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