Astrophotography - Equipment

Transcription

1 Astrophotography - Equipment Written and Edited by David Pearson Some material extracted from Antonio Miro original Beginner s Class Astrophotography handout

2 Astrophotography Astrophotography is taking photographs of objects in the sky Records the objects to see for history Records once in a lifetime celestial events Allows seeing dimmer objects that can not be seen with your eyes or visually with your telescope See objects in color Astrophotography can be performed with Only a tripod with SLR or DSLR (digital camera), or any camera with tripod connector Telescope with SLR/DSLR, or just hold a point and shoot camera up to the eyepiece of a telescope Webcam, video or specialized digital camera can be used with a telescope Computer may be required to see the objects, and to combine or process the images

3 Astrophotography Types of pictures that can be taken Star Trails Asteroids, Meteors and comets Eclipses Transits - Planet transits across Sun Celestial objects (stars, planets, clusters, galaxies, nebula, etc.) Occultation's Sun spots Moon and craters Meteor Planet impacts Constellations Aurora Borealis Rainbows & Sun dogs

4 Necessary Functions to perform Astrophotography Camera Equipment Support Purpose Support weight of all equipment, provide pointing capability to find and track and/or guide sky objects Consists of tripod/pier, manual or motorized mount, Camera and/or telescope, and imaging accessories. Tracking Purpose To rotate the telescope and/or camera at the speed necessary to move at the same rate as the Earth s rotates and keep selected object in camera field-of-view The mount performs this function. Can be manual (user controlled) or motorized. Focusing Purpose Focus the telescope and/or camera for best object resolution/view Consists of a focuser that connects an eyepiece/camera to the telescope. Can be manual or motorized Polar Alignment Purpose Align mount coordinate frame to Sky coordinate frame to allow good tracking performance Depending on polar alignment method, different hardware and/or software is used

5 Necessary Functions to perform Astrophotography Guiding Purpose To actively keep chosen sky object in the field-of-view of the camera or telescope. The mount along with camera/telescope and other accessories are used to perform this function (not all mounts can perform this function) Camera Purpose Capture selected sky object image by leaving camera shuttle open for extended amount of time or to capture video Camera and/or computer with software controls this function Scripting Purpose Automate processes to perform all astrophotography functions Computer and software performs this function Image Processing Purpose To capture, calibrate and process image(s) to bring out detail in the final image. Computer software performs this function

6 Tracking The main purpose of tracking is to keep the selected object in the camera field-of-view. To do this precisely, the mount or the user must move at the combined apparent sky rotational speed of 15 deg/hr (15 arc-sec/sec) and the much smaller effect of the earth s motion around the sun of deg/hr. Some mounts ignore the smaller term or a portion of it. There are two basic mount types that perform tracking differently; Altitude-azimuth and equatorial An altitude-azimuth (alt-az) mount has two perpendicular axes, one axis that rotates 360 degrees around the horizon from north, to east, to south, to west and back to north, and one axis that is perpendicular to the other that rotates from horizontal (0deg) to vertical (90deg) and back to horizontal. Types: Tripod, Tripod with manual alt-az adjustment, and manual or motorized alt-az mount The alt-az mount must mimic the apparent sky motion using two axes and over time an unwanted rotation occurs, called field rotation (see field rotation discussion). The alt-az mount needs to be aligned with the sky to perform tracking. This is performed by performing a star alignment (see polar alignment section) Note: picture of manufacture product is for reference only and does not recommend a specific product

7 Tracking A equatorial mount has two perpendicular axes, the right ascension (RA) axis is pointed at the Earth s rotation axis and is driven at apparent sky rate to allow object tracking. The declination (DEC) axis provides a second axis that when combined together with the RA axis allows pointing at any object in the sky. An equatorial mount only needs one axis (called right ascension (RA)) to move to match the apparent sky rate. The one axis motion can be performed either by the user or can be motorized. Most equatorial mounts come with an azimuth and altitude set of adjustments to precisely allow pointing the right ascension (RA) axis at the Earth s rotation axis (polar alignment). To assist in polar alignment, some mounts come with a polar alignment scope in the mount Types: DIY Barn Door Tracker Dedicated camera mount Telescope equatorial mount Note: picture of manufacture product is for reference only and does not recommend a specific product

8 Tracking DIY Barn Door Tracker A very simple way to track is to use two pieces of wood, a hinge, and a ¼ - 20 bolt. A quarter turn of the bolt every 15 seconds matches Earth rate. It is both cheap and fun to build, and you can have very nice results. Adding a rotating head between the barn door tracker and your camera will help orienting the camera to get the field of view you want. You may even add a one rev/min DC motor to make the tracking more automatic. Do need to point the hinge at the North Star (Polaris) Can allow exposures up to about 2 to 2 ½ minutes

9 Tracking Dedicated Equatorial Camera Mount Attach a DSLR to a camera equatorial mount which is attached to tripod Just a small sample of available products ioptron Sky Tracker Vixen Polarie Do need to point the camera mount at the North Star (Polaris) AstroTrac TT320X AG AutoGuiding Tracking Mount Note: picture of manufacture products is for reference only and does not recommend a specific product

10 Focusing The purpose of focusing is to obtain the best view and resolution of the object being capture by camera. In practice, this can be very difficult. The focuser connects an eyepiece/camera to the telescope, and can be manual, or motorized with manual or computer control. If a DSLR or SLR with a lens is being used than the camera must be focused. Focusing is more critical as the focal length increases, because as the field-of-view decreases the image gets larger and any focus errors will be more apparent leading to shapes of objects that are not desired. Also, note that standard camera lens were designed to be used during the daytime, as a result the optics may not be as good as a telescope. Focusing may be difficult because star will not be pinpoints, but sometime strange shapes, so manual and auto focus may be difficult. Depending on camera type, one of the following focusing method can be used. Four focusing methods (worse to best); 1) do a manual focus at time of exposure, 2) determine the infinity focus setting during the daytime and mark the position on the lens with tape or mark, 3) If camera has live view and magnify-the-image capability, use it to focus the best you can, 4) Same as option 3), except use auto focus mode then switch back to manual (beware this doesn t always work depending upon camera. If it doesn t work use option 3). Manually focusing a telescope with a non-dslr/slr camera is also difficult. Many techniques can be used to assist the process.

11 Focusing Aperture Mask The simplest and cheapest is a simple aperture mask over the front of the scope that has two holes cut 180 degrees apart. The size of these holes varies depending on the aperture of the scope you are using (about 2" diameter for an 8 inch aperture and maybe 3" for a 10"). Unless you have a computerized scope with "Go To" capability, you must use the finder scope to make an accurate mental or hard copy map of the position of the scope while it is on the object to be photographed. You now move the scope to a bright star nearby and look through the camera viewfinder for the star. Most likely, you will see two stars with the same brightness. Move the scope focus in or out and you will notice the stars will begin to converge or diverge. What you want is to converge them into a single stellar image as accurately as you can and after doing so, you are in focus. Now move the scope back to the photographic object and using the locator map you made previously, position the scope EXACTLY as it was before. You can now make the photograph. Bahtinov Mask Hartmann Mask or Scheiner Disk Note: picture of manufacture product is for reference only and does not recommend a specific product

12 Focusing Focault method Focault method - A razor edge to cut a star beam. Use a "knife edge focuser" to get a precise focus. This employs the fact that a well focused star makes a small pinpoint of light on the focal plane of the camera. The knife edge focuser in simple terms is a very thin edge that when placed at the precise prime focus of the scope will cause a well focused star to quickly vanish from view when the scope is moved in a direction that will cause the knife edge to occult the star. An out of focus star will not quickly vanish but will appear to slowly dim out as it is occulted. Mitsuboshi knife-edge focusers Note: picture of manufacture product is for reference only and does not recommend a specific product

13 Focusing Off-axis Guider An off-axis guider allows simultaneous visual viewing of the camera image. Since the eyepiece has the same focal distance as the camera, once you focus using your eye in viewing the image, the camera is also in focus. Mitsuboshi Off-Axis Guiders Note: picture of manufacture product is for reference only and does not recommend a specific product

14 Focusing - Camera Another method of focusing is to take a short exposure of a star field using the camera and then adjust the focus until the smallest stars are achieved. Note that the telescope/lens diameter and optical quality, telescope collimation and atmospheric conditions will limit the minimum star size that is achieved. A more expensive method, but easier is to use is a computer software program (FocusMax or similar) to analyze the star field image and adjust the focus until the smallest star diameter is achieved. In addition to the software, a motorize focuser (FeatherTouch, MoonLite, or similar) and controller must be connected to the computer to use this method.

15 Sample Focuser Manufactures 2, 2.5, 3 and 3.5 format Note: picture of manufacture products is for reference only and does not recommend a specific product

16 Polar Alignment In general, Polar Alignment is required to maximize the viewing time for a given telescope field-of-view. However, for astrophotography a good polar alignment minimizes image smearing and reduces one cause of elongated stars during image capture. The process of obtaining a good polar alignment is different for different type of camera/telescope mounts. For equatorial mounts, the RA axis is made parallel to the Earth s rotational axis. The mount only has to move the RA axis to move at the Earth s rotational speed. For alt-az mounts, a multiple star alignment must be performed for the mount to determine its telescope coordinates relative to the sky coordinates. The mount must move its two axis simultaneous to move at the Earth s rotational speed. Both mount types can be used for astrophotography, but the best choice is an equatorial telescope mount Methods for Polar Alignment (all methods are for equatorial mounts, except for 7.) 1. Manual method using eye and mount or telescope reference aid 2. Crosshair or double crosshair reticule (lighted reticule works better) 3. Polar align reticule 4. On a equatorial telescope mount, use a Polar alignment finder-scope that is mounted inside mount. 5. Star Drift method 6. Polar alignment software using camera 7. Star Alignment

17 Polar Alignment 1. Manual method using eye and mount or telescope reference aid With this method, the user sights along a reference straightedge to point the telescope RA rotation axis of the mount at Polaris. The reference can be the long edge of the telescope, mount s RA axis or two screw heads on opposite ends of the telescope. To adjust use the tripod legs, tripod head, or latitude/azimuth adjustment bolts/knobs on the mount. Note if the telescope is used as the reference straightedge, the telescope must be parallel to the RA axis (DEC = 90) and in the highest position over the RA axis. If mount has latitude scale, adjust for your latitude. Double check using above method. Recommend that the telescope mount is leveled first. 2. Crosshair or double crosshair reticule (lighted reticule works better) Can be used in finderscope or telescope eyepiece(focuser) holder. If using finderscope, the finderscope must be aligned with the telescope. Insert reticule into finderscope or focuser. Put the telescope parallel to the RA axis (DEC = 90) and in the highest position over the RA axis. Adjust the tripod legs, tripod head, or latitude/azimuth adjustment bolts/knobs on the mount until Polaris is center in the crosshair.

18 Polar Alignment 3. Polar align reticule Can be used in finderscope or telescope eyepiece(focuser) holder. If using finderscope, the finderscope must be aligned with the telescope. Insert polar align reticule into finderscope or focuser. Put the telescope parallel to the RA axis (DEC = 90) and in the highest position over the RA axis. Adjust the tripod legs, tripod head, or latitude/azimuth adjustment bolts/knobs on the mount until Polaris is centered in the polar align reticule quadrant shown for current day and time (see picture).

19 Polar Alignment 4. On a equatorial telescope mount, use a Polar alignment finder-scope that is mounted inside mount. Same method as 3), see previous page 5. Star Drift method The star drift method can be used to align your telescope to any accuracy. First, level your tripod and orientate it to Polaris (not necessary, it just helps). If you have a Polar alignment circle, it can also help to use them. Look at a star near the Equator, South. Track the star in RA only, and look if the star goes up or down in your eyepiece (supposing your are looking straight at south with your head vertical). Rotate the azimuth of your telescope to adjust it until the star is not moving anymore. Then, move to a star at East (West works also). Do the same, but adjust the altitude this time (the angle between your telescope RA axis and the horizon). By switching several times from South to East (West), you should be able to adjust your polar alignment quite quickly. Of course, the first time you will spend a lot of time; take notes of what you are doing, and it will be much quicker the next time you do it. Continue star drift process until the star does not move for the time you want to take an exposure

20 Polar Alignment 6. Polar alignment software using camera There are many software programs to help in polar alignment using a web cam or camera. Some mimic the star drift method by displaying the star drift over time. Others require precise star alignments and the software computes the polar error by comparing the star coordinate to what the telescope mount measured. Also some continuous displays the error as the polar axis is being modified by the user to provide real time updates and to know when complete. There are to many programs available to name them all. Here is an incomplete list; Alignmaster, EQAlign, PoleAlignMax, PemPro, WCS, Polar finder, StarTarg Star Alignment (Required for alt-az telescope mounts) Star alignments are performed by choosing a known bright star, placing it in the center of the telescope eyepiece, finder or camera, and then tell the computer which star it is. This is performed one, two or three times. When complete, the computer determines the coordinate transformation between the mount/computer and the sky. This is not technically a polar alignment method, however it basically does the same thing, determines how to move the telescope to rotate at Earth rotational rate.

21 Image Smear without Guiding Nomograph t I f P e s where t = Exposure Length, min I s - Image Smear, microns f = Focal Length, mm P e = Polar error, arc-min Polar axis alignment requirements for astronomical photography, Hook, Richard N., 1989 volume 99 page 19-22, British Astronomical Association provided by NASA Astrophysics data system

22 Image Smear without Guiding (continued) Nomographs

23 Guiding Guiding is the act of continuously correcting track errors in real-time to eliminate image smearing and minimize elongated stars in capturing images. Track errors can consist of polar alignment errors, Earth rotational rate errors, refraction induced errors, atmospheric induced errors, wind induced errors, and mount/telescope induced errors. Although the purpose of guiding is to remove errors, it can also add errors by just the act of guiding, such as sensitivity to telescope vibration, and the interaction with guiding exposure rate, atmosphere fluctuations, and mount periodical tracking errors. There are two methods, manual and automated. Manual Guiding Consists of using a crosshair reticule as an eyepiece in either the finderscope, telescope or off-axis guider that is attached between telescope and camera. The objective is to maintain the guide object in the center of the crosshair by using mount hand controller. Note: picture of manufacture product is for reference only and does not recommend a specific product

24 Guiding Auto Guiding Auto guiding is performed by the computer using guiding software and a guiding camera attached piggy back to the telescope or in-line with the imaging camera and telescope. The use of a guiding camera attached piggy back is best when used with a short focal length telescope. For long focal length telescopes, the in-line guide camera is best with the use of an off-axis guider, inline-axis guider, or a imaging camera that has a build-in guiding port or sensor. on-axis guider off-axis guider Guiding Port There are numerous Guiding software available, such as Maxim DL, PHD, CCDSoft and MetaGuide. Note: picture of manufacture product is for reference only and does not recommend a specific product

25 Guiding - Auto Auto guiding has a sensitivity to guiding exposure rate, atmosphere fluctuations, and mount periodical tracking errors. The atmosphere can cause frequency fluctuations of the guiding star image in the order of less than 1 hz. As long as the guide image exposures are several times longer than 1 sec (3 or 4 sec is better than 2 sec), the ability of the guider to track the guiding star image is very good. The longer exposure averages the star fluctuations over a longer time period and therefore the mount does not try to correct the much higher frequency fluctuations. However, with sever periodic error in the mount, usually due to low quality gears or hard grease balls in the gears, guide exposures of 2-4 sec may not be possible. This can cause the stars in the image to be elongated. To prevent this, the mount should be of good quality, which usually means a higher price. Another option, may be to use multiple star guiding that is available in Maxim DL (not known if other software offer the same capability). Or if using a separate guide camera, adding a focal reducer to brighten up the guide stars has shown to increase guide rates from 1 to 2 seconds.

26 Sample High End Telescope Mounts Note: picture of manufacture product is for reference only and does not recommend a specific product

27 Camera Depending on the setup the imaging camera can be a webcam, DSLR/SLR, or astrophotography CCD. Although a smartphone, or point and shoot camera can also be used by taking a picture through the eyepiece. The difference between the DSLR/SLR and the astrophotography CCD, is the CCD camera can be cooled below the ambient outside temperature which reduces the noise and allows dimmer objects to be exposed with the same exposure duration. A DSLR/SLR has interchangeable lens that changes the focal length which changes the field-of-view and magnification. Usually with this configuration the camera is piggy back on a mount or telescope. Using low focal length lens or fisheye lens provides large field-of-view to capture constellations, aurora borealis, meteor showers, polar star trails, dark sky with lighted foreground or wide view of Star, planet, moon conjunctions. Using an astrophotography CCD or DSLR/SLR without a lens on a telescopes fixes the field-of-view and magnification. Therefore depending on the image or object to be capture, the object size needs to be matched to the telescope field-of-view or telescope focal length (see how to compute field-of-view with camera and telescope topic) Note: picture of manufacture product is for reference only and does not recommend a specific product

28 Camera The webcam can be used for astrophotography, but only in unique situations as the equipment and image processing is different than other cameras. Since the webcam captures images at 10-60Hz or higher, a computer is needed to record the images and the image must be bright since each exposure is 0.1 to seconds in duration. With the exposures so short, the images are susceptible to atmospheric fluctuations and therefore a lot of images must be taken to capture the best images to combine. The video images are combined by tools like RegiStak or PIPP to produce a single image. Astrophotography CCD cameras come in two types; monochrome or one shot color. Monochrome cameras require separate exposures of a sky object through Red, Green and Blue (RGB) filters to obtain a color image. Usually a fourth exposure, called luminance, is also obtained using a clear filter. These four exposures when combined by processing produce a astrophotography picture. To simplify the exposures using multi-filters, a filter wheel can be added to the camera. Note: picture of manufacture product is for reference only and does not recommend a specific product

29 Camera One shot color cameras are similar to a DSLR in that only one exposure is needed to create a color image. However, to achieve this requires four side-by-side pixels to be combined, each with a different color filter sitting over the pixel (called a Bayer matrix). This technique to obtain color images comes at some loss of resolution in exchange for the ability to capture color in one exposure. RGGB BGGR GBRG GRBG Astrophotography one-shot color camera require longer exposures to obtain good color images than required by a monochrome camera shooting with four filters. The advantage with one-shot color cameras is the total image time is substantially less than using a monochrome camera and having to image through four filters. Remember to capture the images in RAW format when using DSLR or CCD cameras. The RAW format is best if image processing is to performed.

30 Scripting Scripting is the process of automating the steps in operating your telescope and camera in taking astrophotographs. There are several computer programs available that automate the steps of focusing, taking and saving pictures, guiding, roof control, camera on/off, slewing to objects and determining where the camera is pointed. Scripting programs include, but are not limited to CCDCommander and CCDAutoPilot. There are also methods to capture DSLR photographs through an intervalometer, tablets, smart phones and computers.

31 Image Processing Imaging processing includes capturing images, image calibration and image processing. Generally, image capture is not included as image processing, however, it is here as the image capture process is directly related to image calibration. Capturing images consists of taking one or multiple exposures (subs) of a fixed length of a single sky object. Images should be saved as RAW. Most DSLR cameras have this capability. For astrophotography cameras save as a FITS file. Image Calibration consists of taking one or multiple dark, bias and flat frames to calibrate the camera/filter/telescope to improve image quality. See following pages for definitions. Image processing consists of aligning sub images, combine or stack subs, and photo processing the combined image to obtain an image that can be seen in detail with appropriate brightness, contrast, range of grayscale, and (if applicable) color balance. Image processing computer programs, such as photoshop or PixInsight can be used. Also software experts such as Richard Berry, Bruce Johnston, Michael Newberry, Douglas George, and Christian Buil (and others) have all written full-range programs which may be capable of reading and manipulating the raw images from your camera.