Astrophotography: An Intermediary

[ravnostic]

So I’m getting used to the wedge-mount (it’s been nearly a year); I’ve got guidance down to up to 4 minutes when the fates are kind; I’ve advanced in my stackings, I took my first 50 biases last night. Time for a new album, me thinks.

I’ll start it off with this one. It’s just a link; leads to a page with a 14 Mb jpg of the moon, a composite of 46 individual frames, about 4800×4800 pixels. I’m quite happy with it.

http://fossilspringsaz.com/pics/2012/oct/28/moon_stitcheditjpgfull.jpg

[CauseISaidSo]

With all the recent advances in photography and imaging technology, when are we going to start seeing color photos of the moon? 🙄

(Kidding, of course – just repeating a much-mocked question I saw someone on the interwebs ask a couple of weeks ago.)

Seriously, that’s an awesome composition, rav. It’s really cool to see the ejecta patterns around the craters. One can almost imagine the rock impacting the surface.

[ravnostic]

As you wish, CISS (fully saturated, saved, then doubly saturated, but elsewise, true-color):

[ravnostic]

I think this will hit up the G+ post, if I’m not mistaken. 28 15-second frames, 50 bias, some 20ish darks, flats, and dark flats; under a full moon, but I made the most of it as best I could. I’ll do better next week under the 1/2 moon. Still, I think it’s my best Ring Nebula yet. It’s only 2.5 arc minutes, which is very, very small.

https://plus.google.com/photos/107857888121727893520/albums/5805068148489255089/5805068154546037666

[chupathingie]

Ya can damn near see the footprints on the moon! 😆 Very nice composite. And you got the central star in the ring, as well…I’ve been jonesing lately… it’s supposed to 37 and clear fri/sat nights here. I may get out and shoot some. I’m tempted to run off a bunch from inside the city, just so I can see what I can do with LP and gradients.

[ravnostic]

Hey, if I managed as well under a full moon, you can at least do as well with LP. Here’s the Orion Nebula, I have to say I appreciate the moonlight for the blue tones that would elsewise be absent. This shot is doing very well over at G+.

http://fossilspringsaz.com/pics/2012/oct/28/orionfullmoon.jpg

It’s my best-ever Orion Nebula, but I think I’ll out-do it next week.

Had some fun, too; shown a green-light laser upon it and while it obviously didn’t reach that far, the atmosphere picked it up and gave it a cool green hue (not posted here); I might use it (or something similar) whenever the farkstronomy II contest gets scheduled (if ever).

[chupathingie]

Quite an improvement over a year. Very nice detail in the dark, dusty areas.

[ravnostic]

If I can keep that U-Man guy from using his children, I may fare well, Chupa.

//damn that U-Man, he has lenzez and everything….

The detail is severely lacking, catually (actually); the moon washes a lot out. In the next two weekends I expect to do much better, but I thank ye for the compliments and I do like it myself!!

[Zero_Exponent]

Very impressive work, rav! I don’t know a ton about astrophotography, but I know that it takes a lot of time, money, and hard work to get results like you’re getting.

[ravnostic]

Thanks, Z^0. Incidently, came across this the other minute, and it’s the best explanation I’ve seen for darks, flats, and biases. I don’t completely agree on every point, but the guts of it is perfect, and to post, I need to do so in it’s entirety (legally, at least)

Copyright Pertti Tapola 2005

This text may be freely distributed without charge to anyone. For
that purpose it may be put on a web site, provided that this header is
included and the contents is not changed.

This is not a complete guide but rather a checklist. For deeper
understanding of astronomical image calibration, I recommend reading a
good book, e.g. “Handbook of Astronomical Image Processing” which is a
very thorough and complete treatment of the subject.

---

Here is a short reference guide into different kinds of frames:

Dark
====

What

---

– reflects the camera-induced noise,
– temperature, exposure time and ISO setting -sensitive,
– camera-specific,
– aperture, telescope etc. have no effect so they are not needed.

How

---

– no light coming in (use lens or body cap) and block the viewfinder as well,
– exposure time and ISO setting the same as for light frames,
– near the same temperature as light frames,
– take as many as you can and average-combine them.

Use

---

– whenever the temperature, ISO and exposure time is the same,
– for the camera they were taken with (with whatever scope, filters, etc),
– ImagesPlus “Auto” dark calibration matches across exposure times and
also somewhat across ISO and temperature,
– if you have perfect tracking (=autoguiding) you may need upto 5 times
as many darks as light frames,
– with nonperfect tracking (some movement between each light frame),
or dithering between each light frame even 10 darks are quite enough,
– I have been trying different approaches but have not yet settled on
one (a huge number of darks collected over longer time vs 8-20 darks
collected after each imaging session), but in both cases the “Auto”
feature is great.

Flats
====

What

---

– records the uneven illumination of the image,
– dependent on the camera (sensor size and type) and the optical system
(scope, including barlow lenses, extension tubes etc),
– even filters can change it (usually not their color but their size
and position),
– dust in the sensor is also recorded.

How

---

– use an evenly illuminated source of light (evening sky opposite the
setting sun, double T-shirt in front of the lens, a lightbox etc),
– preferably light should be bright enough to keep the exposure time
under one second (otherwise, stack more or blur the noise out),
– check that the scope/lens is focused to infinity (no need to be perfect
but should be very near),
– use the lowest sensitivity (usually ISO 100),
– use camera autoexposure (Av mode in Canon),
– take a few (4-10) and average-combine them,
– for lenses, one set for each aperture setting,
– for scopes, one set for each combination of filters/extension tubes/barlows
etc, in some cases even for different camera orientations,
– I prefer to keep the camera sensor clean – especially when taking
the flat frames, to get the dust out of the equation,
– the ImagesPlus from version 2.75 does not need changing flats into
grayscale any more, but other programs usually need that.

Use

---

– whenever the optical chain is the same (scope, filters etc),
– some telescope/camera combinations “sag”, so you may need to make sure
the orientation is the same,
– for lenses use the one with the same aperture,
– keep the sensor clean also when imaging,
– I have taken four sky flats, rotated 90 degrees, for each lens and
aperture (upto 1 stop closed down for 10D, full frame cameras require
more) and averaged each set together.

Bias
===

What

---

– reflects the camera “zero” level,
– different for each pixel, usually has a pattern over the image area,
– camera-dependent,
– ISO sensitive,
– not dependent on optics.

How

---

– cap the lens (or the camera without a lens),
– use the same ISO setting as for light frames,
– pick the shortest possible exposure,
– take a few (1-10) and average-combine them.

Use

---

– usually not required, because unscaled dark-calibration subtracts
the bias as well,
– if you have a remaining weak fixed pattern all over the image, bias
frames may help,
– you can always reuse them when the ISO setting is the same (for the
same camera),
– I have taken one set (10 images for each ISO, average-combined) which
I use repeatedly.

Dark and bias frames change slowly over the time, so you probably should
not use anything more than one year old.

[ravnostic]

I’m happy with this image of the Pinwheel Galaxy.

https://plus.google.com/photos/107857888121727893520/albums/5643774762113128657/5807412143156232466 😀

[chupathingie]

wow, only 60sec subs? I wouldn’t have expected so much detail…

[caradoc]

Helping my wife get rid of some of her (now deceased) father’s stuff – anyone interested in a Bushnell “Jason” 618S (I think) 675 x 4.75″ reflector?

[ravnostic]

Sorry for your mom’s loss, caradoc; that must be hard for her. 🙁 My dad’s 4th cancer surgery is tomorrow and I really get emotional when I hear of others who’ve lost their folks; it’s inevitable, of course, but that doesn’t make it easy.

I’m not familiar with the model; will have to look it up; might be good for a piggy-back.

[caradoc]

‘Twas my wife’s father – so I’m helping my wife and her mother (his ex-wife) deal with stuff. If you need more details, I can get them. It looks like a pretty bog-standard “student” telescope.

[Author]

Posts