Astro, How to

Taking Astronomical Photographs with the Coolpix cameras

I've had some questions about how I took the astronomical photos on this site with the Nikon CP900, CP950 and CP990 cameras. I'll try to outline what I did here; it is pretty simple really!

Introduction:

All of the Coolpix cameras have the ability to take pictures of the Moon, and brighter planets and stars through a telescope. In addition the CP950/990/995 can take wide angle shots of the stars to show constellations. None of these cameras is a true astronomical CCD camera; those are far more sensitive and able to take much longer exposures. However, it is possible to have quite a bit of fun with what the cameras can do.

Constellation shots:

Well, nothing special here. I used a tripod. The CP900 is not able to take star photos due to the limitation of 1/4 second on the slowest shutter speed. The CP950/990/995 can. On the 950, set the camera to aperture priority, and select the largest aperture. The camera will automatically select an 8 second exposure. I do it this way because you can't use shutter priority with the higher ISO settings. You can try higher ISO if you like. On the CP990/995, you can use manual mode. Be sure to set infinity focus. You will probably want to subtract a dark frame to clean up the shot.  I generally use wide angle so that I can get the lowest f number. It is not necessary to guide the camera during an 8 second exposure, but stars will trail with longer exposures with the 990/995. To minimize vibration, you might want to try the "hat trick". This is where you cover the lens with a hat (or your hand or piece of paper, etc), start the exposure, count to 1, then remove the hat. This gets rid of vibrations in the camera. Also, keep in mind that the infinity setting of the CP950 only might not be in perfect focus for some cameras at the telephoto end of the range. See this page for details. You will have to figure out how to get the camera focused if you want to use the telephoto.

The Moon and planets:

It seems to be "common knowledge" that you are supposed to buy a camera adapter, and attach the camera to the telescope to take pictures. To start with,  I simply held the camera up to the eyepiece and snapped away. This actually works very well for the Moon and planets if you are fairly steady. If you feel that you want to attach the camera using an adapter, go right ahead. Lately, I have tried using an adapter and a cable release, but I'm not sure that the results are any better than simple hand holding. See them here.  Here is a link to a page where the camera is attached. Attached Camera. Take a look at it, there is a nice photo of a globular cluster taken with an 8 second exposure. Note that on this page the camera appears to be supported by the lens, which although Nikon does not recommend it, seems to be harmless to me, so make your own decision about it. Here is another page, where Denny Cannon shows how he made a bracket for his C8 that supports the camera with the tripod socket and attaches to the eyepiece itself.. It looks pretty easy to make.

 For the Moon and planets, you should consider just taking pictures through the eyepiece. If you want to scout out what can be done with longer exposures, then you will have to attach the camera.

* Nikon says that the camera should not be supported by the lens. This is what you would have to do if you used a camera T adapter. You would have to figure out some way to support the camera. However, many people have done this with no problems. The swivel joint in the 950 is very robust, one guy even bought an extra joint and supported a 50 lb weight with it. The 990/995 design is even stronger,

* The camera lens is not removable, so you have to use the "afocal" technique anyway. Yes, you can buy an adapter that will let you do this if you wanted to, but it makes the entire assembly a couple inches longer. I used the meade eyepiece projection tube.

* When using the afocal method, the camera has to be zoomed in all the way or the shot will be cut off at the edges. In addition, since the camera lens is some distance from the eyepiece, the field of view is VERY small, maybe only one arc minute with some eyepieces. This can make getting the camera aligned to take your shot difficult.

* Your selection of eyepieces is very important. Most modern eyepieces are not a good match to digital camera photography for several reasons. First, they have large eye relief. Low power eyepieces may even have a built in light shield that prevents you from getting the camera lens close enough to the eyepiece lens to avoid vignetting. Second, modern eyepieces have a large apparent field of view (how big the field looks when you look through the eyepiece). This actually reduces the amount of the field that the camera will be able to see. Thrid, modern eyepieces take advantage of their large apparent fields to allow some non-flatness near the edges since your eye can't take in the whole field at once anyway, but this works against the attached camera. The best eyepiece is an older eyepiece with crummy eye relief and a small apparent field!

* Only the brightest objects can be seen on the camera LCD anyway. This is much imporved on the 950, but you will still have to put in an eyepiece to get your subject centered. This is very true of faint stars.

* I did make a camera attachment (see the picture at the end) and try to attach the camera to the telescope. Because of the narrow field, dim LCD, and lack of a cable release for the camera, it is hard to get an object perfectly centered and then swing the camera in for the shot without disturbing it. In addition, the disturbance of the camera, even with a home made cable release, was just transmitted to the image. I found that the best technique was to hold the dust cap over the telescope for 1/2 or 1 sec when starting the exposure to let the vibrations die out..

* Taking hand held pictrues is not has hard or bad as it sounds. Try this experiment. Point your telescope at the Moon. Now back off from the eyepiece an  inch so that you just see a portion of the image; this is where your camera lens will be. Now move your head around a bit; you just see a different part of the image. You are actually just looking at the image that was formed by the telescope, moving your viewpoint a little does not move the image. When you take a picture of the image, it is just like when you take a picture of anything, and because your camera is not touching the telescope, the motion of your camera is not transmitted to the image. Anyway, try it for yourself.

* There are flip up mirror arrangements that you can buy. They are intended for attaching an astronomical CCD camera, which has an even narrower field than the Nikon camera. The mirror selects between the camera and the eyepiece. You might be able to use one of these to center objects in the eyepiece, then flip to the camera. I think this whole arrangement is going to be too large and heavy; especially if you somehow support the camera, and with the extra length that will be needed to do eyepiece projection on the camera port.

* Yes, the CAME program can trip the camera shutter through the serial port. You could use that if you wanted to. I was all set up to do this, using the HP200 version of CAME, but the limitations of the serial port (no manual control) make it impossible to use.. I have modified the HP200 version of CAME to work with the 950, email me if you want a copy. (HP200 only)

* Yes, for longer exposures , you are going to have to figure out some way to attach the camera. If you have a large massive telescope, you will probably be able to do it. A good guide scope or flip mirror could help you center your objects. When you are all done, though, you probably should have bought a CCD camera!  I have ended up using the modified Meade eyepiece projection tube, see a picture and details at the bottom of this page...

Moon and Planet photo tips:

* Even if you use spot metering, the metering on the camera will not meter that small dot of a planet. You will have to set your exposure manually. I usually use shutter priority setting. The lens will open up all the way because it is so dark.

* If you can, use a higher ISO to get a shorter exposure. The problem with it is that the 950 does not allow you to set shutter priority at high ISO, and if the planet is small, like Mars, the camera thinks that the field is totally black and sets an 8 second exposure. Because of this, you can really only use the higher ISO on the Moon where the camera will meter correctly. I experimented with the "hat trick" using a piece of cardboard over the telescope to obtain a shorter exposure, but no real luck. This is not a problem with the 990/995

* While I avoid using the digital zoom for normal shots, you can use it on planets. There is no information lost in using the digital zoom to blow that tiny planet up a bit so that you can focus better on the LCD screen. Using the digital zoom to focus is a great idea.

* The auto focus will not work on planets. It will sometimes work with the moon. What I do to avoid focusing problems is to set the camera for infiinty, and do all of my focusing with the telescope.

* On the Moon, you can use spot metering to target a spot on the daylight side, and then shift the camera over to take your shot.

* Take a lot of shots. Bracket your exposure and your focus. Planet shots are mostly black space, so they compress down nicely in the camera.

* Even a fairly low power eyepiece will result in high power because of the added zoom on the camera. You will not need your 4mm eyepiece.

* The CP950 "best shot selector" will not work on planet shots, and does not really seem to work on the moon.

* Use a rubber eyeguard on your eyepiece so that you don't jam the camera lens into the eyepiece in the dark and scratch it. Many rubber eyeguards can be folded over to get the camera closer to the eyepiece.

* Don't forget to turn off the flash, ha ha!

 

Picture

My first attempt at a home made camera support. It doesn't look like much here... but this is a camera tripod head bolted to a flat camera mount attached to my old Meade 2080 scope. The head is at a funny angle to allow the CP900 to come right up to the eyepiece. This also can work without the star diagonal. I used the tripod quick release to take the camera on and off quickly. It is possible to take longer exposures of stars through the telescope with this setup, see below. That is a junk eyepiece to keep the dust out, not the eyepiece I was using. I have also tried using an eyepiece projection tube, read about it on this page.

To take shots, I found the best technique was to have the finder lined up perfectly, and to just use the finder to center the object in the camera. Generally, if I could not see it in the finder, I could not take a picture of it anyway with just 8 seconds.

I no longer use this setup, I use the Meade eyepiece projection tube mentioned above.

Picture

Here is M13 in Hercules. 8 second exposure at ISO 320 with the CP950. On the original shot, there was a LOT of noise and you can barely see the stars. I have enhanced this shot considerably to bring out the stars, as well as subtracting a dark frame that was taken at the same time. I also tried to take shots of more diffuse objects such as the ring nebula without any luck.  This seems to be of limited usefullness since there aren't many things you will be able to capture. This shot is right at the limit of what you can do with the 950 and deep sky objects. Obviously, a CCD camera would do a much better job.