Im­age Pro­cess­ing

Stack­ing comets with Deep­SkyS­tacker.

Sky at Night Magazine - - CONTENTS - With Dave Ea­gle

Deep­SkyS­tacker is a fab­u­lous piece of soft­ware for stack­ing im­ages to­gether. It works very well for static deep-sky tar­gets, but when it comes to comets, you need to do a bit more work. That’s be­cause comets do one thing other deep-sky ob­jects don’t: they ap­pear to move against the back­ground stars. Al­ways con­sider that when plan­ning on imag­ing comets. If the move­ment of the comet is greater than the res­o­lu­tion of your op­ti­cal setup, then un­less you are guid­ing on the comet it­self you will need to use shorter sub-ex­po­sures to pre­vent blur­ring. Comet 45P/Honda-MrkosPa­j­dusakova, which came very close to Earth ear­lier this year, had a rapid ap­par­ent move­ment forc­ing many im­agers to use sub-ex­po­sures of less than 30 sec­onds. Sub­se­quently, each sub-ex­po­sure showed the comet in a slightly dif­fer­ent po­si­tion. Af­ter reg­is­ter­ing your im­ages, the stack­ing process must take the po­si­tion of the comet into ac­count and stack on the comet it­self to avoid blur­ring de­tails in the coma and tail.

An­other thing to note is that the de­tails within a very ac­tive comet can change over the course of a few min­utes. Try to avoid tak­ing your subs over an ex­tended pe­riod, as when stacked, they will once again blur if the comet’s fea­tures have changed dur­ing the pe­riod the ex­po­sures were taken.

Once you have used Deep­SkyS­tacker to reg­is­ter your subs (don’t for­get to cap­ture them in RAW in the first place), the stack­ing process out­lined here needs to be fol­lowed so the pro­gram ‘knows’ where your comet lies on each ex­po­sure.

If the comet is fairly faint you may need to in­crease the star de­tec­tion thresh­old when reg­is­ter­ing to in­clude the comet in the de­tected stars. If the comet can­not be de­tected, don’t worry, we’ll show you later how we can give Deep­SkyS­tacker this in­for­ma­tion be­fore stack­ing.

Now find the comet again

Once your subs are reg­is­tered, click on the first im­age in the win­dow be­low the pre­view panel. At the side of the im­age pre­view, four but­tons will be vis­i­ble. Click the third one down, a green comet shape.

When pressed, tiny green rings will ap­pear around the po­si­tion of the stars the reg­is­ter­ing process has mapped. Hover your mouse over a star and you will get a prompt to set the comet here. In other

words, Deep­SkyS­tacker is ask­ing you: ‘Is this the comet?’ The point you can click will jump from star to star as you move your mouse across the im­age. Hover the mouse over the bright­est spot within the comet’s coma. A ‘set comet here no­ti­fi­ca­tion’ will ap­pear.

If the comet in your im­age is too faint to see, look just above the top-right of the pre­view im­age and move the mid­dle grey marker closer to the black point on the left. This will brighten the view with­out af­fect­ing the im­age. Find the comet’s po­si­tion and hover the mouse over it. Zoom in on the im­age by rolling the mouse scroll wheel for­ward. It takes a while to get to grips with this. Tar­get the mouse pointer di­rectly over the bright spot; a mag­ni­fied view (top left) also helps

with this. Hold down the shift key and press the left-hand mouse but­ton. The po­si­tion of the comet will now have been se­lected in this sub.

It’s worth get­ting the po­si­tion spot on as it im­proves the qual­ity of the fi­nal im­age, re­sult­ing in straight star trails. Click on the next im­age and re­peat the process, work­ing your way through all your subs. Note that the #Stars col­umn to­tal will now have a (C) be­side it, in­di­cat­ing that a comet’s po­si­tion has been added. Be­fore mov­ing onto the stack­ing process, check that all your subs show this.

Ev­ery­thing is now ready for stack­ing. Click Stack Checked Pic­tures, then

Stack­ing Pa­ram­e­ters. The Comet tab will now be vis­i­ble. Sev­eral stack­ing op­tions are avail­able: Stan­dard Stack­ing ig­nores

the comet and stacks on stars, while Comet Stack­ing ig­nores stars and stacks on the comet po­si­tion marked in each sub. Stars + Comet Stack­ing at­tempts to stack on both. Un­der the Light tab, se­lect the best ‘stack­ing mode’ set­tings for your im­ages. Sim­plest to use is the En­tropy Weighted Av­er­age

Stack­ing Mode, though be care­ful if us­ing this mode with full frame RAW DSLR im­ages. These fre­quently crash the soft­ware due to their sheer size.

Once you are happy, click OK to close the stack­ing pa­ram­e­ters win­dow, then OK again. Your im­ages will now start stack­ing on the comet’s po­si­tion and au­to­mat­i­cally save the stacked TIFF im­age.

Comet C/2014 Q2 Love­joy in Jan­uary 2015, stacked on the stars – re­sult­ing in a trailed and blurred comet

If the comet is too dim to iden­tify in a given frame, you can in­crease the bright­ness us­ing the slider at the top-right of the pre­view pane

Hover your mouse over a star and you will be able to set it as the comet; you’ll need to do this for ev­ery frame you want to stack

An al­ter­nate view of C/2014 Q2 Love­joy in Jan­uary 2015 from the same data; this time stacked on the comet with trail­ing stars

The Comet tab shows the three stack­ing op­tions avail­able; switch to the ad­ja­cent Light tab to pick the stack­ing mode

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