David's Astronomy Pages Notes - Session 351 (2009-02-27) Notes (S324)   Notes (Main)   Home Page   Notes (S355)

Goto Images from 2009-02-27

Ted Agos Universal Focal Reducer Adapter Tube

In early 2009 I purchased a Ted Agos Universal Adapter Tube fitted with an Antares focal reducer.  This will become a main part of my new imaging setup from 2009 onwards.  

Ted Agos Adapter Tube fitted with Antares 6.3 focal reducer

The advantages of the Adapter tube are several

• it allows the focal reducer to be placed as close to the back plane of the telescope as possible
       thereby improving the image quality and reducing the effects of vigneting
  
• it reduces the total length (overhang) of the imaging train 
      thereby reducing problems or issues  
          - camera sag  / off-centre optical axis / variable illumination depending of scope position.
          - camera sag /relaxation leading to loosening of connections
          - telescope balance problems
          - fork clearance issues - may not necessarily be eliminated but can gain several extra degrees in Dec. access
  
• it permits the focal reducer to be placed at the right (optimal) spacing from the CCD camera
       thereby optimizing the performance of the focal reducer / reducing off-axis distortion

The Focal Reducer was tried out for the first time during session S351.

Experiences are listed below

1. Based on my equipment train ST-7/CFW8a, I slightly modified the position of the focal reducer using the push tube. Exactly as you said, the reducer moved relatively easily in the adapter, yet all the time it was held nice and tight in the slips. 
   
2. I then screwed the tube into the front of my CFW8a filter wheel. The adapter T-Threads fitted very well and the ST7/CFW8a/AdapterTube together formed a nice and solid unit. 
   
3. Inserting the Adpater tube/CFW8A/ST7 into the Optec TCF-S however showed up a problem. The adapter tube wouldn't go all the way in, but stopped around 10 mm short of the expected position (which I had imagined would be shoulder at the end of the adapter tube). Without taking off the Optec TCF-S to look (I will one day) I assumed that the end of the tube was hitting against a smaller opening at the back of 8" LX200. That's a pain I thought, I won't get quite the saving in imaging train length that I was hoping, but still the train was a lot shorter than my previous setup, so I pushed the adapter in as far as it would go and tightened the three locking screws on the TCF-S to hold it in place. Almost straightaway I could see that there was going to be a problem. Luckily I was doing this during the daytime with a clear head. My TCF-S focuser was positioned near the middle of it range (say at ~3500 steps, out of 7000 step range). Now I know that when I turn on the focuser it immediately goes through a routine of moving to step 0 position to initialise itself before moving back to either its previous or a temperature compensated position. This means pulling everything into the tube by a distance of 4-5mm. If I had the adapter tube at it maximum insert depth and the TCF-S had tried to initialise it would have desperately tried to pull the tube in further and possibly ground up its gears in the process (I would have been even more desperately trying to hit the off button !) It's good that I picked this up, but someone else might not ! I don't think this will be a problem with my new 12" LX200R as the 12" model (like 10" & 14" models) has a larger opening at the back of the scope which would be able to accommodate the 2" adapter tube. (I am of course assuming that the 'block' is not some component of the Optec TCF-S focuser) 
   
4. I moved the focuser to the 0 position and then inserted the adapter to its limit, and then pulled it out a few mm. as a safety margin, and tightened by 3 locking screws to hold the adapter in the focuser. Overall my imaging train was some 14mm longer than the length I was expected it. But still the train is a lot shorter than it was before. I' haven't done do, but I can feel that I will need to take off /adjust some of my scope's balance weight 
   
5. At night, I refocussed the primary mirror for the new CCD position and started to take some pictures. Image quality looks fine at first glance. Obviously I have the new Anatares reducer now rather than my previous Meade reducer, Based on the new image scale (2.77 arc sec/pixel at 2x2 binning compared to 2.46 arc sec/pixel before) I could calculate that I had moved my setup from f/7.6 to a new focal length of f/6.6. and had obviously moved my CCD closer to the back plate of the scope. I have clearly gain back some of the field of view I lost when I put on my longer TCF-F focuser and extra length adapters, and have moved the focal reducer back into the region where distortion should be less. 
   
6. With the shorter imaging train I should be able to image closer to the poles without the camera hitting the telescope base. Carefully moving the scope towards the pole I found that declination limit for safe unattended slewing had indeed increased, but only from 58 deg to 64 deg. This was less than hoped, mainly due to the loss of 14 mm or so because I couldn't insert the tube as far as I would have liked. Another 10 mm would have had a big difference (and should have got me to around 70-73 deg). If I can insert the tube in further with my new 12" scope, which has larger clearance anyway, I should eventually get back to 75 deg Dec or so. 
   
7. Examining the image orientation by linking them via TheSky. I found that my images where orientated at c. 180.7 deg, rather than 180.0 deg which I normally like to aim for. This then raised the problem for me that unlocking the adapter and rotating the camera, would potentially move the tube either in or out during rotation which might either change focus (annoying but fixable) or accidentally cause me to loose by safety margin for when the focuser moved to 0 position at next start-up. Since I don't have the shoulder available to hold the adapter in position whilst rotating I decided to continue by session at 180.7 orientation rather than mess with it.
   
8. Whilst I didn't notice it in my first session or indeed in dark sky images in my second session, I did find that images of the slightly bright sky at the start of my second session showed increased brightening in the centre of the image compared to what I might normally see. I haven't looked at these closely yet, so not sure if this is increased vignetting (corners darker than I had before) or it is brightening in the center due to focusing of reflections. 

So there we go. My overall feedback is that a) you may wish to note the warning regarding the issue of using Adapter with Optec TC-S and 8" LX200 b) you might consider whether to use a slightly shorter tube (or promote a shorter tube option in such cases) Some of the above issues will (should) go when I moved to 12" LX200, ST-10 combination (they're ready to move into my observatory with its new taller roof). 

Overall I think I'm happy with the Adapter tube (or at least will be shortly) 

If I was staying with my 8" LX200 I think I would be looking to cut around 15mm off the bottom end of the adapter tube with a hack saw (or better still in a machine workshop) 

Moving forward. I was wondering about the vignetting issue which I'm already seeing with my smaller ST-7e camera (if vignetting is what the problem is). The issue might simply reduce or be eliminated by adjusting the focal reducer spacing to a more optimum position, as this has large effects as you have documented. I'm a bit concerned however that if/when I move to using a AO-8 unit (when I will push the reducer up to the AO-8 end of the adapter tube to reduce the extra spacing to its minimum), I will be left with long section of tune before the adapter with its less than 2" ID (1-7/8" ID ?), which might cause more vignetting than a shorter tube, where I would have access to full 2" or 2.5" of aperture at the back of the scope. So I still might need to cut off the end of the Adapter tube. 

Ted Agos Adapter Tube fitted with Antares 6.3 focal reducer Perspective view		Side View
T-Threads lightly filed (left),  Tube without End-Shoulder (right)		Push Rod, modified with simple scale bar (mm)
Adapter with End-Shoulder removed		Push Rod inserted into Tube to move sliding insert

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ST-10XME CCD Tests, Gain, ReadOut Noise, Dark Current

A series of tests were performed on my new ST-10XME CCD camera on/around 2009-02-18 to 27, with the goal of characterising the performance the camera to confirm its general health and provide parameters to input into advanced photometric SNR calculations.

Camera Expectations

The ST-10XME camera is 16 bit, and uses the KAF 3200ME CCD.  This is a Non-ABG CCD and has is specified as having high QE (peak QE 85%) due to micro lens over each pixel. and low dark current,

The full well capacity is understood to be 77,000  e- and gain is 1.32 e-/ADU.
The camera is specified to have a dark current of 0.5e-/pixel/sec at 0 deg C

Saturation Level

At 1x1 binning, saturation level in the camera was measured to be around x ADU.  This compares with an expected figure of ~58333 ADU.
At 2x2 binning, saturation level in camera is ~ x ADU. This compares well with the expected value of ~x ADU

Basic CCD Test

A basic CCD test was conducted based around the methodology given in "The Handbook of Astronomical Image Processing", Section 8.2 by Richard Berry and James Burnell  [http://www.willbell.com/aip/index.htm]

• Camera turned on and allowed to reach thermal equilibrium at -25 deg C
• 2 flat frames obtained using the same identical integration time, and using 2x2 binning (F1 and F2)
• 2 bias frames taken (B1 and B2)
• 1 dark frame taken with 60s integration time (D)

The flats were taken using white illuminated board on the wall of the observatory and a tungsten light source, with the integration time selected to provide a suitable Flat Frame count; around 50% of saturation level. 2x2 is the normal binning mode that I use for taking light frames at night.   Frames were also collected at 1x1 and 3x3 binning for comparison.

In practice 3 flat, 3 bias and 3 dark frames were taken allowing 3 pairs of bias and flat frames to be created.
 

Manipulation of Frames  (Mean and Standard Deviation)

Frames were analysed using AIP4WIN. (AIP4WIN software is available with the "The Handbook of Astronomical Image Processing". It has superior analysis tools compared to those of CCDsoft ) 

The two bias frames were added together and the mean pixel value was measured  μ_(B1+B2). Next one bias frame was subtracted from the other and the standard deviation measured  σ_(B1-B2).    The left-hand image below shows features in the total bias. The right-hand image below shows that subtracting one bias frame from the other removes any features in the bias leaving the total noise in the two bias frames. The noise is √2 times the noise in a single bias frame

The two flat frames were added together and the mean pixel value was measured  μ_(F1+F2). Next one flat frame was subtracted from the other and the standard deviation measured  σ_(F1-F2).   The left-hand image below shows features in the total flat. The right-hand image below shows that subtracting one flat frame from the other removes any features in the flat leaving the total noise in the two flat frames. The noise is √2 times the noise in a single flat frame.

The first bias frame was subtracted from the dark frame to provide the dark (thermal) current recorded over the integration period (D-B).  The mean value was measured  μ_(D-B).

Calculation of Gain  (e-/ADU)

For a frame containing a high signal level such as a flat field frame, it can be expected to display Poisson statistics when measured in electrons,   i.e.  
         σ _electrons =  √μ _electrons
however since both σ and μ  have been multiplied by the conversion factor or gain, g (in e^-/ADU) we have actually measured
         g.σ _electrons =  √g.μ _electrons

solving for g
        g = μ _electrons   /  σ² _electrons

           g =  μ_{(F) ADU}   /  σ² _{(F) ADU}

A slightly more accurate figure for g can be obtained by subtracting the influence of the bias mean and noise

    g =  μ_(F1+F2) - μ_(B1+B2)          (in   e^-/ADU)
              _{-------------------------------}
                σ_(F1-F2) - σ_(B1-B2)   

 Calculations provided the following results

These figures compare with an expected gain value of 1.32 e^-/ADU. 

Calculation of Read-Out Noise (σ _ron)

The only source of noise in a bias frame should be the read-out noise. The sum of the readout noise in two bias frame has been measured from the standard deviation of the difference of two bias frames σ_(B1-B2).   And we know that this value is √2 times the noise in a single bias frame.  Therefore we can calculate readout noise as follows :

    σ_readout     =     g.σ_(B1-B2)     /   √2    [ in e^-/pixel ]

Based on measurements and the gain values calculated above, the following readout noise results were obtained based on the average of 3 tests :

These figures compare with an expected read-out noise of 15 e^-/pixel, and indicate that the camera is in good health.

A separate run in which Read-Out Noise was calculated for 2x2 binning at a 29 different set-point temperatures indicated a mean read-out noise of  14.1 +/- 0.2  e-/pixel 

Calculation of Dark Current

The mean dark current value of the 60s dark frame was converted to a dark current per second by dividing it by the integration time

    D _ADU  = μ_(D-B) / 60    [ in ADU/pixel/sec ]

or in electrons

    D _electrons = g.μ_(D-B) / 60    [ in e^-/pixel/sec ]

The following results were obtained based on 3 tests.

The figure of 0.43 compares with an expected dark current of  0.06  e-/pixel/sec  (based on expected 1 e-/pixel/sec at 0 degC and a 6 degC doubling rate temperature).   Notionally it would seem that the camera is underperforming in terms of dark current

Further Testing

A further sets of tests were carried out to understand the behavior of the camera under various states of cooling.
 

Bias/Dark Testing (2009-02-18)

Using an automated routine sets of 2 bias and 2 dark frames were acquired at 1x1, 2x2 and 3x3 binning over a range of set point temperature between 16 and -30 deg C. The data was acquired in two batches: one indoors (ambient temperature 20 deg C, where CCD was cooled through the range 16 to -11 degC ) and one outdoors  (ambient temperature 4 deg C, where CCD was cooled through the range -11 to -30  degC).  Five minute stabilisation periods were employed after each temperature set-point change.  Dark exposure times were  90s (1x1), 90s (2x2) and 60s (3x3) respectively.

Bias 

The Bias value for each temperature set point was recorded and plotted.  

Whilst there is a degree of fluctuation (related to instability in CCD temperature and Camera Body Temperature ?) there is reasonable match up between the data sets collected at ambient temperatures of 20 deg C and 4 deg C.   The data show that mean bias decreases with decreasing CCD temperature, with bias values that tend towards an almost constant value below around -20 degC.    (Each point is the average of the mean from 2 bias frames). 

Graphs of Bias vs CCD temperature  (S351 Tests)

Bias values for 2x2 and 3x3 binned frames are incrementally higher than for 1x1 binned frames, but show similar trends of decreasing mean bias with decreasing CCD temperature.  The data can be reasonably well modelled assuming a 'baseline' minimum with a doubling rate value of around 6.5 deg C

Besides understanding the performance of the camera under different operating conditions the principle purpose of generating functions that model the mean bias values is to use them in my observatory software to subtract estimated mean bias and dark values from raw light frames in order to provide realtime measure of sky brightness (in ADU/sq arc sec).

Dark Current

The Mean Dark value for each temperature set point was recorded and plotted.

Whilst again there is a degree of fluctuation (related to instability in CCD temperature and Camera Body Temperature ?) there is reasonable match up between the data sets collected at ambient temperatures of 20 deg C and 4 deg C.   The data show that mean dark value decrease rapidly with decreasing CCD temperature, with dark values that tend towards the mean bias value at temperatures below around -15 degC. This indicates that dark current is exceedingly small (and practicallyt insignificant) at the CCD temperatures that will be normally be used for observing (typically -25 degC).

Graphs of Dark Values vs CCD temperature  (S351 Tests)

Subtracting Bias values from Dark values for each temperature set point and dividing by the Dark Exposure time gives Dark Current values in ADU/sec/pixel.
This confirm that the Dark Current of the ST-10XME is exceedingly low and become almost zero below around -15 deg C. 

At -25 deg C the Dark Current is 0.01 ADU/sec/pixel (1x1 binned darks)  [ or 0.015ADU/sec/pixel (2x2),  0.02 ADU/sec/pixel (3x3) ].  
With gain of 1.32 e-/ADU the dark current of 0.013 e-/sec/pixel is significantly lower than suggested by the camera specification - a welcome surprise but consistent with measurements made on the KAF 3200ME CCD by Richard Berry in a different brand of camera. [ reference here ]

Graphs of Dark Current vs CCD temperature  (S351 Tests)

The dark current becomes so low that its level falls to much less that the read-out noise, such that dark current measurements made on individual frames are sometimes negative.  (i.e. the uncertainty level associated with the read-out noise and very small dark values are such that the mean pixel value of a 90s dark exposure can sometimes be less than the mean pixel value of a bias frame taken immediately before or after the dark).

Graphs of Dark Current vs CCD temperature  (S351 Tests)

Read-out noise

For each temperature step the read-out noise was calculated from the standard deviation of the difference between pairs of bias frames and using a gain of 2.69 e-/ADU.   Average readout noise for the 29 steps was 14.1 +/- 0.2 e-/pixel. A graph of read-out noise vs CCD temperature is shown below.  As can be seen the data suggests that readout noise is pretty independent of temperature . However the mean bias itself is dependant on temperature and seems associated with dark current in the camera electronics as the bias frame is being read and counted.

Graphs of Mean Bias and Readout Noise vs CCD temperature  (S324)

Linearity Tests

Graphs of Mean Flat value vs Exposure  under 'constant' light conditions showing linearity up to ~ 50,000 ADU for 1x1 binning, (66,000 e-) surprising the 2x2 and 3x3 binning are also non-linear from around 50,000 ADU

Linearity/Well Capacity Testing

Single Bias Frame (3x3 binning)

Bias Frame 3x3 binning,  2009-02-18

Hot Pixels (3x3 binning)

Bias Frame 17 x 60s (Average median), 3x3 binning,  2009-02-18

Graphs of Mean Dark Current vs CCD temperature  (1x1 binning)

Graphs of Dark Current vs CCD temperature  with simple model curves (S324)

Graphs of Dark Current vs CCD temperature  with best fitting models (S324)

Gain Measurements / CCD Transfer Curves  (S324)

Linearity Tests (S324)

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Cosmic Rays / other radioactive decays

In early 2009 I purchased a Ted Agos Universal Adapter Tube fitted with an Antares focal reducer.  This will become a main part of my new imaging setup from 2009 onwards.  

Cosmic Rays captured during acquisition of Dark Frames
Cosmic Rays captured during 85 minutes
		Maximum of  Dark Frame - Dark Median (50% size reduction)  17 x 300s, 3x3 binning 2009-02-20 (#352287-303) Full Size
Cosmic Rays captured during 17 minutes
		Maximum of  Dark Frame - Dark Median (50% size reduction)  17 x 60s, 3x3 binning 2009-02-20 (#352219-35)
Cosmic Rays captured during 3 minutes
		Maximum of  Dark Frame - Dark Median (50% size reduction)  17 x 10s, 3x3 binning 2009-02-20 (#352134-50)

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