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 dioptrecorrection 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 =LFe

−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.

• UN-DAMPED VIBRATION - ELECTROMAGNETIC SHUTTER RELEASE

It is important that the telescope is properly balanced, both about the polar axis and the Declination axis. Any imbalance, especially about the Dec. axis, will result in un-damped shutter vibration. The electromagnetic shutter release produces a sharp recoil. Because the shutter blind is raised vertically, it maybe effective to rotate the DSLR body so the direction of shutter travel is perpendicular to the Dec. axis.

Image vibration due to shutter release recoil manifests itself as duplicated or elongated image details, aligned in the same direction (typically in the plane of the dec. axis). Un-damped vibration will prevent the critical focus being found.

• CHECKING FOCUS

The Sony a900 LCD view-screen can be used to check focus at x19 (raw file). It has sufficient resolution to make a precise check on focus at night. The only difficulty in using the view-screen to check focus is when photographing the Sun in H-alpha. The H-alpha image is low contrast and it is difficult judging the precise focus owing to reflections off the view-screen. Reflections maybe reduced by using a screen cover PCK-LH4AM which is plane polarised. Additionally a Hoodman LCD screen Loupe will enable the view-screen to be seen @ x1 without any reflections marring the image. The screen Loupe does not magnify the view-screen pixels, it fits over the 3” screen to remove glare.

• LAPTOP REMOTE CAPTURE & CONTROL vs LCD VIEW SCREEN

The Sony a900 maybe controlled from a remote laptop using the bundled software package “Remote Camera Control”. The camera is supplied with a USB cable but it will not be long enough. You require a 5m repeater cable. All the functions of the camera can be controlled using a laptop and this software package. The LCD screen resolution is 784x1176, so unless the laptop monitor has a resolution of WXGA or better, their is no advantage.

• LAPTOP REMOTE CAPTURE & CONTROL vs LCD VIEW SCREEN (cont.)

The disadvantage is the USB upload time which can be well over a minute. The twin BIONZ processors in the Sony a900 can transfer the raw 26Mb image file to the LCD view-screen in one or two seconds. By reducing the image size to <S> 2x2 binning, the upload time can be reduced to less than a minute, which is still too long compared to the upoad time to the view screen. The Sony “Image Data Suite” also only allows you to view the uploaded image @ x4. Where a laptop does have a decided advantage is when photographing the Sun in H-alpha. Providing the laptop can be placed either in the shade or within a nearby shed, it will be much easier to check the uploaded image is precisely focussed. But because the upload times are so much longer for the raw files, than checking them on the LCD view-screen, it is better to try to obtain a close focus on the view-screen first.

• EXPOSURE - MANUAL MODE OPTION

Set the lens lever to <M> and the PASM mode dial to <M>. The telescope adaptor does not have a confirm feature, so the camera does not receive any information, either focal ratio or the presence of a lens. It is possible to set the PASM mode dial to <A> aperture priority and set the focal ratio, but for all the sophistication of the metering system, it is better to calculate exposure times and bracket ±2 stops if necessary. When photographing the Moon pay particular attention to the terminator. The dark areas near the terminator, if underexposed will be exhibit Chroma noise. Increase the exposure time if necessary until the dark areas are correctly exposed, even if it causes the limb to be burnt out. Frames may be stacked during the processing phase to increase dynamic range.

• EXPOSURE - CALCULATING EXPOSURE TIMES

The exposure time is calculated using the following formula:

€

t =f /#2

S *B where f/# is the objective focal ratio or the effective focal ratio

S is the ISO rating of the sensorB is the Brightness Value

B values are published in “Astrophotography” by Barry Gordon 1985 Willmann Bell Inc.An exposure time calculator is available from Glen LeDrew at “The Starry Room” <g_ledrew@hotmail.com>For example: Moon @ First / Last Quarter, TEC140APO @ prime focus + x2 Barlow ef/# = 14; B = 40; ISO100

€

t =142

100* 40=1964000

≅120

sec. Bracket the exposures ± 2 stops

• EXPOSURE - ISO SETTING & RESOLUTION

Raising the ISO setting amplifies the gain on the sensor, it does not increase the sensor’s Quantum Efficiency. Amplifying the signal increases the random noise, which for each photosite is a fixed proportion of the well depth, at any particular temperature. The Exmor CMOS sensor’s ISO range extends from ISO100 to ISO6400. The native sensitivity of the sensor is ISO100. Sensor noise affects the sensor’s resolution and so does shooting jpegs. For the best resolution you should always shoot raw frames @ ISO100 to ISO800. Do not set ISO to <AUTO>.

The Sony a900 ISO range is ISO100 to ISO6400. The ISO setting merely amplifies the photosite electron count, it does not increase the (QE) of the sensor, and the amplification greatly increases random noise. Below ISO800 noise is acceptable, and can be handled either in camera or post exposure. Faster than ISO800 the Exmor CMOS sensor noise becomes intrusive and above ISO3200 objectionable. Unless there are compelling reasons ISO100 to ISO800 should be selected. On no account use the <ISO RANGE><AUTO> option. It is advantageous to minimize seeing effects during short exposures to increase the ISO rating and reduce the exposure time concomitantly. For exposure times shorter than 1/100s set ISO100.

• EXPOSURE - ISO SETTING & RESOLUTION (cont.)

Noise between ISO100 & ISO800 is acceptable, between ISO800 & ISO1600 noise is tolerable; between ISO1600 & ISO6400 noise is objectionable. Dark frame subtraction reduces thermal noise and this can be done in two ways. A set of dark frames can be shot before and after the session, averaged, and then subtracted from each frame. The camera’s BIONZ processors can also be set to do a dark frame subtraction after each exposure. Because dark frames must be taken at the same temperature as the image frames, it is better to use the in-camera dark frame subtraction since it is taken immediately after each exposure and will be at the same temperature. The temperature will fall throughout a lengthy imaging session, so the averaged before and after dark frames will only be a close approximation. Where the exposure times are brief, in-camera dark frame subtraction is preferable.The signal to noise ratio may be decreased using raw file noise reduction and by stacking frames post exposure.

• EXPOSURE - ISO vs SEEING

It is tempting to increase the ISO rating and reduce the exposure time so as to “beat the seeing”. Where exposure times are shorter than 1/100s there is no point in doing so. In Seeing AI the image wavefront is ostensibly plane, so there is no advantage to increasing the ISO rating. In Seeing AII increase the ISO rating to ISO200 or ISO400, and in Seeing AIII increase it to ISO800.

• EXPOSURE BRACKETING vs DR-O

The dynamic range of the Exmor CMOS sensor is 12EV @ ISO100 with DR-O off. Increasing the ISO rating decreases the dynamic range. The Sony a900 has a Dynamic Range Optimizer (DR-O). For Solar & Lunar photography this should be set either to <OFF> or <STANDARD>. For deep sky photography of nebula set DR-O to <ADVANCED AUTO>. For deep sky photography of star fields or clusters set DR-O to <ADVANCED Lv5>.

High dynamic range imaging of the Moon is best done by stacking bracketed exposures during the processing phase.