Shooting the moon and stars with a Rebel Xti ....lens help

[Smulls]

I have a Rebel XTi with a 17-85mm EF-s lense and a tripod. I was wondering if there was any way to take decent pictures of the night sky and moon with this set up. (im trying to learn but am very new) Also what lense uprade would you reccommend for this? (Im not loaded but suggestions of different price ranges would be very helpful.)

[TCampbell]

Two guidelines will help you.

1)  The moon and the "Loony 11" rule.

When taking a photograph of the moon, you can get a correct exposure using the "Loony 11" rule which suggests that if you use f/11, then the correct shutter speed is always the inverse of the ISO setting.  In other words, if you use ISO 100, then set the shutter speed to 1/100th.  If you use ISO 400, set the shutter speed to 1/400th.  This rule only works at f/11.  If you use a different f-stop (suppose you use f/8 -- which allows exactly twice as much light flow through the lens) then you trade a a stop of aperture for a stop of shutter speed (or ISO).  Since f/8 allows exactly twice as much light to flow through the lens as comapred to f/11, then you might set the shuter speed to expose for only half as long (e.g. ISO 100, f/8, and 1/200th would also be a correct exposure.)  With a camera, I'd keep it simple and just use f/11.   If the camera is connected to a telescpe then you can't control the focal ratio of the telescope (it is whatever it is) -- in which case you do a bit of math to work out the shutter speed.

2)  The stars and the "Rule of 600" (which you should think of as the "rule of 375" (or just round it to "rule of 400")

Since the Earth is spinning on it's axis, the stars appear to move across the sky.  Stars positioned roughly above Earth's equator move at a rate of 15 arc-seconds (angular movement) per 1 second of real time.  This assumes the camera is on a stationary tripod and not on a special "tracking" mount.

This rule was used for 35mm film cameras -- but the size of the image frame is important to the rule.  A "full frame" digital camera (like a 1D X, 5D series, or 6D) have a digital sensor which is the same size as a single frame of 35mm film.  But a Rebel XTi has a sensor which is a bit smaller... it's an "APS-C" size sensor and it's smaller by a factor of 1.6.  This means we have to divide 600 by that crop factor (1.6) and this gets us to 375 as our base value.

Here's the rule and how it works... you take the base value (in your case, it's 375 due to the sensor size of your camera) and DIVIDE that by the focal length of your lens (in millimeters).  E.g. if you are trying to capture as much sky as possible then you're probably using the 17mm end of the focal length.  So the math is 375 ÷ 17 = about 22.  That means... if your camera is on a SOLID tripod and not moving (not tracking the stars) then you can take an exposure which is 22 seconds long and you should not be able to notice the stars beginning to elongate (growing "tails") due to the movement of the Earth.

To take photos of the stars, you probably want to find the darkest skies possible... well away from urban lighting (light pollution) and also select a night when the moon is not in the night sky (e.g. find a date near the "new moon").  

You will still likely need to crank the ISO on your camera.  This is going to be a challenge because the max ISO on your camera is ISO 1600 and you'll probably need 1600 (it would be better if you could use ISO 3200).

These will likely be noisy images at ISO 1600.  

You can use lower ISOs and longer exposures if you want to capture an image of "star trails" (use a remote wired release to take 30 second exposures and just keep taking them.  Normally you'd use the remote trigger, put the camera into "continuous" shooting mode with that 30 second exposure per shot, and "lock" the shutter button on the remote trigger (the on-camera shutter doesn't have a lock but the remote trigger does.)  And just hang out for a few hours while the camera keeps shooting.  

You can use a free program called "Star Stax" to merge the images into an image which shows the long star trails.

If you do NOT want star trails but still want long images, then you have to use a "tracking" head or tracking mount.   These are devices such as:

• Vixen Polarie
• iOptron SkyTracker
• AstroTrac
• Losmandy StarLapse

As one might guess, the more expensive devices tend to do the best work and/or are the most adaptable.  All of these tracking heads attach to a normal tripod (tripod should be SOLID -- you don't want anything too "springy" or wobbly or it'll show in your images.)

[ebiggs1]

Again Tim's explaination is like a text book.  He is the man!  

"I have a Rebel XTi with a 17-85mm EF-s lense ..."

As you might think or already know this isn't the best combo for astrophotography but it will work.  But I did use a XTi for a long time and a good friend of mine, Tom Martinez, got many of his XTi sky photos published.

The first thing most people think is they need a long telephoto lens to photograph the night sky. The oppisite is actually ture.

A lens that is getting a lot of talk and use in the astro fireld is a pretty cheap lens.  The Rokinon 14mm f/2.8 IF ED UMC Lens.

It is MF but extremely sharp.  There is also a 24mm f1.4 model.  You might try one of these.  In astro photograhpy your ef-s 17-85mm will show its weakness.  The XTi will work!

[Smulls]

Now if I wanted to upgrade my xti what would be your suggestion without completely breaking the bank? Or would you recommend just upgrading my lense instead?

[ebiggs1]

You know I don't remember exactly how much the Rokinon 14mm is but if I remember correctly it is around $300 bucks.

It is also something you may want anyway if you are really serious about starting this facinating hobby.

I think I might try it first.  I know all my astro buds like it very much.  It is MF and not AE.  Keep that in mind.

About the camera, if you like the XTi, take a look at the newest Rebel T6i.  They have made tons of improvements since the XTi days.  Plus I assume you aren't solely interested in astrophtography?  You do use it for other stuff?    Yeah, I thought so, most of us do.

[ebiggs1]

BTW, the Rokinon 14mm will not be good for shooting the Moon.  You did mention that but we seem to have gotten off on just night sky (stars).    For the Moon you will need a good telephoto lens.  The best buy, as of today, is the Sigma and/or Tamron 150-600mm f5-6.3 models. Both in the $1100 range and very, very good lenses.  You choice as they are nearly identical.

[TCampbell]

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@ebiggs1 wrote:

BTW, the Rokinon 14mm will not be good for shooting the Moon.  You did mention that but we seem to have gotten off on just night sky (stars).    For the Moon you will need a good telephoto lens.  The best buy, as of today, is the Sigma and/or Tamron 150-600mm f5-6.3 models. Both in the $1100 range and very, very good lenses.  You choice as they are nearly identical.

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It is also possible to connect the camera to a telescope if you have either a refractor or compound (e.g. catadioptric scope such as a schmidt-cassegrain type or maksutov-cassegrain type scope.)   It "might" be possible if you had a newtonian reflector (including a dobsonian-mounted reflector) but reflectors sometimes have problems with the focal length (the camera adds about 2" to the focal length and the scope may not be able to focus with a camera attached.  This isn't a problem with a refractor or compound scope because those scopes typically employ a 90º "diagonal" at the back to insert the eyepiece for better viewing comfort and the diagonal also occupies about 2" of the focal length path... so you remove the diagonal and instead attach the camera and you're back to the original focal length (hence the refractors and compound scopes usually never have a problem with a camera attached.)

If you do have a telescope, then the accessories you typically need are a "nosepiece" -- which has either a standard 1.25" diameter or 2" diameter tube that inserts into the eyepiece seat (most eyepeices have either 1.25" or 2" barrels (there are _some_ exceptions).  The "nosepiece" has a standard "t-thread" at the back.  From there you get something called a "t-ring" specific to Canon EOS mount.  This is a ring which has t-threads on the front (to thread onto the nosepiece) and the back of the ring has the standard Canon EOS bayonet type attachment to connect the camera.  With these two pieces, the camera slips into the eyepiece seat of the telescope and you focus it just as if you had an eyepiece on the scope.

As for the Rokinon... it's low cost because it's a COMPLETELY manual lens (it has no electronics at all).  It has a manual f-stop ring and is manually focused.  But that's fine for images of the night sky because the camera would never be able to auto-focus on the stars (they aren't bright enough to lock focus) and must be manually focused anyway... and you're likely to set the aperture to wide-open to gather as much light as possible.

When ANYTHING in the night sky is sharply focused... then EVERYTHING in the night sky is focused (the distances are so vast that every ojbect is effectively at "infinity").  But do take time to carefully refine focus.  It's easy to think you focused the lens... shoot lots of images... unload the images on your computer only to discover that they were all just a little out of focus.

[ebiggs1]

"It's easy to think you focused the lens..."

This is so true.  Infininty does not necessarily mean infinity on a lens.  You may need to go past it or just atad short of it.