by Jeroen Smaal

Introduction

These pages are about astrophotography with a digital camera in general, and the Nikon CoolPix 995 in particular.

Before I bought my current telescope(s), I made the decision that it had to be suited for occasional astrophotography. This meant I should be looking for a suitable camera and a solid equatorial mount.

Having owned a Nikon Coolpix 995 camera for some time, it was a logical choice to begin using it as a camera for astrophotography. Resources and pictures I found on the web about using this camera to photograph planets and deep-sky objects were very encouraging. A suitable mount was found in the form of a Vixen Super Polaris DX, which, as you can read on other parts of this site, has been superceded by the Vixen Sphinx (SX).

I’m very interested in getting feedback about these pages (send your feedback to astro@smaal.info) !

Astrophotography Equipment

Telescope and mount

A description of the telescope and mount I use for astrophotography can be found elsewhere on the site. What make the C8, and the Super Polaris DX (and SX) particularly suited for astrophotography, are the following properties:

1. •The Celestron C8 is a capable telescope with a useful aperture for astrophotography.
   

2. •The Vixen SP/DX and SX mounts are solid and offer many features that are useful for astrophotography.
   

The only thing that limits the C8’s usefulness for photographic work is its long focal length (f/10), which makes it a ‘slow’ scope with a relatively small true field of view (TFoV); It’s very easy to make the C8 into a faster scope by attaching an f/6.3 reducer/corrector, something that I will certainly try.

Digital Camera

Although I was not looking for an astrophotography camera when I bought the Nikon Coolpix 995, I found it to be suitable for astrophotography for these reasons:

1. •Small, threaded lens that can easily be connected to an eyepiece.
   

2. •All settings can be manually adjusted to suit the situation.
   

3. •Timed exposures of up to 60 seconds are possible.
   

4. •Built-in dark frame subtraction for noise reduction.
   

5. •Sensitivities of ISO100 up to ISO800 (up to ISO400 useable for long exposures).
   

6. •Relatively low-noise CCD.
   

Digi-T eyepiece adapter

Because the Coolpix does not have an interchangeable lens, using it for astrophotography meant I had to be using afocal projection (‘eyepiece projection’) to connect the camera to the telescope. The term ‘afocal’ comes from the fact that using this construction, there is no focal plane for the telescope & eyepiece to project an image (the image is projected at infinity; the light rays leaving the eyepiece are parallel); this is the same way your eyes ‘focus’ on objects very far away. For this to work, the camera has to have its focus set to infinity.

There are a some challenges to overcome when attaching (digital) cameras to telescope eyepieces:

1. 1.The digital camera has to be very securely mounted to either an eyepiece, or some other fixed part of the telescope.
   

2. 2.The camera’s objective lens has to be as close to the eyepiece lens as possible to prevent vignetting (restricted field of view, like looking through a toilet paper roll).
   

3. 3.The eyepiece lens has to be as large as possible, or at least as close as possible to the size of the camera’s objective lens, also to prevent vignetting.
   

4. 4.The telescope has to be able to bear the extra weight of the camera at the eyepiece end.
   

ScopeTronix Digi-T system

There are a number of products available to mount cameras to a telescope eyepiece. I found ScopeTronix’s Digi-T adapter to be suitable because it fitted to a number of eyepieces I already owned, provided a solid connection to the eyepiece, and could use a standard T-adapter to connect my camera. For my Coolpix I needed a 28mm T-adapter.

Please note: It has come to my attention that ScopeTronix is unfortunately no longer in business. There are various alternatives to the ScopeTronix Digi-T system.

Eyepieces

The Digi-T adapter uses three small screws that must be tightened to attach it to a notch in the eyepiece which is revealed when the rubber eye-cup is removed. The outside of the Digi-t adapter is a standard 42mm T-thread. The correct notch needed to properly fit a Digi-T eyepiece can be found on a number of eyepieces. A (partial) list can be found below:

1. •GTO Optics Plossls and Super Plossls (not the Silver Top Plossls!)
   

2. •Orion Sirius Plossls
   

3. •Meade 4000 series Plossls
   

I found that when the camera is securely fitted to the eyepiece, which in turn is securely fitted to the telescope, the scope and mount are easily able to handle the extra weight of the camera. I prefer not to use a diagonal during photography, but when photographing near the Zenit, it’s more convenient to use one. With the C8 it’s also possible to adjust the balance of the scope on the mount. Some combinations of telescopes and mounts do not allow this.

PC controlled shutter release cable

Although there are ways to do exposures of up to 8 seconds without touching the camera, longer exposures of up to 1 minute are only possible while keeping the shutter release pressed for that period of time (bulb mode). This introduces an intolerable amount of shaking in the image, so I have ruled out using this technique.

Enter the shutter release cable for the Coolpix 995. On the internet I found some sites that described using a serial protocol originally meant for the Coolpix 995’s predecessor (the Coolpix 990) for long-time exposure astrophotography (this site). What was needed to make this work was a modification to the original Coolpix 995 USB cable, or an additional serial cable originally meant for the Coolpix 990 (the SC-EW3). I chose the SC-EW3 serial cable. When this serial cable is connected to a (laptop) PC, automated, computer controlled imaging with exposure times of up to 1 minute per image becomes possible.

There are several pieces of free software, both for Windows and Linux environments, that can automatically take multiple images using this setup:

TheForce is very versatile because its functions can be accessed by scripts (VBS etc. – see the TheForce website).

What I did was take an old 486 laptop with Windows 95b installed that I still had lying around, and install TheForce, PhotoPC and the PhotoPC GUI on it (as well as TheSky and some other useful astronomy software). This allows me maximum flexibility during astrophotography and observing sessions.

See my astrophotography album page for some results (currently only the M3 and M82 image). In time, will be creating many more.

The learning process

OK, so I have the equipment and the software. Now how do I go about making those exciting planetary and deep-sky images? It’s a skill I’m still developing, but here are a few things I have learned so far:

Polar alignment is essential to good photography

When doing exposures of more than a small fraction of a second, it is evident that the object being photographed has to stay perfectly centered in the eyepiece for that amount of time. Even when doing several short exposures that would later be combined to one image using a stacking procedure, precise polar alignment is important because field rotation* caused by alignment errors will creep into your pictures, making stacking very hard (though not impossible).

It’s clear that the longer the exposure times, the more accurate your polar alignment needs to be. Note that I am not doing guided astrophotography (yet), so the accuracy of the polar alignment will be the most important factor in achieving perfect photographic results.

It’s also nice not to have to chase the object as it leaves the field of view of the eyepiece, as it is not very easy to make adjustments to the telescope when looking at the LCD of the camera.

At the least, learn how to use your mount’s polar axis scope. I found accuracy of the polar axis scope of my Vixen Super Polaris DX mount good enough for exposures of up to 8 seconds (which is the camera’s maximum without additional tweaking), and possibly much longer. I found that just using the polar finderscope, objects will stay centered in the eyepiece for longer than 15 minutes at 200x before noticeably drifting. Exposure times of up to 1 minute are possible without noticeable star trails**. Much better is to use the star drift method of polar alignment. Once you have mastered this technique, you can achieve a near-perfect polar alignment in a short time.

Take the time to focus optimally

When doing deep-sky photography, it’s very important the focus is as near-perfect as possible. With deep-sky (nebulas and clusters), the camera should be set to focus at infinity because the camera’s autofocus will not be able to focus on faint objects. With planetary and Lunar work this is much less of an issue, as these objects are bright enough to let the camera achieve perfect focus automatically.

I usually use the eyepiece I will be attaching the camera to, to first try to achieve optimal focus visually. This is the most important step. Make sure that when focusing visually, your eyes will also be staring into ‘infinity’; that is, the muscles of your eye lens should be perfectly relaxed (although I do not wear glasses, I found this not to be trivial). Attach the eyepiece to the camera, and securely attach the camera and eyepiece to the telescope, making sure the eyepiece slides all the way backward in the diagonal or visual back. Set the camera to the self timer by pressing the ‘focus’ button 4 times, then set it to focus at infinity by pressing the focus button while turning the selection knob one position to the right. The display will show ‘INF’.

If you find that images of stars are discs with a hole in the center, this is a sure sign your initial focus has been off. This effect can only occur in telescopes with a central obstruction such as Newtonians, Schmidt-Cassegrains and the like; The hole in the center of a star image is the shadow of the telescopes' central obstruction. The image to the right of this text gives an example of how this will look.

This effect will not always be clearly visible in the camera's LCD, so it makes sense to review the first images taken during a session and go to maximum in-camera review magnification (6x with the CoolPix 995) to check whether focus is OK.

Important: If you notice holes in the star-discs that are off-center, your scope's collimation is off!

Learn how to use your camera

Astrophotography is very different to daytime photography. There are a few camera features you would do well to become familiar with before starting the astrophotography session. A few things that you should try out beforehand are: The camera’s self-timer; Noise Reduction; Manual aperture and shutter settings; Manual focus; Manual ISO settings.

Use some form of noise reduction

I have found that the camera’s built-in noise reduction is quite effective, but I have also done some experimenting with manually taking dark frames (see below), and using them during post-processing to reduce image noise and imperfections (hot pixels). The results I get with manual dark frames are quite encouraging. I have found that when taking one dark frame per 5 to 10 exposures, the results are at least as good as with the built-in Noise Reduction, while it's also saving a great deal of time.

A dark frame can be thought of as an image that registers all imperfections in the camera’s CCD and electronic signal processing in a single image. A dark frame can be used to correct images during post-processing by taking the imaged pixels and subtracting the corresponding pixels from the dark frame, to even out any imperfections introduced by the camera’s CCD and signal processing. A dark frame is simply an image taken under the same circumstances as the original image (same temperature, exposure time), while preventing any actual light to fall onto the camera’s CCD (blocking the lens). The noise reduction built into the Coolpix camera simpy takes an empty image after every normal image to carry out these corrections automatically.

Although I found the camera’s image noise quite tolerable, there is noticeable image noise in the red channel during longer exposures, which can be filtered during post-processing. With the fainter objects, I found that ISO400 produces the best, though noisiest results. There also is a noticeable temperature effect. The camera performs best when it’s cooled down and the outside temperature is low.

Use a low-power eyepiece

A low power eyepiece has the following advantages:

1. •More light enters the camera, which means shorter exposures are possible.
   

2. •Less vignetting because of the larger eye lens.
   

The camera’s zoom range (1 – 4x) is sufficient for a wide range of magnifications if you want the smallest details (especially on planets). When more magnification is needed, a Barlow lens can be used, keeping the advantage of the larger eye lens.

Do NOT touch the camera during exposures

Touching the camera (for example when pressing and releasing the shutter button) causes slight vibrations of the telescope. Even the sturdiest of mounts cannot prevent this. This will ruin your image, especially at high magnifications.

The easiest way to prevent this is to use the camera’s self timer. On my Coolpix 995 this is done by pressing the Macro/Self timer button a few times (the self timer can only be used in Macro mode). After this, you can manually set the focus to infinity. Setting the self-time will introduce a delay of 10 seconds between pressing the shutter release button and taking the image, which is more than enough time to dampen any vibrations. The camera’s bright pre-flash light will flash during this time; you might want to put some tape over it to preserve your night vision. This will restrict the maximum exposure time to 8 seconds, which is still enough to image the moon and planets, and many of the brighter deep-sky objects (depending on your scope’s aperture). Remember that on the Coolpix 995 you have to set the self timer before every exposure.

The alternative way is by using the PC serial control as described on the photography equipment page. This will give you maximum flexibility and convenience and allows exposures of up to 1 minute.

Charge your camera’s battery

As you will be experimenting a lot, your camera will be continuously on for a long time. You will need every bit of charge your camera’s batteries hold. If you have one or more spare batteries, also keep them at hand. It might be convenient to use a wall adapter, if you have one (which I don’t), providing you are close enough to a wall outlet of course.

Take as many images as possible

The convenience of a digital camera is that an exposure costs nothing (only time). You will find that of all the images you will take during a session, some will be excellent, and some will be worthless, especially when the seeing is unsteady. The more images you take, the greater the chance of one or more excellent images. For stacking purposes, you will also need as many images as you can take to get the best results. Also make sure that you have enough free space on the Compact Flash card to accommodate your pictures, although I found pictures with a lot of dark sky in them to be significantly smaller in size, bytewise, than daytime pictures.

Experiment!

Try out as much camera settings (zoom, exposure times and ISO) as you can. The possibilities to review your photographs ‘on the spot’, are limited when using just the camera’s LCD. On the planets, often a photograph taken will look fine in the camera’s display, but will be overexposed or fuzzy when you review them on your PC. I usually take my photos with ISO100 or 200, but I found that sometimes ISO400 can produces very nice (though somewhat more noisy) pictures. Noise can be filtered out during post-processing reasonably well.

Learn where and how to find interesting objects

While this is true for visual astronomy as well, for long-exposure astrophotography this is especially the case. Often objects show their true beauty only when photographed. While the Orion Nebula (M42) may look blue-grey with a hint of green in all but the largest amateur telescopes, an astrophoto made with even a modest telescope will reveal many colors and shades. The same is true for such a well known object as M31. M31 will look like an elongated grey blob without much structure when observed visually in most telescopes. When photographed, it will easily show its spiral structure, dust lanes and many interesting features and details impossible to see with a medium-aperture telescope. The point is: you will need to look at the night sky in a whole different way. Objects that seemed out of reach visually from your back yard with your small telescope might be very interesting when photographed. Astrophotography adds a new dimension to your hobby.

I use the excellent TheSky to print out maps when preparing to observe the more elusive deep-sky objects. Aligning the scopes’ setting circles as described elsewhere on this page will also help.

Getting it to work

How I plan an evening of observing and astrophotography:

Keeping the batteries charged

I use batteries for the scopes’ drives as well as for my digital camera. It’s essential they are fully charged before starting a night out doing astrophotography.

Wait for a clear night

Clear nights are a rare occurrence where I live, especially in winter. There usually is one mediocre night every two weeks. Really good nights (with good seeing and good transparency) are even rarer.

Decide which objects I’m going to image

Usually I print out some star maps from TheSky of the area surrounding the objects I’m going to look at or image that night.

Get the scope out

Getting the scope out was, in the case of my C8 SP-DX, a multi-step procedure. First I get the tripod and mount out of the house (roughly pointing the mount North). Then I will get the C8 OTA out of its case, attach the OTA to the mount, add the counterweights to the shaft, level the mount, and balance it. Then I will attach cables, batteries, finder scopes and other accessories. This will take about 5 minutes.

Of course, with my new Vixen SX mount, I can take the tripod, OTA and mount out in one piece.

Polar alignment with the polar finder scope

While the C8 is still cooling down, a process that takes ½ hour to an hour, I will do a polar alignment using the mounts’ built-in polar finderscope [for instructions on using the SP and GP/DX polar finderscope see here]. This will take a few minutes. For visual and short-exposure photography purposes, this is often accurate enough. I’m amazed at how easy and accurate this is with the SP/DX mount (and now also with the SX mount).

Accurate polar alignment using the star drift method

To get a better polar alignment I will perform an alignment using the star drift method. During this period the scope is usually still cooling down. This will take about 15 minutes. This step can be skipped if a perfect polar alignment is not needed, however.

Align the finderscopes

Following the polar alignment, I will align both the red dot finder and the 8x50 right-angle finder to a bright star in a convenient location.

Align the setting circles

After polar alignment, the scopes’ setting circles can also be aligned. Choose a bright star with known coordinates to point the scope at, and turn the Right Ascension setting circle to match the RA coordinate of the star. The scale can be set with an accuracy of one minute of arc using the Vernier scale. Choose a star close to the Celestial Equator (at a right angle to the pole) for the most accurate alignment. Here is a list of suitable stars (for use in the Northern hemisphere):

download brightstars.pdf

The declination setting circle should already be aligned. If it’s not, use the same star as in the previous step and set its correct declination as accurately as possible on the declination scale.

Aligning the setting circles will make it easier to find the smaller and fainter deep-sky objects, by allowing you to find objects by their celestial coordinates.

Of course, those of us who use go-to mounts, should align their mounts at this time

Wait for cooldown, check collimation

I usually go inside to let the scope cool down during this period. After it has cooled down, I will check the scopes’ collimation. As collimation is usually fine, I will only have to tweak it occasionally.

Find the object to image

This speaks for itself. I use a low-power eyepiece or the mounts’ setting circles to locate the object, put it in the middle of the field of view, and start tracking it. Then I will remove the diagonal and switch to an eyepiece with the Digi-T adapter already attached, and carefully focus.

Attach camera to eyepiece

Using the same eyepiece I ended up with in the last step, I attach the camera to the eyepiece and slide the eyepiece back into the visual back of the telescope, making sure it’s fitted very securely (you would not want your expensive eyepiece and camera fall out of the scope!).

Set the camera’s features

Adjust the camera to suit the object you are trying to image. Usually this means enabling Noise Reduction, setting a different ISO, setting the camera to manual (M) mode using the mode button and turning the selection knob.

Shoot!

You can start the photography session. Occasionaly make sure the object you are trying to photograph stays in the field of view of the camera, since many deep-sky objects are not directly visible in the camera’s LCD.

I've been able to do a photometry of an 8 second ISO 400 exposure of M42, and found the faintest star visible to be of magnitude 13.3 !

Post Processing your images

The most exciting (and potentially the most depressing) moment comes when you download the images from a photography session to your PC. Usually I have 50 or more images to download.

I use a number of tools to achieve the most pleasing results; i.e. a noise-free, detailed, contrasty and correctly colored image:

Adobe Photoshop 6.0 +

I use Adobe Photoshop to do all kinds of image corrections, manual aligning and stacking, contrast enhancement and noise removal. I plan to describe some of these procedures on these pages at a later time. Especially the possibilities for using curves to separate signal from noise, and layers to combine multple photographs into one, are essential for post processing.

RegiStax (very good freeware)

I use RegiStax for aligning, stacking and processing large numbers of images. It's freeware, available from http://registax.astronomy.net/. This is a well-known and very useable tool. The built-in post-processing Wavelet filter is a great feature. It produces the best end-results with a large number of raw frames to work on (more than 20), but it can be used with fewer frames.