Beginning Astrophotography: Session 6 - PowerPoint Presentation

Beginning Astrophotography: Session 6 Selecting, finding and Focusing an object Geoff Smith

Selecting an object The type of object — g alaxies, bright and dark nebulae, clusters, dust, planetary nebulae —is for you to choose. Bright and dark nebulae may be the easiest subjects. Star clusters, especially globular clusters, need good seeing and spot on tracking. Dust is difficult —you need long exposures and excellent processing Planetary nebulae are often small, so long focal lengths are needed or else they are large and very faint so long exposures are needed Galaxies offer a wide choice, from really large ( Magellanic Clouds) to zilch. Size matters — You need to match the object to your equipment

Size Matters Too small is can be OK as long as you have the resolution. You can always crop. Don’t try to image a 2 arc minute galaxy with a 200mm focal length

Size Matters Too large may be a problem But you can always consider a mosaic FLI 35mm x 35mm chip QSI 15mm x 15mm chip

Know your FOV.Know the size of your object. Use an online calculator for FOV. For example https://www.bintel.com.au/use-bintel-astrophotography-calculator/ Or do a simple calculation Here is a sensor dimension and is the focal length. This gives FOV in arc minutes and is accurate to better than 0.03% for focal lengths greater than 200mm and sensor sizes less than 36mm  

Finding an object Just point the telescope: works OK for bright objects. Setting circles — a dying art. In fact, most mounts now have uselessly small ones or none at all. Use a GoTo telescope —w orks for me Planetarium program

Is the object there? Some objects are so faint that you may not see them in a short exposure. Maybe your GoTo is less than perfect and the object is just outside the FOV Is it there? If not, where is it? Use Plate Solve to check.

Plate SolvingA name hangover from the days of glass photographic plates A plate solving routine determines the exact coordinates of the centre of an image, as well as various other parameters. You can use it to: Check if you are pointing in the correct place Ensure that you point to the same location on multiple nights If your pointing is a little off, you can use a plate solve to synchronise the mount

Plate Solving Software Astrophotography software often has plate solving capabilities MaximDL (Has Pinpoint Lite) SGPro (Various compatible plate solvers) May also be used as stand-alonePinpoint http://pinpoint.dc3.com/ (US$149) Platesolve2 (free). Works with SGPro Astrotortilla (free) https://sourceforge.net/p/astrotortilla/home/Home/

Focusing It is extremely important to get as sharp a focus as possible It’s hard work: it takes practice and proper equipment, and it will take you quite some time and effort to become good at it. Large focal ratios are more forgiving than small focal ratios One criterion is the critical focus zone (CFZ). In microns it is given by At f/11, CFZ = 0.266mm At f/5, CFZ = 0.055  

Approximate focusing Point telescope at a 3 –4 magnitude star. Display continuous exposures of around 1sec. You should see an out of focus star. Adjust focus to make star as small as possible without fussing too much.

Manual Focusing Methods: Live View: DSLR Basics of Live View Focusing Use manual exposure and manual focus. Use ISO 1600, bulb shutter-speed, and a wide aperture. Use a bright star or planet to focus on. Get close to focus before you use Live View. ( Previous slide)Zoom in to 10x to focus.See http://www.astropix.com/html/i_astrop/focus/live_view_focusing.html for more detail

Manual Focusing Methods: Bahtinov Mask

Manual Focusing Methods: Bahtinov Mask Make one yourself: http://www.deepskywatch.com/Articles/make-bahtinov-mask.html Buy it: https://www.bintel.com.au/product-category/imaging/imaging-accessories/

Manual Focusing Methods: Software assisted Astrophotography software will often give information about star images. This can be used to achieve optimal focus. Here is an example from MaximDL

Focuser Control Get an electric focuser! Operating a focuser by hand is extremely frustrating (although it can be done) The image quivers at every touch and probably things will be out of reach.

Autofocusing methods You need to have approximate focus to begin with Autofocusing relies on moving the focuser in and out, taking images and then measuring star sizes to determine the best focus position. The software will then move the focuser to this position

Autofocusing software: FocusMax http://www.ccdware.com/products/focusmax/ FocusMax works with MaxImDL , CCDSoft V5, and TheSkyX Professional Camera Add OnFocuses on a single starIf you need to refocus you will have to leave your object and go to a suitable star.

Autofocusing software: Sequence Generator Pro http://mainsequencesoftware.com/Products/SGPro SGP contains an autofocusing routine: Stay in focus even when the temperature doesn't want you to! Our powerful focus metrics will be able to determine proper focus for almost any deep space target. No need to slew off target to find suitable focus stars! Simply start your sequence in good focus and auto focus will take care of the rest!

Next Time: Image Calibration Improve the quality of your images by using: Flat frames Bias frames Dark frames What does it all mean? How do we take calibration frames?How do we use them?

Questions?