Nebula Photos Sunday Chat #13 Recording
Added 2022-05-09 00:16:57 +0000 UTCRecording of today's live chat session. Thank you all for coming, and special thanks to those who presented on their M78 processing workflows.
Comments
Reading your reply again, I must say that you gave an important key point: "Undersampling requires less expensive equipment and is more tolerant of poor sky conditions which is why it is rightly recommended for beginners".
A.Linhares
2022-05-14 08:58:34 +0000 UTCCompletely agree, it is a complex, confusing and large topic. Thanks for your reply.
A.Linhares
2022-05-11 18:54:36 +0000 UTCTo take advantage of oversampling (or even 'perfect sampling') requires more expensive equipment (larger telescopes/mounts) and the right conditions. The biggest advantage is to resolve small details which is great for galaxies, stars, etc. Undersampling requires less expensive equipment and is more tolerant of poor sky conditions which is why it is rightly recommended for beginners. Masters of astrophotography still might choose to undersample in order to gather lots of light quickly on a relatively large object. For example, see Adam Block's APOD Blue Horsehead pic here (https://apod.nasa.gov/apod/image/2107/ic4592_block_2132.jpg) which was done at over 4"/px. Note: the usual complaint I hear about undersamplng is 'blocky stars', but they look fine to me here. Best of both worlds is high resolution (oversampled) and great light gathering power (fast optics) under great skies, but that is prohibitively expensive for most amateurs since it requires large, heavy telescopes on big mounts in remote locations. There are some intriguing telescopes now available off-the-shelf for amateurs like the Planewave Delta Rho 350 ($18.5k) and the Celestron 36cm RASA ($14k), but even these pale in comparison to the size of the telescopes in professional observatories. Of course, the professionals have run into the same problems with the atmosphere we run into, so they devised adaptive optics where the mirror can deform into millions of configurations many times per second to counteract the seeing. Probably won't see that in the amateur market in our lifetimes. Just realized I didn't cover lucky imaging. It is a technique that amateurs actually can use to try to 'beat the seeing' and resolve small details. It is definitely useful for solar system imaging, and there are some promising results using it for deep sky too, at least with bright objects like PNe. So yes, confusing and large topic, and hope this reply helps, more than hurts.
Nico Carver
2022-05-11 18:13:59 +0000 UTCHi Nico, thanks for your reply. I will look deeper into the entendue topic. The idea that I have on entendue is light gathering power" (maybe not the best definition). This way, as the pixel size increases the trend is to entendue to increase but with increase of pixel size one would go for a situation of undersampling (where the trend is to lose dimmer starts). Thus, where is the middle point? Sorry to make all these questions, but for someone that is starting (or little experience) all those topics are confusing... and my advice to future readers is to not overthinking :)
A.Linhares
2022-05-11 17:47:34 +0000 UTCYes, more faint stars (and possibly extended details) will definitely show up with more aperture and better sampling. But here's the rub. In order to actually photograph those fainter stars and details, all other conditions (atmospheric seeing, light pollution level, tracking accuracy, etc.) must also be met. For most amateurs living with some light pollution and average seeing, it is difficult to ever use all the resolution that a telescope can theoretically achieve. This means in practice that those fainter/smaller objects (stars mostly) would be invisible whether you sampled at 1"/px or 4"/px. That's what I say it's often not worth sweating too much over perfect sampling. And we haven't yet delved into the topic of focal ratio or quantum efficiency. If you are really into the math aspect of characterizing different systems, there were some good discussions on Cloudy Nights in the past few years on this topic, if you search 'entendue'.
Nico Carver
2022-05-10 19:43:53 +0000 UTCMy mind is still processing the topic on the importance of undersampling vs oversampling. Shall we consider the next hypothetical situation: start size 1 arc-sec, sensor A 1 arc-sec/pixel and sensor B 4 arc-sec/pixel. On the sensor A, the star would fulfil completely the pixel (because the size of the star matches the pixel size). But on the sensor B, only 1/16 (ratio of areas) of the pixel will receive the light of the star. Thus, the difference of magnitudes as seen between sensor A and B is 3 magnitudes! ( mA-mB = -2.5 Log10( fA / fB), where m= magnitude and f = light flow). If my interpretation is correct, this would means that fainter objects may be invisible when undersampling… What is your opinion on this?
A.Linhares
2022-05-10 19:27:33 +0000 UTC
