Sony A900 - Alpha 24.6MP DSLR Camera User manual

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.
<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.
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.

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:
€
A=L
Fe −1





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
FDA-FL1AM
Type L focusing screen
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.

•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
Feather Touch® 3545 3".5 Dual Speed 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.

•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
OPTEC TCF-S 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.
The technique I use is first focus on a bright
star, checking the image on the LCD view
screen, RAW image @ x19. Then point the
telescope at the object of interest & take a
sample shot. Check the focus on the LCD screen
@ x19 & make systematic adjustments in 10 &
then 5 step increments until the focus is as
sharp as the seeing permits. Because there is
backlash in the spur gear train between the
stepper motor and the Crayford focus spindle,
(~±20 steps) you must rack in @ 10 or 5 step
increments until it is evident the focus is
becoming worse, and then backtrack until you
home in on the sharpest possible focus. Seeing
will limit your ability to locate the precise focal
plane.

•FINE FOCUS -DEPTH OF FOCUS
Conrady deduced a formula for depth of focus (DoF) based on the geometric circle of confusion. For a 1/4wave wave-
front error the depth of focus is given by:
DoF = 4 x Lambda x f/#^2 where Lambda is the wavelength (22 micro-inches)
For the TEC140APO prime focus f/7 DoF = 0".000 088 x 7^2 = 0".004312 (54 steps on the TCF-S focuser)
However in Seeing: Antoniadi I the wave-front is essentially plane, and the only significant wave-front error arises from
OG. In the case of the TEC140APO the P-V wave-front error is 1/28 wave, which is 7 times less than Conrady's
formula. (~±4 steps in the TCF-S focuser)
In achieving critical focus you are fighting three things: the depth-of-focus; the seeing; focus shift with falling
temperature.
•TRACKING FOCUS
A refractor's focus will shift inwards as the ambient temperature falls. For example my TEC140APO focus shifts ~-
0".100 during a night. The easiest way to track & follow this focus shift is by using the Optec TCF-S focuser piggy
backed on the TEC's StarLight Instruments FeatherTouch 3545 rack & pinion focuser.
It is possible to focus the Sony a900 using the Optec TCF-S to within ±4 steps (1 step = 80 micro-inches). This is the
limit, it is impossible to obtain a finer focus. The Exmor CMOS sensor is flat to about 5 micro-inches, so the focus
tolerance is approximately 64 times the sensor's flatness.
•SEEING
Seeing distorts the object wave-front & takes the image in & out of focus. Once the critical focus has been found, a
sequence of exposures will produce some frames more sharply focused than the rest. This is why it is worthwhile
shooting dozens of frames & selecting the sharpest.