first.
Apple iMac G5 w/ 1gb RAM
everytime I try to do anything with my wife's 3ghz Dell Laptop I just want to throw it like a frisbee across the room. I've never seen someoen spend more time defragmenting, Registry fixing, Virus repairing than actually using the unit. I don't have to do any of that with a Mac
Posted 21 December 2005 - 03:54 PM
I did... he's clueless about the Mac. He (and 99% of the PC world) have no idea how far Apple has come in the past 5 years. OSX Tiger would dazzle any computer geek... if they would give it try! I've been working on PC's since 1988 and I am blown away by Tiger.
Tiger (OSX 10.4) is by far the most advanced and USER FRIENDLY of any OS available today - including Linux
what most people don't realize is that Windows is a copy-cat of Apple's earlier OS's. IMHO a Windows OS will NEVER be superior to a Mac OS. the architecture is different and there's no 3rd party hardware to foul stuff up
it's just amazing what you can accomplish when all the internal hardware relates to itself