Astrophotography Equipment

Equipment

I have a variety of digital cameras, but only the "bigger" SLR-style cameras are applicable to astrophotograpy (AP) tasks.  There are several reasons for this observation including the ability to use a remote control for the shutter and the availability to set the camera in a manual mode that allows taking extended duration photos.  Generally, but not always, the DSLR cameras meet these criterion.

For the last 12+ years I have been using Canon equipment and currently have 3 Canon bodies: a 20D 8mp, a 1Ds Mark 2 16mp and a 1Ds Mark 3 21mp.  The Mark 2 and Mark 3 bodies are full-sized (AKA very heavy) pro cameras while the 20D is a consumer-grade body.  I have only used the Mark 2/3 because my remote control dongle matches these bodies and provides accurate interval and exposure control.

In addition, I recently purchased an Olympus OM-D EM-5 16mp camera.  The EM-5 is based on the micro four thirds (MFT) format and has a number of endearing attributes that were both unknown and unexpected when I bought the camera.  First, the MFT design does not use a mirror in the camera.  The standard SLR (single lens reflex) camera has a diagonal mirror that is in the optical path that reflects the light through the lens to the user's eye during the framing and focusing process.  This mirror is retracted when you trigger the shutter producing the loud characteristic shutter noise associated with the SLR-type camera.  The MFT design eliminated the mirror and instead uses the imaging chip in real time to present what the lens sees to the user via either the rear LCD display or via a smaller LCD display located inside the camera that provides a so-called Electronic View Finder (EVF).  To the casual observer, these MFT cameras look just like a DSLR except that the eye piece is acting as a miniature video display.  The consequence of this design is two-fold.  First, it makes the form factor of the camera smaller and thus lighter in weight and more portable.  Second, and more importantly for this application, the physical distance from the lens mating flange to the focal plane is quite a bit shorter thus allowing the use of a wide variety of legacy lenses by using a simple mechanical adapter that provides an offset equal to this difference in distance (and the mounting lugs).  Because of this, there are a number of excellent used lenses that are available "for cheap" that prior could only be used on the native cameras.  For instance, I have several Zeiss lenses designed for the Leica M-mount.  With a simple mechanical adapter that provides the correct offset and is machined for the M-mount lugs, I can use these lenses on the Oly.  And it produces excellent results, but only in manual focus and manual aperture modes.

The second remarkable quality of these MFT cameras is that manual focusing is done in real time using the EVF or LCD panel on the back of the camera.  These displays show exactly what the camera will record when the shutter is triggered, thus allowing viewing of the focus and exposure parameters easily.  Focusing old SLR film cameras or even newer DSLR cameras without the live view capability was a bitch and required many wasted shots or addition of an extra prism-based focusing aid to get it correct with fewer tries.  The crux is attainment of critical focus and as the photos will show, it is harder, or impossible, to reach critical focus without using some kind of aid.  The LCD panel provides this aid and it is built into the design of the camera.

Generally, to do any AP shots of stars, you have to use some kind of tracking mount or another method of compensating for the movement of the stars relative to the earth.  The standard way to do this is to use a tracking mount.  More specifically, an equatorial mount where the rotational axis of the mount is parallel to the earth's rotational axis.  These mounts turn the telescope/camera at the same rate as the sky causing the stars to "hold still" while you take a long exposure photo.  The longer the photo, the more light that can be gathered.  But, the longer the photo, the higher the chance for errors in your alignment.  This process of alignment is generally called "polar alignment" because the scope is pointed at the pole star, Polaris.

As it turns out Polaris is not really at the celestial pole.  It is close, but not close enough for taking long photos.  Consequentially, the process of achieving a good polar alignment becomes quite a bit more complex requiring both optical aids like a special aiming scope and a mechanical procedure for fine adjustment.  In early 2013 I purchased an Astrotrac mount that provides both portability and low tracking error.  This assumes, of course, that you adhere to the alignment process with high fidelity and attention to detail.

The Astrotrac mount comes in a compact carrying case to provide high portability.  I had to augment the basic tube with a "battery box" and a VCR bag that contains the extras needed.  Note the 3 plugs on the top of the battery box.

I fabricated the battery box from a jet ski battery, an ammo can and a 3-plug cigarette lighter socket.  The can also stores the lens heater (to prevent dew from collecting on the lens), extra wires, a magnetic compass for coarse alignment of the mount and a small plug-in light.  The Astrotrac comes with a small AA battery array that powers the mount.  But, extra 12V is needed for other things and the Astrotrac comes with a cable set that allows using a cigarette plug, so the decision to make an all-in-one was easy.

Inside the tube is a roll that contains the tracker head, 3 legs with guy wires, turn buckles and screw feet.  The adjustable screw feet are used for for leveling the pier.  The mounting wedge is stored upside down at the end of the tube. Note the machined attachments on the bottom for the support legs.

The wedge is placed on the open end of the tube and secured with the latches.  The legs are then plugged onto the machined attachments, the guy wires are attached to holes in the wedge and the turn buckles are tightened until sufficiently taught. The tracker head is attached to the wedge mount via stainless steel socket head cap screws using the 3 holes in the wedge.  The tracker provides a 3/8" screw mount for attaching standard tripod heads.  Your camera is then connected to the tripod head.

When the mount is deployed, the screw feet are used to level the head of the pier using the bubble level built into the base of the wedge. It should be noted that the design of the Astrotrac allows filling the main tube with ballast such as bags of sand, gravel, rocks or a water bladder to provide extra stability for the mount.  In the photo above, the tracker head is deployed in the correct position for the northern hemisphere; if tilted to the opposite side, it would track for the southern hemisphere.  The tracking head is attached to the 12V supply and it is ready to go.  A microprocessor in the head controls the rotation of the lead screw.  The lead screw then separates the 2 arms of the tracker providing rotation for the head of the mount.

In this photo, the polar alignment scope can be seen.  The swing arm can be rotated to provide a clear view and the scope is held to the arm by a magnetic attachment.  The rotary switch on the scope controls the illumination of the reticle used for sighting.

I use a gear-drive tripod head attached to the tracker to provide fine positioning control.  A standard dove tail mount was attached to the removable portion of the tripod head.  The camera and sighting tool (a Telrad 1X LED sight) are attached to the dove tail bar.  The tripod head and camera are shown in a south-facing configuration in the photo above.

The tripod head has to be rotated 180 degrees and the dove tail bar reversed to allow looking at the zenith.  The knobs of the head interfere with the arms of the Astrotrac and therefore the head has to be rotated.  The camera shown is the Olympus OM-D EM-5 With Voightlander f/0.9 17.5mm lens.

A reversal of the zenith position will allow a view of the southern sky.  The Telrad LED sight is clearly visible in the photo above.

The Astrotrac head uses a tangent approach to tracking.  Inside the head is a lead screw that is controlled by a stepper motor and a microprocessor.  The processor knows where it is relative to the length of the screw and adjusts its rate to compensate for tangential error.  This tracker has very low periodic error, but can only track for about 2 hours before needing to be reset as that is the length of the lead screw.  So far, this time has not proven to be a limitation.

Lunt 60mm (500mm focal length) Solar Scope with Sony A7RM2 mounted on Astrotrac mount.

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