Technical Intro to Digital Astrophotography Part 2

1. Technical Intro to DigitalAstrophotographyPart 2

2. Quick Recap of Pt 1 About 30-75% of photons, depending on thesensor (also frequency - color), are converted to electrons. This, the sensitivity to light of a sensor, is called Quantum Efficiency (QE) and is fixed for a sensor. The ADC converts the voltage to numbers, the Analog/Digital Units (ADUs). The ADCs vary in range of ADUs they can store, e.g.0-4096 for 12bit ADCs, 0-65563 for 16bit. Gain is an amplification of the voltage before it enters the Analog to Digital Converter (ADC). On DSLRs we control it with the ISO settings.

3. Noise & SNR • Noise is the enemy of a good image. There are various noise sources, from the low photon flux (low light), to noise from electronics, light pollution and heat (dark current). • Maximizing the Signal to Noise Ratio is our main concern for getting the best possible images. • The main way for increasing signal is by taking more and/or longer exposures and stacking them, or going to a faster optical system (perhaps with a reducer). • For reducing the noise, we take more exposures, use calibration frames, dithering, cooling, filters, larger/binned pixels.

4. Color vs Monochrome • All sensors are mono: color sensors justadd color filters in a “bayer matrix”. • A processcalled debayering or demosaicing tries to “guess” all3 colors for a pixel from the informationfrom neighbor pixels.

5. Astro-modified DSLRs • DSLR’s built-in filter blocks 75% of the Ha and S-II spectral lines (red emission nebulae). • We remove the filter completely, or replace it with a steep UV/IR cut:

6. DSLR ISO • ISO is an amplification of the voltage from ALREADY detected photons – it has nothing to do with sensitivity to light. It amplifies both the signal and the existing read noise equally, but does not amplify ADC noise. • Increasing ISO reduces our dynamic range and can limit our exposure (we hit 100% intensity faster), so be aware that more exposure is ALWAYS better than higher ISO. • What to do: Aim for the longer exposure you can, given your telescope/mount/guiding/sky. You can increase the ISO as much as you can without washing out the sky – lower the ISO if it is limiting your exposure length. • Don’t go over ISO 6400 for relatively recent cameras (e.g. circa 2010+) or over ISO 3200 for older ones – useless digital amplification is applied to reach the high end of the ISO range of a camera.

7. Part deux

8. Light Frames & Calibration Light Frames: The actual images of your subject. Taking several, all at the same ISO and exposure time will reduce noise and bring out more detail after they are stacked together by a program like Deep Sky Stacker. No limit (more = better). Before stacking them, Light Frames need to be calibrated with Dark Frames, Flat Frames and Bias/Offset Frames or Dark Flat (Flat Dark) Frames. As we saw, with debayeringthe original single-color pixelsare used to guess full-colorpixels. Since the defects thatcalibration fixes apply to theORIGINAL monochrome pixels,calibration has much better results when done before de-bayering, e.g. by using RAW files without conversion in DSS.

9. Dark Frames Dark Frames: Use the same ISO, exposure time and (sensor) temperature as Lights, but with cap on. Aim for 20 or so, especially if you don’t Dither. For Canon you don’t have to take the Darks at the same time as Lights, as the sensor temperature is recorded in RAW. The software Dark Master (https://sourceforge.net/p/darkmaster) will match your Lights/Darks and export to DSS. Both with darks and lights, theLiveView and screen in generalshould be disabled and yourDSLR viewfinder covered (that’swhat that rectangular plastic onthe strap is there for). Also disabled: in-camera long exposure noise reduction.

10. Flat Frames Probably the most important calibrationframes – they record defects of the opticaltrain. They require a uniform light source in front of telescope/lens with EVERYTHING, including camera focus, exact rotation, filters, ISO etc the same as with the Light Frames. You can use Auto Exposure (Av) for help (you want about 50% histogram), except when using narrowband filters. Common methods are: shooting the twilight (can add gradients), shooting a uniform wall (requires moving the scope), stretch a white t-shirt tightly on your aperture, point a torch (single LED is better) to it from a distance. For small apertures, a tablet or laptop in front of the telescope is usually enough. Some laptops at some exposures produce horizontal scan lines, for those you add the t-shirt. Dedicated light-boxes that use LED strips are the best solution, and it is a good DIY project if you are into that.

11. Bias/Offset & Dark Flat Frames Both your Darks and your Flats contain the Read Noise & the Offset, so if you subtract them from your Lights, you are removing it twice, hence you are adding the noise back. To be able to use Darks and Flats effectively, you need either or both these types of calibration frames: Bias/Offset Frames: Same ISO as the rest, smallest possible exposure (1/4000s for most) with cap on. 20+ are enough. You can reuse them for months. Dark Flat (Flat Dark) Frames: The same rules as the Darks, but for exposure you use the Flat Frame exposure time.

12. Dithering Dithering involves making sure that the same parts of the image do not fall on the same pixels, so that random defects/noise will be removed after we use Median or Kappa-Sigma stacking. For autoguiding with PHD2 or Metaguide, we can use a free program called APT (ideiki.com) to control our exposures (with CCD or Canon DSLR) and move the aim a bit between each exposure. For unguided, unless we have perfect PA, we have some movement between frames, but it is not random (and can introduce ugly pattern noise). So, we could manually tap a direction on our control pad every 2-3 or so frames (obviously not during an actual exposure), following a sort of spiraling pattern – i.e. left, then up, right, down, then a but farther left etc. We are aiming for movements in the order of 10px or so.

13. DSS settings No bkg calibration if you use StarTools. Median: Safe choice for any number of frames. Kappa-Sigma can do well with many frames. Can help a bit with noise, could cause trouble. You can try. Don’t drizzle unless you know what you are doing. Don’t Align RGB if you use StarTools. Median, or perhaps Median Kappa-Sigma If you have many calibration frames.

14. DSS settings • Register settings: If you don’t get enough stars recognized, lower threshold (2-5%) and try “median filter”. • Groups: Allow you to use light frames of different exposure or ISO – you just add them in different groups each with its own set of calibration frames.HOWEVER: It is not something that isrecommended to do – try to use the samesettings for all light frames. In VERY few situations, most notably M42,you need shorter exposures for the very brightparts and DSS groups can do that, howeveryou will still get better results if you stack eachset and process it separately and do thecombining at the very end (e.g. via HDR inPixInsight or Layer module in StarTools).

15. Broadband Filters • Generic LP filters: Baader Neodymium, Skywatcher LP filter, other Moon & Skyglow filters etc. • Relatively inexpensive, not very effective. Cut some of the light pollution, along with an almost similar amount of stellar light. Help a little with nebulae in the city. May require UV/IR-block filter for mono CCD/full spectrum DSLR.

16. Broadband Filters • Expensive Multi-layer LP filters: IDAS LPS-P2 (preferred over similar LPS-D1), Orion Imaging filter etc. • Expensive, target specific light pollution spectral lines. Don’t block too much stellar light, so fine even for galaxies etc. Work as UV/IR block, relatively color balanced. Can’t do much for heavy light pollution.

17. Broadband Filters • CLS type filters: Astronomic CLS, Optolong CLS etc. • Not too expensive. Cut a lot of light pollution, but along with stellar light. Still a benefit even for galaxies in heavy light pollution – as always emission & planetary nebulae benefit the most. Also come in UV/IR blocking “CCD” versions.

18. UHC/Narrowband/Line Filters • Narrowband filters are for emission and planetary nebulae. • Astronomik UHC (some other “UHC” filters are broadband), DGM NPB: More “extreme” than CLS, good for visual as well. • Orion UltraBlock, Lumicon UHC: More visual filters, since they allow the O-III & Hβ lines (blue/green nebulae), but not Hα. • O-III, Hα, Hβ, S-II etc: Line filters isolate a single emission. Cut through most light pollution. Combination of S-II, Hα, O-III in place of R, G, B gives us the Hubble palette.

19. Filter Comp. – M42 from Stockport UV/IR Moon & SG Orion Imaging Optolong CLS Optolong Ha 12nm UHC

20. Filter Comp. – M31 from Stockport UV/IR Moon & SG Orion Imaging Optolong CLS Optolong Ha 12nm UHC

21. Which DSLR? The one you have is fine. If you are buying one specifically for Astrophotography, it makes sense to buy an inexpensive crop-sensor, perhaps even second hand. Otherwise your budget will reach cooled-CMOS/CCD levels which perform better. I suggest Canon, even though you can often find better sensors in Nikon and Sony, because they work with much more (and mostly free) software, they can do planetary imaging (again with free software – or without even a PC for a couple of models), there are many modding services for them, they make Dark libraries possible (i.e. reusing Dark Frames) and give you more back-focus than Nikon (enabling e.g. M42 manual lenses). Canon 450D is the minimum suggested, and the 500D slightly improved. The 550D, 600D, 650D, 700D, 100D, 60D have pretty much the same (better than the 450D & 500D) sensor. Of those: 550D & 600D can do planetary imaging without a PC (the 550D a bit sharper), the 600D, 650D, 700D have an articulated screen, the 100D is smaller/lighter, the 60D is heavier and more expensive (due to extra non astro-related features). Canon 1100D, 1200D and 1300D lack some features like mirror lock-up, so the previously mentioned should be preferred at least at similar prices. The 750D/760D have an improved sensor, but are expensive. Also, some units (early ones?) exhibit some horizontal banding due to the Phase Detection AF pixels (dithering would be required).