USING SONY a900 for ASTROPHOTOGRAPHY • TELESCOPE ADAPTORS - PRIME FOCUS - EYEPIECE PROJECTION • REMOTE RELEASE - SONY CABLE RELEASE RM-S1AM - SONY REMOTE COMMANDER RMT-DSLR1 - GADGET INFINITY RADIO S1 REMOTE - JJC-JR-C IR MODULAR REMOTE - HÄHNEL HW433S80 RF WiFi REMOTE - JJC TM-F LCD DIGITAL TIMER / INTERVALOMETER • DRIVE FUNCTION - MIRROR LOCK-UP • FOCUS - SCREEN L - DIOPTRE CORRECTION - RIGHT ANGLE MAGNIFIER • FINE FOCUS - RACK & PINION FOCUSER - MOTORISED FOCUS - REMOTE DIGITAL FOCUSER - DEPTH OF FOCUS • TRACKING FOCUS • SEEING • UN-DAMPED VIBRATION - ELECTROMAGNETIC SHUTTER RELEASE • CHECKING FOCUS • LAPTOP REMOTE CAPTURE & CONTROL vs LCD VIEW SCREEN • EXPOSURE - MANUAL MODE OPTION - CALCULATING EXPOSURE TIMES - ISO SETTING & RESOLUTION - ISO vs SEEING - BRACKETING vs DR-O USING SONY a900 for ASTROPHOTOGRAPHY The Sony a900 DSLR is capable of taking fine photographs of the Sun in H-alpha and white light, the Moon, and deep sky objects. It has a 35mm Fx format CMOS sensor specifically designed for low light level photography. The twin BIONZ processors are designed to produce high resolution, low noise, high dynamic range images. The 3-inch LCD view screen may enlarge raw images x19, and its resolution is sufficient to judge precise focus without having to resort to a remote laptop monitor. The optical viewfinder affords 100% frame coverage, and has internal dioptre correction and interchangeable focusing screens. • TELESCOPE ADAPTORS - PRIME FOCUS The Sony 'Exmor' CMOS sensor size is 35.9mm x 24.0mm, giving a frame diagonal 43.2mm. The throat clearance I.D. of the prime focus adaptor needs to accommodate an image circle 43.3mm diameter (allowing 0.1mm clearance) otherwise the frame corners will be mechanically vignetted. Prime focus adaptors come in two basic types: the one piece CNC machined with a throat I.D. greater than 45mm, as manufactured by 'Telescope Adapters', & the two piece that comprises a 2-inch push fit sleeve or nose-piece & a standard T coupling. Both have a standard 48mm filter thread at the end of the sleeve I.D. The advantage of the two piece prime focus adaptor is that the T-adaptor may be swapped for another SLR/DSLR T-adaptor. The disadvantage is the restricted throat depth. <www.TelescopeAdapters.com> WILLIAM OPTICS 2-INCH PHOTO-ADAPTOR The one piece adaptor has a decided advantage when the focal ratio is slower than f/7. Despite the Fx frame diagonal being 43.2mm, with a fast APO it is possible to accommodate the image circle, unvignetted, with a two piece adaptor. For example: TEC140APO f/7 prime focus & WO Sony alpha T-mount & 2-inch sleeve The above diagram demonstrates that the WO adaptor's I.D. is just sufficient to fill the Fx frame at prime focus at f/7. This however is not the case at slower f/ratios, and when a Barlow is employed. For instance a x2 Barlow decreases the convergence angle to ~2º & contracts the unvignetted image circle to ~38mm. This is an example of a T-Adaptor & 2-inch sleeve: • TELESCOPE ADAPTORS - EYEPIECE PROJECTION Eyepiece projection enables a far larger image scale at the expense of a slower effective focal ratio. The adaptor needs to have three parts: the focusing mount nosepiece that fits into the 2-inch eyepiece collet; the eyepiece section, preferably adjustable so the amplification may be varied; the DSLR bayonet adaptor. I have examined numerous commercial eyepiece projection adaptors & not one of them was a well thought out design, or well made, so I designed & had the late Ron Irving machine it for me. The camera bayonet section is identical to the prime focus sleeve adaptor. The amplification factor of an eyepiece projection adaptor is calculated from: L A = −1 Fe where L is the projection distance (field stop to image € plane) Fe is eyepiece focal length The effective focal ratio is prime focal ratio times the amplification. The best type of projection eyepieces are either Plössl or Orthoscopic. I use a 25mm Plössl. • REMOTE RELEASE Exposures have to be made without touching the camera. It is not possible to depress the shutter release button without imparting vibration. You could use the Sony a900 2s delay, but this would not completely address the issue of induced vibration. The electromagnetic mirror release also produces a recoil that imparts vibration. It is necessary to lock-up the mirror prior to releasing the shutter. You cannot lock-up the mirror & then use the timer delay manually. The only remedy is to use a shutter release remote and herein lies several issues. • REMOTE RELEASE - SONY REMOTE COMMANDER RMT-DSLR1 The IR Sony Remote Commander comes with the Sony a900 and enables most of the camera functions to be controlled remotely. However it cannot be used to use the mirror lock-up before opening the shutter because the <DRIVE> menu option <REMOTE> has to be selected, and both the <REMOTE> & <LOCK-UP> options cannot be selected simultaneously, nor can they be selected consecutively. • REMOTE RELEASE - GADGET INFINITY RADIO S1 REMOTE The S1 RF WiFi remote release enables shutter release from up to 15m, & also supports <BULB> mode, but does not enable mirror lock-up. It comprises a receiver that connects to the camera body via a short connector cable, & a transmitter. The transmitter is powered by a L1028 12v 23A battery & the receiver by a CR2 3v battery. Battery life is ~20,000 exposures. • REMOTE RELEASE - PRACTICABLE REMOTE OPTIONS • REMOTE RELEASE - SONY CABLE RELEASE RM-S1AM The standard remote cable release for the Sony a900 is the RM-S1AM. It can control the shutter according to whichever <DRIVE> option is selected, <LOCK-UP> allows you to lock-up the mirror & then open the shutter. The accessory however is grossly over-priced There are three different types of remote which will enable the firmware <DRIVE> options to be overrided: IR Modular; WiFi RF; TIMER/INTERVALOMETER Cable Release. • REMOTE RELEASE - JJC-JR-C IR MODULAR REMOTE 4 pin Hot Shoe Adapter for DSLR/SLR cameras as FS-100; JJC-JR-C IR MODULAR REMOTE This IR remote is Sony RM-S1AM compatible. It comprises a bi-directional IR self powered receiver (which requires an ISO hot shoe adaptor 4-pin Hot Shoe Adapter for DSLR/SLR cameras as FS-100;) & a self powered transmitter, range 8m, with a direct 80cm cable option. It can operate mirror lock-up <DRIVE><LOCK-UP> option with one press of the button & then can open the shutter with a second press of the button. The only limitation is the CR2032 coin Li-ion cells which have a limited capacity and need replacing after ~500 exposures. • REMOTE RELEASE - HÄHNEL HW433S80 RF WiFi REMOTE The Hähnel HW433S80 RF wireless remote enables mirror lock-up & shutter release & <BULB> mode from up to 80m, & the HF signal will penetrate concrete walls. It also can be used as a 2.8m cable release. Battery life is similar to the S1 remote. • REMOTE RELEASE - JJC TM-F LCD DIGITAL TIMER / INTERVALOMETER The JJC TM-F LCD Timer/Intervalometer cable release is Sony RM-S1AM compatible, enabling mirror lock-up, shutter release, self timer, interval shutter release, exposure timer, multiple exposure timer up to 399 exposures. It has a long battery life. Set <DRIVE> option to <LOCK-UP>. • DRIVE FUNCTION - MIRROR LOCK-UP With these latter three IR & WiFi & cable remotes, <DRIVE> menu option has to be set to <LOCK-UP>. Because either of these remotes use receivers connected to the camera remote shutter port, the option <DRIVE> <REMOTE> does not need to be selected. • FOCUS Having made all the necessary menu settings & exposure & drive options, the next aspect in securing a digital image is focusing. Focusing the image is the most awkward aspect of astrophotography with a DSLR. • FOCUS - SCREEN L For focal ratios slower than f/4 gridded focusing screen L should be fitted. Screen L has a clear centre & a focusing frame with fine chevron marks within the target sections. FDA-FL1AM Type L focusing screen • FOCUS - DIOPTRE CORRECTION Adjust the dioptric correction scroll wheel on the side of the optical viewfinder until the screen chevron markings appear crisp & clear. Do this against a bright daylight sky. • FOCUS - RIGHT ANGLE MAGNIFIER Fit Sony right angle x2 viewfinder accessory to the viewfinder eyepiece, FDA-A1AM. Adjust the eye lens focuser until the screen grads are in focus. Use a bright star & adjust the rack & pinion focuser to bring it into sharp focus. Take a short exposure & check the image on the LCD screen @ x19. Adjust the fine focus until the star image appears sharply focused. Do this by systematically racking the fine focus in & then out until the stellar image is smallest. • FINE FOCUS - RACK & PINION FOCUSER Focusing a DSLR is far more critical than focusing an SLR because the DSLR sensor is flatter than the depth-of- focus, whereas 35mm film is bowed towards the pressure plate by several hundredths of an inch (by approximately twice the film thickness). The only way to critically focus a DSLR is by making small incremental adjustments of the focuser's fine focus knob. A high quality rack & pinion or Crayford focuser, with a 10:1 fine focus knob is the minimum requirement. Feather Touch® 3545 3".5 Dual Speed Focuser • FINE FOCUS - MOTORISED FOCUS Fitting a motor focus to the fine focus knob can help find & track focus. However unless the motor controller has a digital read out it can still be a hit & miss affair. • FINE FOCUS - REMOTE DIGITAL FOCUSER The focuser I recommend for focusing a DSLR is the Optec TCF-S temperature compensating Crayford focuser, with digital read out & PC remote control. It has a red LED digital read out and is adjustable in 80 micro-inch steps, over a 0".7000 range.