Im­age Pro­cess­ing

Re­duce the ther­mal noise in your as­tropho­tos.

Sky at Night Magazine - - CONTENTS - With Sara Wa­ger

Cal­i­bra­tion frames should play a key role in your data pro­cess­ing. With Astro Pixel Pro­ces­sor soft­ware (APP) us­ing cal­i­bra­tion frames is easy and they make such a dif­fer­ence to the im­ages you pro­duce that you’d be mad not to use them.

The cal­i­bra­tion frames we’ll be us­ing are flats, darks and bi­ases. Bias frames re­move the elec­tronic sig­nal in­her­ent in the sen­sor and should be taken with the dust cap on and at the fastest ex­po­sure pos­si­ble. Darks are taken with the dust cap on and at the same ex­po­sure length as the light frames

(the shots of the ob­ject you’re imag­ing); they’re used to re­move hot pix­els and amp glow within the sen­sor. Flats are taken to counter in­her­ent vi­gnetting and any dust on the fil­ters or op­tics. These aren’t taken at a spe­cific ex­po­sure length as the aim with flats is to cap­ture data around a third to halfway across the his­togram. As such, flats need to be taken against a light source, for ex­am­ple a light box.

It’s worth not­ing that cal­i­bra­tion frames ben­e­fit mas­sively from be­ing taken at the same tem­per­a­ture as the light frames. This can be al­most im­pos­si­ble for DSLR users, but easy if you cam­era is astro-cooled.

Cal­i­bra­tion be­gins by cre­at­ing a Bad Pixel Map (BPM) so you can deal with any hot and cold pix­els. Af­ter that you’re go­ing to use the in­te­gra­tion process to cre­ate mas­ter cal­i­bra­tion frames. If noth­ing in the imag­ing train changes then your flat frames can be reused for sub­se­quent imag­ing ses­sions; bias and dark cal­i­bra­tion frames can be reused for many months. We used our cal­i­bra­tion li­brary for more than eight months.

Get started

Start by open­ing APP and load­ing your light frames by se­lect­ing the ‘Load’ op­tion from the menu on the left and ‘light’ from the frame op­tions be­low the menu.

Add your flat, dark and bias frames by fol­low­ing the same process (‘Load>flat’; ‘Load>darks’; and ‘Load>bias’). There are no lim­its for how many of these frames you can load – we like to add as many as pos­si­ble, per­haps 30 flats, 30 darks and 100 bias frames. You can check that all your cal­i­bra­tion frames have loaded by us­ing the right scroll bar on the lower con­sole.

Pro­duc­ing a BPM can take a bit of trial and er­ror. Se­lect the ‘Cal­i­bra­tion’ op­tion on the menu. Keep ev­ery­thing at its de­fault set­ting but en­sure that the ‘cre­ate bad pixel map’ and ‘de­tect hot col­umns’ tick boxes are ticked, and that the ‘Cre­ate MasterBias, -dark -flat’ tick box is not. Then click ‘Cal­i­brate’ to cre­ate your BPM.

Dou­ble click the ‘bad pixel map 1’ file at the bot­tom of the lower con­sole to see the BPM, then click the ‘de­tails’ tick box at the top of the win­dow to see the read­ings. What you want to see is about three per cent hot and cold pix­els. Ad­just the ‘hot pix­els kappa’ rat­ing us­ing the drop-down menu on the cal­i­bra­tion panel (lower the value to in­crease the per­cent­age) and click ‘Cal­i­brate’ again, then re-check your bad pixel map. Re­peat this process un­til you reach about three per cent. We find that a Kappa fig­ure of 1.9 works well.

Make your mas­ter frames

To cre­ate reuse­able mas­ter cal­i­bra­tion frames there are a cou­ple of steps that need to be fol­lowed at the start of your first in­te­gra­tion process. Once the mas­ter frames have been cre­ated, you can load them up in­stead of all the in­di­vid­ual frames on each sub­se­quent in­te­gra­tion.

En­sure that the ‘Cre­ate MasterBias, -dark -flat’ tick box is ticked and the ‘cre­ate bad pixel map’ and ‘de­tect hot col­umns’ are not, then click ‘Save cal­i­brated frames’. APP will cre­ate mas­ter cal­i­bra­tion frames in the lower con­sole. These will be saved au­to­mat­i­cally in your orig­i­nal work­ing di­rec­tory along with your cal­i­brated lights. We like to save these cal­i­brated lights so that they can be used and added to sub­se­quent imag­ing runs.

Add the bad pixel map you cre­ated ear­lier by go­ing to ‘Load>flats’ and se­lect your bad pixel map. This will load into the lower con­sole along with your light, flat, dark and bias frame as well as your masters.

It’s al­ways worth­while check­ing how well your cal­i­bra­tion frames have worked. You can do this by open­ing a light frame in the lower con­sole by dou­ble click­ing it. The im­age will open in the mid­dle screen. Along the top of this you’ll find a drop­down ‘im­age’ menu. Click on this and se­lect ‘l-cal­i­brated’ and the im­age will change to the cal­i­brated im­age. This is a use­ful vis­ual check. You can zoom in by click­ing the left but­ton on your mouse to see if most of the hot pix­els have gone.

From this point, ev­ery­thing can be left at de­fault ex­cept ‘Out­lier re­jec­tion’ in the ‘In­te­gra­tion’ tab. Se­lect ‘win­sor sigma clip’ and ‘kappa 2.5’. This will en­sure that any fi­nal hot pix­els are elim­i­nated. Click ‘in­te­grate’ and APP will cre­ate a cal­i­brated, in­te­grated stack of your lights data, which can be viewed in the lower con­sole.

Now when you next in­te­grate any data you just need to load in the mas­ter dark, bias and flat frames, and the BPM, then in­te­grate them as above.

The flat frame (left) shows dust; the mas­ter bias frame (cen­tre) re­moves any light read­ings in the sen­sor; mas­ter dark frames (right) show hot pix­els

Choose ‘load’ and then se­lect whether you want to add your flat, dark and bias frames

Post-cal­i­bra­tion im­age Hot pix­els vis­i­ble in the pre-cal­i­bra­tion im­age (cir­cled red) have been vir­tu­ally elim­i­nated in the post-cal­i­bra­tion im­age

Check all of your cal­i­bra­tion frames have loaded by scrolling down through the lower con­sole

Click ‘de­tails’ at the top of the screen to see your bad pixel map in­for­ma­tion

Pre-cal­i­bra­tion im­age

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