Imaging bright objects

How I take images depends on the object in question. I usually use a webcam for bright objects inside our solar system, and a sensitive cooled camera for everything else.

The reason is that even though the webcam is much less sensitive than my main camera and has a much smaller dynamic range (brightness levels), it can take up to 25 frames a second, whereas the main camera is limited to one 0.1 second frame each second. To minimize the effects of seeing (air turbulence), I can therefore use the technique of "lucky imaging" with the webcam. This is a technique with exposure times short enough so that the changes in the atmosphere during the exposure are minimal. From these images (a movie really), I select the frames least affected by the atmosphere and combine them into a single image by shifting and adding the short exposures. This yields a much higher resolution than would be possible with a single, longer exposure and allows me to reach the diffraction limit of my telescope, about 0.5 arc seconds (one arc second is the apparent size of a dime about 3.7 kilometers away). By adding hundreds of individual frames like this, the effective dynamic range of the webcam increases, reducing the effects of noise, and I can apply advanced image processing techniques to further increase the resolution of the final image.

Because the exposures are short, I can also use the simpler ALT-AZ setup for the telescope, which is less sensitive to disturbances and vibrations by the wind. During the exposures, the telescope is passively tracking the object to counter the effects of the rotation of the Earth. The Earth's rotation moves objects with a speed of up to 15 arc seconds each second out of view or, with the image scale generally used for these images, between 30-60 pixels each second. The telescope mount can counter the effects of this rotation, but with my telescope, the remaining tracking errors have an eight minute periodic component of 21 arc seconds in them. With the built-in software of the telescope mount I reduced that to an eight minute periodic error of 7 arc seconds peak to peak (in polar mode; I never measured it in ALT-AZ mode but it sure is much higher).

But, because the individual exposures are very short, this remaining periodic tracking error in an 8 minute time period does not lead to image smearing.


Another benefit of the webcam for these kinds of objects is that, unlike my main camera, it is not a monochrome but a color camera. The 640x480 pixel CCD contains a Bayer filter (50% of the pixels have a green filter, 25% have a red filter and 25% have a blue filter). Although this means that each RGB pixel has at least two interpolated color components, it does have the benefit that the colors are shot at the same time. The only post processing required is relative shifting of the color channels, as the Earth's atmosphere refracts light at a slightly different angle for each color, which amounts up to several pixels on the image scale used.

The maximum number of frames that can be combined using this method is limited to a few thousand. Only a certain percentage, like 10%, of the frames can be used to maximize the resolution of the result. My wish to use an ALT-AZ setup for these kind of images also limits the total exposure time because of the effects of field-rotation in this mode. In addition, the imaged objects themselves rotate too, which leads to smearing if the movies are longer than about 10 minutes.

Saturn

Saturn on March 8, 2004.

Jupiter

Planet Jupiter floating in space on March 8, 2004. A combination of 662 frames taken with a ToUCam Pro webcam through the 254mm SCT. A total exposure time of 44 seconds.

Please note how this exposure is expressed in seconds, while most others of this blog are expressed in minutes or even hours. Jupiter's apparent brightness is extremely high compared to objects outside our solar system. In fact, only the Sun, Moon, Venus and Mars can reach a higher apparent brightness.

I clearly improved my technique compared to my first attempt of this planet with this telescope.

Eye Opener II

On several occasions when using the SCT visually, I noticed a significant glare around bright objects (e.g. planets). This is caused by internal reflections in the optical system. I also ran into this problem during imaging several time. For example the January 21 raw frames of M109 are completely ruined by the reflections of the light of a nearby star.
I found that the standard visual back of the SCT is a major cause of this. Flocking it is however not an option as this increases vignetting. To test this theory, I took some flats on December 26, 2003. Indeed, with the standard visual back my replacement f/3.3 focal reducer has an ADU drop of 7% across the field when used with the ST-7XE camera. But, after flocking the inside of this original visual back, this increased to 12%.
I figured that although an Eye Opener II would theoretically not have any benefits with my 1.25 inch eye pieces, nor with my cameras, it may actually lead to a reduction of the reflections. So I bought one, and found that it was actually even more reflective than the original visual back. Indeed, it is slightly worse than the original visual back in regard to the glare.
However, after flocking the inside of the Eye Opener II, it is actually much better than the original visual back. When using the camera with the f/3.3 focal reducer flocking the Eye Opener II doesn't have any negative effects (the 7% ADU drop remains). However, the glare is very significantly reduced when using it visually with bright objects.

So, for my purposes the Eye Opener II isn't beneficial because it reduces vignetting, but it is only because I have more room to add some flocking to suppress the reflections.

Determining Declination Backlash Using a CCD Camera

By using a 4mm eyepiece, speed 3 (8x sidereal) in ALT-AZ mode I found that my DEC axis backlash can be compensated by entering a value of 30 . This is the value I have been using until now. But, most of the time

• The telescope is used in a completely different configuration while imaging (heavy camera, counter weights, etc);
  
• I am using polar mode, not ALT-AZ;
  
• I am using a guiding speed of 66% sidereal (sidereal is 15 arcsec/sec, so I am guiding at 10 arcsec/sec), not 8x sidereal while imaging;
  
• For any speed other than guiding speed, the UP/DOWN buttons behave different. For guiding speed UP/DOWN moves the Declination axis only, but for any other speed, this moves the telescope Altitude. In ALT-AZ mode, Dec movements cause both axis to move, while in Polar mode, an Altitude movement causes both axis to move.
  

So, for a proper backlash compensation, the amount of backlash should really be determined in the same configuration as that will be used for imaging.

A more accurate approach to determine the DEC backlash compensation in polar mode is:

1. Set up the telescope with the camera attached as usual (but not with focal reducer for increased accuracy).
   
2. Turn off tracking (SETTINGS - TELESCOPE - ...).
   
3. Point the telescope to east.
   
4. Set the guiding speed to 66% (or whatever what you usually use).
   
5. Select guiding speed (MODE - 1)
   
6. Take an image of 60 second, while doing that, do the following:
   
   
   
   1. Press UP key for 10 seconds.
      
   2. Wait 10 seconds
      
   3. Press DOWN key for 10 seconds
      
   4. Wait 10 seconds
      
   5. Press UP key for 10 seconds
      
   
   
7. TODO
   

This is how I came to use the value 100 instead.

Of course, backlash is only a problem if the the guiding direction is changed, so, with a proper polar alignment and an AO7 all this is not really a problem. Also, as long as the direction is not change, the issue of retrograde motion also doesn't appear (this is the problem of a drive, usually the Dec drive, to move a bit in the opposite direction selected by the keypad, before resuming in the correct direction).

On the night of 2004-02-07 I set out to improve my DEC backlash correction using the Single Axis Mount Dynamics command of CCDOps. The telescope was mounted in polar mode.

TODO

No backlash compensation Backlash 8s, Y [-7.87,34.56]	Backlash compensation 25 Backlash 6s, Y [-7.33,48.56]
Backlash compensation 50 Backlash 4.5s, Y [-10.57,45.69]	Backlash compensation 75 Backlash 2.5s, Y [-33.86,33.80]
Backlash compensation 100 Backlash 1.5s, Y [-25.32,38.82]	Backlash compensation 150

Galaxy M109 in Ursa Major

The aurora in this image is the result of a reflection in the optical system caused by a star just outside the field of view. After taking this image I have been flocking the inside of the AO-7 adapter ring, which was a major contributor to this problem.

For this image, I have used a longer exposure time for the blue filter than for the other two, to compensate for the lower sensitivity of the main CCD for this color.

This is the first image I used my newly built light box for to do flat field correction.

2004-01-21: Luminance 50 minute exposure (5x600s). R and G binned 2x2 10 minutes (1x600s), B binned 2x2 16 minutes (1x960s). Camera cooled to -20°C.

MPL 2860 Pasacenten

A preliminary light curve spanning 4 hours for Minor Planet 2860 Pasacenten. Determined from 81 frames obtained on 2003-11-01. Each 3 minute frame was binned 2x2. The camera was cooled to -20°C. The telescope used was the 254mm SCT in a f/12 configuration (focuser/AO-7/CFW-8/ST-7XE). No focal reducer was used to minimize vignetting. The image scale is 1.23 arc seconds per binned pixel.

As can be seen from the light curve, the rotational period of this minor planet is around 2.5 hours. I used GSC3674:761 (mag 14.1) and GSC3674:3 (mag 13.5) as K,C stars for the photometry.

I also determined from these Pasacenten frames that I can go until magnitude 18.5 in 3 minutes when binning 2x2 at F/12 from my San Jose patio, and possibly a bit deeper. By combining all 81 images (243 minutes = 4 hours), I determined that I can go until magnitude 20 in that period. Possibly even deeper if during the last hour the frames were not out of focus due to the dropping temperature.