High dy­namic range Orion Neb­ula

Sky at Night Magazine - - THE SKY GUIDE -

Messier 42, the Orion Neb­ula, is a very pop­u­lar tar­get for as­tropho­tog­ra­phy, and for good rea­son too. It’s bright, colour­ful, easy to find and shows a wealth of de­tail. If you’re not that ex­pe­ri­enced, the bright core re­gion, known as the Thrust, pro­vides a good prac­tice tar­get. Then, as ex­pe­ri­ence grows, you can con­cen­trate on the glow­ing outer wisps of ne­bu­los­ity.

Here you’ll run into a prob­lem; how can you re­tain the beau­ti­ful de­tail and colour in the Thrust re­gion while pulling out the faint outer wisps? Mod­ern cam­eras can go some way to­ward do­ing this when used in

con­junc­tion with image pro­cess­ing tech­niques. These in­volve stretch­ing the image to ac­cen­tu­ate its dim­mer parts while not los­ing the brighter core.

How­ever, a sim­i­lar, and pos­si­bly more vis­ually nat­u­ral, re­sult can be achieved by sac­ri­fi­cial imag­ing. Here you con­cen­trate on one area at the ex­pense of the other. For ex­am­ple, longer ex­po­sures can be used to pull out the faint wispy stuff al­low­ing the core to over-ex­pose to white. It’s some­times dif­fi­cult to do this as an im­ager be­cause it just feels wrong to let it hap­pen. The core is even eas­ier to image be­cause you just need to make sure the in­ner re­gion ex­poses cor­rectly and not worry about cap­tur­ing the outer stuff.

Both bright and dim images should ideally be ap­proached us­ing reg­u­lar deep-sky imag­ing tech­niques. This re­quires tak­ing mul­ti­ple images, which are then cal­i­brated us­ing dark frames and flat fields to en­sure the best qual­ity. How­ever, if you’re new to imag­ing and don’t feel com­fort­able with cal­i­bra­tion, regis­tra­tion and stack­ing, there’s no rea­son why this can’t be done with sin­gle images to start with.

If you do de­cide to go the ad­vanced route, ba­sic cal­i­bra­tion in­volves re­mov­ing hot-pix­els and vi­gnetting from your light frames – that’s the term used to de­scribe the images of the ac­tual sub­ject mat­ter. Free­ware such as Deep­SkyS­tacker (deep­skys­tacker.free.fr/ english/in­dex. html) can do this for you but you still need to cre­ate the cal­i­bra­tion data. Hot pix­els – un­nat­u­rally light pix­els caused by tiny sen­sor im­per­fec­tions – can be cor­rected by cov­er­ing the tele­scope aper­ture af­ter you’ve taken a set of light frames. By tak­ing an­other set of shots at ex­actly the same ex­po­sure, the re­sult­ing dark frames will iso­late the hot pix­els present in the light frames (they’re al­ways in the same place); they can now be eas­ily sub­tracted from the light frames.

Flats are a lit­tle more com­plex. You need to point the imag­ing tele­scope at an evenly il­lu­mi­nated light source and take images that sat­u­rate the cam­era sen­sor to about 50-70%. The sat­u­ra­tion can be de­ter­mined by the po­si­tion of the peak spike in the his­togram dis­play for the image.

Ran­dom ther­mal noise in each image re­quires a num­ber of darks and flats to be taken and av­er­aged to­gether. This re­duces the back­ground noise ac­cord­ing to the square root of the num­ber of images in­volved. For ex­am­ple, if four images are used, the noise is re­duced to a half its orig­i­nal value. If nine images are used, the noise is re­duced to one third.

Al­though this may sound com­pli­cated if you’re just start­ing out, pro­grams such as Deep­SkyS­tacker do much of the don­key work. Your job is to make sure the images you sup­ply it are fo­cused and tracked as ac­cu­rately as your equip­ment will al­low.

When dif­fer­ent re­gions of a DSO re­quire dif­fer­ent ex­po­sures, you need to com­bine two shots

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