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03-26-2020, 08:18 AM   #1
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Astrophotography queries - K70 + O-GPS1

Still very much a beginner in this field. I think I've got the basics covered. I know how to calibrate with the O-GPS1 and how to focus using a bright star in manual focus. I can find and make an almost acceptable image of Orion's sword with my 200mm f/2.8, for example, this being a nice bright subject, and I can set up a wide angle lens (28mm f/2, 35mm f/1.9 and a couple of wider angle but smaller aperture lenses), to produce images of whole constellations or "nightscapes" without too much trouble, but how do I go about photographing a somewhat dimmer subject if I can't even see it in the viewfinder or in LiveView?

I'm basically familiar with using stacking software (DeepSkyStacker) to improve my signal-to-noise ratio, but this isn't of much use if I can't point the camera accurately at the subject in the first place. Is there some trick, technique or necessary piece of hardware I'm missing to enable me to point my camera approximately at M31 (Andromeda Galaxy), for example, even though I can't see it in the viewfinder? I know I can photograph it, it features as a small smudge in a wider-angle shot of the sky I took, but I'd like to be able to replicate something like the example shown in the tutorial ... which brings me to the second part of my query.

When taking multiple shots for the purposes of stacking, does the Noise Reduction feature(s) in the camera obviate the need for "dark frames"? Can I switch Noise Reduction off and use "dark frames" instead, or do the two functions serve to enhance each other? Further to this, should Noise Reduction be enabled when generating "dark frames"? Simply trying if not to save time (I accept that some sort of dark frame is recommended) but to maximise limited shooting opportunities and/or not to waste time unnecessarily. If I can take the required number of pictures in quick succession with Noise Reduction disabled, whilst the sky is clear or there's no aeroplanes passing, then take the "dark frames" later when conditions might not be quite so suitable, that could be advantageous


Last edited by kypfer; 03-26-2020 at 08:29 AM. Reason: Afterthought
03-26-2020, 08:27 AM   #2
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What I do to shoot dimmer areas is find a few key stars that indicate where I should be looking. Stellarium is a great resource for this. Live View is infinitely more useful than the viewfinder, and use manual focusing with the zoom function. Turn focus peaking off. Take a trial snap with ISO set to as high as you want (1/2 second exposures are fine) to see whether you've got the correct region in your image.

You're right about being able to turn off the NR in the settings. It's pretty much a forced dark frame.

What you should be aware of is how dark frames are affected by conditions such as ambient and sensor temperature. Make sure that your camera is at operating temperature before shooting your lights and maybe shoot some darks in any downtime you get. You might want to shoot some bias shots as well as flats, and again getting everything to the right temperature is key to avoid any influences of thermal expansion etc.
03-26-2020, 09:21 AM   #3
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If you're going to use a stacker and dark frames, turn off the Long Exposure NR entirely.

Google Sky Map on your phone, or other similar Augmented Reality apps will help show you roughly what you're pointing at. A laser pointer aligned with your lens can also help you get close.
03-26-2020, 11:51 AM   #4
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When you say dimmer object what are you referring to?
Where I am (a bright bortle 8 area with a 9 not far) there are lots of things that are not visible in the view finder, even with a 400mm f/2.8 lens. For things like this I like to know where they are and will usually start by using a phone planetarium app. The one that has annoyed me the least is one called Sky Safari. On the free end it has a much more complete catalog of objects than most other ones and also has the official names (M???, NGC?????, etc) so looking them up is easy. Basically if you are new to navigating the night sky use it it compass mode to orient your self and become familiar with the sky. Then once you know where to look take the app out of compass mode and see what bright constelations and stars are near by. At this point I usually start with a star in a constellation that I can find in the view finder and walk it from one star to the next until I think I have the object I am after in frame. At this point I push the ISO up real high if it is really dim and take an astro tracer shot. By real high with my K-3 or K-3ii I will run it at 12800 or 25600. This will allow me to see the object if it is in the frame but with such high ISOs it will be a mess but I am going to frame it up and won't use that shot as one in the stack. If I didn't get it in the frame I will look carefully in the view finder and take a mental note of the stars I see in it and will move it close to one frame in a direction (up, down, right, left) and take another shot until I get it in the frame. Between frames I return it to my starting position before moving to a different framing. Once I get the object in the frame I will center it and take note of what the start look like in the view finder so that when I readjust things I can make it look the same. At this point I start shooting normally. With a 200mm lens you will have a much easier go of things than I do with a 400 as you see an area about 4x what I do.

As far as framing up M31, (one of the objects I can't see from by horriably light polluted backyard with the naked eye) I will find the constellations of Cassiopeia and Pegasus. I start at the great square of Pegasus. If I were using this star map I would go 2 stars to the left and then up 1 star (actually when it is up in my backyard it is high enough and in a dark enough part of the sky that there is a barely bright enough to see star real close to it is actually up 2 stars). With your 200mm lens and the KP you should have it mostly in the frame. Through my 400mm f/2.8 I can barely see the dim central core of Andromeda if I don't look directly at it. I would imagine that you could also see it with your 200/2.8 especially if you are in a less light polluted area. M31 is huge as it it takes up about 6x the area in the night sky as the full moon does so if you are somewhat close you will likely be able to get the galactic disk to show up in frame if you shoot at ISO 12800

On the subject of in camera noise reduction, don't use it for astro shots. You would be better served by taking a pile of dark frames at the end as you tear stuff down and pack stuff up. The goal with a dark frame is not to remove random noise as it can't do that but it to remove specific types of systematic noise. A single dark frame in camera does some but the truly random noise still dominates the shot so while there is some improvement in a single shot you have now spent twice the time taking that single shot instead of taking 2 light frames. By avoiding the in camera dark frame noise reduction you can come close to taking double the amount of light frames which will provide a greater benefit for noise reduction than those in camera noise reduction shots. When one includes a pile of dark frames taken while packing up and tearing down you will get better results because having a pile of them removes a lot of the truly random noise from each one so that the systematic noise or error starts to show and when DSS does the master dark subtraction it will subtract the actual real systematic error instead of also subtracting a bunch of true random noise. Also by working in DSS for dark frame subtraction it will do it in 32bpc instead of the 14bpc that the camera does. This will help greatly in avoiding the loss of faint signal. Dark frames are session dependent and really can't be reused from one session to another. In theory you could if you knew the temperature\ and wind conditions and happened to find a close match but dark frames are trying to capture the error generated by the heating of the sensor as it is used during a long exposure. Where I am nigh temps can vary from about -30F to +90F which is a huge range so I don't even bother trying to create a library of dark frames.

Subtracting random noise from random noise at best works out to be a wash but in most cases will eat some of the signal too. However you will be subtracting more of the systematic error which is why there is a net benefit from the single shot noise reduction but the taking a pile of darks works better because you are driving down the random noise in each of the darks to get a better picture of the systematic error.

You mention taking dark frames when when planes are flying by, I don't do that. When stacking in DSS I use the sigma clipping method of stacking so if a plane flies by the pixels that captured it will be removed because they are outliers. I prefer to take my darks at the end since I'm not taking real pictures then and the camera is the last thing to get put away, especially since my astro tripod takes a good 10 minutes to tear down and pack up. However if I am getting a cloud that is passing I will shoot some darks then as well because I can and nothing of value was lost.

The most beneficial types of corrective frames to shoot would be a huge pile of flat frames to create a master flat for each f-stop you use. These are more important than darks and I've found are best shot on a cloudy day. Just stretch a white t-shirt across the lens, set the f-stop to what you use when shooting astro, focus to infinity, use ISO 100, put the camera into aperture priority, point it at a fairly uniform illuminated area of the sky (opposite the sun works best) and shoot a hundred of them.

03-26-2020, 04:17 PM   #5
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QuoteOriginally posted by HarisF1 Quote
You're right about being able to turn off the NR in the settings. It's pretty much a forced dark frame.
QuoteOriginally posted by SteveinSLC Quote
If you're going to use a stacker and dark frames, turn off the Long Exposure NR entirely.
Google Sky Map on your phone, or other similar Augmented Reality apps will help show you roughly what you're pointing at. A laser pointer aligned with your lens can also help you get close.
Many thanks for the detailed and informative replies, the laser pointer is a concept worth considering
QuoteOriginally posted by MossyRocks Quote
When you say dimmer object what are you referring to?
My "dimmer object" was a generic "anything I couldn't see in my viewfinder or on the LiveView screen" ... thanks again for all the other extensive detail, I'll be making written notes and referring to them next time I'm out. I've started making a few workflow notes, doubtless I'll be expanding them further as time goes by
03-27-2020, 04:24 AM   #6
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I use Liveview, crank up the ISO, shoot and adjust. Then I notice where the visible star(s) are located in relation to the subject I want to shoot. I did an 18x50sec on the Whirlpool Galaxy. M51, that's a tough one to get in the frame, then you have to keep it there - that's where the visible star(s) comes in handy.
1x50sec, search and you shall find, I guess it's faint enough? (K-70, M200 f4, ISO1600, O-GPS1)

You can see Alkaid cut in half at the top, I tried to keep it in that area using Liveview.

The final result: 18x50sec, stacked in DSS, cropped.
03-27-2020, 05:55 AM   #7
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QuoteOriginally posted by kypfer Quote
My "dimmer object" was a generic "anything I couldn't see in my viewfinder or on the LiveView screen"


Well there are dim objects like M31 and then there are dim objects like M51, there are dimmer objects but M51 is about as dim as I've gone with M101 and the Veil Nebula being fairly close. Even in the best of cases I don't have a lens with enough aperture to see M51. However the camera does see it just fine with a long tracked exposure, especially with my 400/2.8. Once I started going after things that weren't M42 and M45 ir became a lot harder to frame them up as they don't show up in the view finder or even at lower ISO shots.
03-27-2020, 11:32 AM   #8
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QuoteOriginally posted by DrawsACircle Quote
I use Liveview, crank up the ISO, shoot and adjust.

QuoteOriginally posted by MossyRocks Quote
Well there are dim objects like M31 and then there are dim objects like M51, there are dimmer objects but M51 is about as dim as I've gone with M101 and the Veil Nebula being fairly close ...

Thanks guys for the further details and examples, I'll have plenty to try out next time we get a clear night. For the time being the cloud is back, for the next few days at least, by which time there'll be the moon to contend with ... hey ho

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