Comet C/2014 E2 (Jaques) images during the night 23rd - 24th August 2014

Part 2 of this tutorial can be found here.

INTRODUCTION

This is presented as a panel of key processing steps to record how I do star freeze images of comets. I use Deep Sky Stacker (DSS) to align and stack comet images.

I've been successfully applying star freeze procedures since 2004. A couple of methods (MinSubMax and Min2SubMax) are my own invention but are not generally applicable unless you have the right software (Images Plus to be specific). Other methods I've regularly used have been published by others or I have adapted and improved them. This time I'm using part of the Berhard Hubl method with my own "comet sieve" procedure and comet rejection method added on top.

All types of star freeze processing methods aim to do the following three steps: a) isolate the comet with statistical outlier rejection methods; b) isolate the stars - either using statistical rejection OR via background/artefact subtraction (or a combination of statistical rejection and artefact subtraction ) OR reshoot the star field when the comet is gone; c) combine the two images of stars only and comet only to create the final image. The final alignment should use the stacking reference frame from your original raw images. Each method only differs in its approach to these three crucial steps.

[TIP - if the comet is against nebulosity or Milky Way dust clouds - then opt to reshoot the field for the "stars-only" image. The comet rejection/subtraction methods tend to damage any extended object]

PRELIMINARY STEPS

After acquiring the images and the calibration frames, I star align and calibrate using the DSS "Register Checked Images" function. This is just a preliminary step to grade the images (don't stack them yet). After they are registered check the scores and remove the frames with low scores (non round stars, clouds intervened etc).

[TIP - I do the frame acquisition of the comet continuously - no time gaps - don't follow the Hubl suggestion that you leave a longish time gap between images - the time can be used wisely to gain a maximum number of frames, you just change the processing to use my sieving procedure - see below]

COMET ISOLATION - SIEVING THE STACK

[TIP - you might be able to avoid sieving if you have a lot of frames and the comet has moved a critical distance. See the NOTE at the end of this section that explains what sieving is trying to do]

Now we prepare a preliminary stacked image to work out how we are going to sieve the stack for the comet isolation step. Go to the first image (in the time sequence) and carefully mark the comet position (we are still using DSS here). Repeat this with the last image in the time sequence. Now chose the reference frame - usually this should be the frame with the highest score but equally you could choose it based on the framing you want (i.e. position the comet well with respect to the frame edges - possibly aiming for rule of thirds) - and mark the comet in this frame as well. To recap you mark the comet position in just three frames of the stack - not all frames. DSS is smart and uses the frame timestamps to accurately position all the intermediate frames. Now make a comet aligned and average combined image. The result is shown on the left hand panel (see above).

Now we look for the fattest star in the average stacked image and note if it overlaps with the time adjacent images of the same star in the trail of that star. Note in this example most stars do not form a continuous trail but are instead a series of dots with gaps between. This is because the comet moved by a significant amount during each frame. The distance between each star image in its star trail is the amount that the comet moved. Take a close look at the comet - its elongated. This is not a worry as there is a simple correction that I'll apply later.

[NOTE I don't guide on the comet and I wanted to reach long enough to record the very faint tail this comet sported]

If you look at the bottom middle area of the left hand panel image, the image of the largest star does touch its neighbour image in the trail of that star. This touching will interfere with the kappa-sigma procedure we are going to used to isolate the comet. So we must now make a decision on how to divvy (divide) up the stack for the sieve as it needs to avoid these star image overlaps. Count how many frames are needed for the fattest star to separate from a prior image of the same star. In this case we get non touching pairs after 2 frames - so we are going to create TWO separate stacks for the "comet-only" processing.

[TIP - if you are going to crop the image - pick the fattest star that will remain in the image]

Now here is the sieve procedure.... Deal each frame in strict time order into each of the N piles in turn (N = 2 in this case). If you have discarded frames, then for the purposes of the sieve you must pretend you still have them for the purposes of allocating each frame to the correct pile. So in this case the first frame goes into pile 1, the second frame into pile 2, the third frame into pile 1 and the forth into pile 2 ... and so on ... just repeat the procedure until all the frames are dealt out (including the phantom discarded ones).

[NOTE The idea behind the sieve is to improve the chances that each pixel in a comet aligned stack that has a star passing through it, has more frames with just background (or comet) occupying it compared to the number of frames where some part of a star is occupying it.]

Sieving can be avoided if A) there are no big bright stars passing through or near any part of the comet. These leave a difficult to remove residual streak if you stacked the lot together or B) the comet has moved during the course of the entire sequence by more than 4 coma widths in the direction it is travelling. In this situation the chances that each pixel in the image has more occasions of being occupied by background than a trailing star (for comet aligned images) or a trailing comet (for star aligned images) is greatly improved. If this is the case just kappa-sigma stacking all images in both star aligned and comet aligned modes may suffice - you may just have to try it first and then decide if sieving is worth a go]

COMET ISOLATION - REJECTING THE STARS

Take each pile in turn and in DSS choose "comet stacking" (on the "stacking" dialog box), choose to align on the comet and pick kappa-sigma stacking. Set kappa to 1.15 (but experiment) and iterations to 10 to 15 (again experiment) and stack.

The resultant stacked images are collected and aligned and combined (use average). I do this step in Photoshop using layers and an opacity sequence of 100%, 50%, 33%, 25%, 20% etc (you rarely ever need more than 3 sieved stacks - unless the comet is moving very slowly, only then would you be forced to use more piles). The result is shown in the middle top panel.

[NOTE this alignment and average stacking wasn't done in DSS as DSS is not designed to automatically align starless images. You will need software that supports manual alignment]

[TIP - if each pile has less than 9 frames then kappa-sigma has a hard time - switch to median stacking in this case. The results are not as good overall compared to those from longer imaging sequences.]

[NOTE - it is important to shoot a long time sequence of images to get Star-Freeze to work well. In this case we are not so interested in signal/noise ratios as in how far the comet moves in the overall time interval. So even if you think you have shot enough images for good noise suppression don't stop. A good measure is shoot long enough for the comet to have shifted at least 3 coma-widths. Slow moving comets will always be difficult if the night is too short!]

COMET ISOLATION - POSTPROCESSING THE "COMET-ONLY" IMAGE

This procedure is only needed if the comet's bright condensation moved noticeably during each frame exposure. The elongated comet was corrected by using "darken" blending in Photoshop. Duplicate the comet only image, set the duplicate to "darken" blending mode. Then use the "Offset" filter (under "filters -> Other") to shift the top image pixel by pixel to align the start of the elongated comet condensation trail to the end of the trail with maximum roundness.

STAR ISOLATION - MAKING THE "STARS-ONLY" IMAGE

The "stars-only" image was also created using the Kappa-Sigma combining method in DSS - in the comet stacking tab (on the "stacking" dialog) do that same as before but choose "align on stars". You should now do this on the entire collection of frames as the sieve is not practical (the coma diameter is too wide to find enough piles with enough frames in each to make Kappa-Sigma work here). The result is shown in the middle bottom panel.

Most of the comet does get removed - the tail certainly did in this case (its thin enough to separate frame by frame). You will see strange artefacts left by the overlapping portions of the comet coma (green and purple in this case - this is usual and expected). These artefacts are removed using Hubl's background extraction method (its a standard method - see Hubl Tutorial - it uses Photoshop in this case). Some changes I made are: a) the Hubl method results in a pair of images and one is subtracted from the other. In the relevant Photoshop dialog, I add 25 to the "offset" field before completing this step as otherwise the background is set at or very close to RGB (0,0,0) which is far too black and look clipped. b) I deliberately add noise (the Add Noise Filter) using a value of 0.1 to 0.2 - the subtracted background is unnaturally smooth and you need to add noise to match the levels found in the comet only image - otherwise the combined images look - well - "combined".

COMBINING THE "COMET-ONLY" AND "STARS-ONLY" IMAGES

Now you have the two images - "comet-only" - and "stars-only". If you have poor results isolating the stars you can always fall back to re-imaging the same star field later (I'm improving some of my early efforts this way).

Now the trick is to combine the images. First have the single reference frame (the stacking reference frame) at hand to guide the alignment. I use Photoshop for this. There are two methods - one based on "screen" blending and the other using the standard "lighten" blending approach. The former correctly blends star and coma colours but is difficult to use (so I'll ignore for now), the latter is easy but the method just picks the brightest pixels from the two blended images - these can create strange "punched through" stars that otherwise lie behind the coma - the colour boundaries are not blended well - too sharp!.

[TIP for the next step the background brightness of the "comet-only" image should be slightly brighter than the "stars-only" image - if its not then reverse the "comet-only" and "stars-only" images in the following description]

For lighten blending you create 5 layers in Photoshop (one is temporary). Bottom layer is the "stars-only" image. Then add a "levels" layer, then the "comet-only" image set to "lighten blending" and then another "levels" layer. Finally add the reference frame as the 5th layer with opacity set to 50% - you will use this to get the first and third layers aligned (then turn off the reference image). Now use the 2nd layer (levels) to set the stars only layer to match the background of the comet-only image - use the black point value in the "Output Levels" control (under the levels histogram) to do this. With careful adjustment of the output level you can get the two images perfectly blended (you should not need to cut out the comet from the background - that can always be detected on at least some ones monitor!!). Don't forget to turn off the reference image visibility when you do this. Finally the 4th layer (the top levels) is used to reset the black point on the histogram to darken the image to the required level - I choose a dark neutral grey background (25,25,25) to (30, 30, 30) when using a 0-255 brightness scale)

The result of the above processing is shown in the right hand panel. I haven't said anything about noise reduction, colour boosting and such like - that's up to you to do, whether on the pre-combined images, or on the final result.

Part 2 summarises the above but also discusses "stars only" and "comet" only image blending.

SUMMARY

So in summary the key elements were:

DSS for time stamp interpolation (Pixinsight can do this too) - for perfect comet alignment.

DSS for Kappa-Sigma - allows control of the 2 key parameters to control what is rejected. Decide if sieving might help based on the presence of big bright stars and how far the comet has moved compared to the size of the comet. To help with this decision, do an average comet aligned stack to chose the number of piles for the sieve using the "fattest star" test. If you decide to sieve then deal the images into the sieved piles on strict time sequence (accounting for discarded images).

Comet align and kappa-sigma stack each pile to generate intermediate combined frames, then manually average combine these intermediate results from the sieved piles to create the "comet only" image.

Star align and kappa-sigma stack all the selected images (you don't sieve these) - then use the Hubl method to do background extraction and artefact removal to create the "stars only" image.

Do your own processing on the comet-only and stars-only images to clean up (try and do the same actions for both) and then combine the result (cleverly!!!).

Try not to cut out parts of images - it always looks bad and can always be spotted - use smart blending instead.