SonnyE Loc: Communist California, USA

How do you guys focus for doing your darks?
I honestly can't see what I'm doing.

rgrenaderphoto Loc: Hollywood, CA

I do not bother with the focus on darks; with the lens cap on, the focus is meaningless. I set up my RedCat51 and ASI533MC camera, connect to the ASI Air + and run a series of dark images. 100 ea in 30, 60, 120, and 180 sec exposures. Never had to worry about focus.

Oh wait, you're making a joke, right?

robertjerl Loc: Corona, California

Some cameras will focus in near dark, esp with moonlight.

Otherwise, if your lens has a distance scale (they are getting away from that on new lenses) then set to manual focus and set it for the distance.

Another trick when the subject is close shine a light on it (if feasible and having someone else do it is easier), do a half press to get focus, turn the light off and finish the press.

Marc G Loc: East Grinstead, West Sussex, England

Mate, focus doesn't matter when acquiring dark's, you don't actually need a lens or scope connected either.

All that matters, is same iso / gain, same temperature, same exposure duration as light frames & obviously the sensor covered.

Europa Loc: West Hills, CA

It depends on how many glasses of whiskey I’ve had…I find a correlation between my focus and the number of drinks.

SonnyE Loc: Communist California, USA

Yes, I was making a joke.



Actually, I have an 8-position filter wheel. I had 7 filters.
So, I took one of my very first Orion filters, deem it useless to me, Masked off the outside and threads, and GASP....
I painted it flat black.
It is in Position 8, and gets run in the sequence as my Darks.
My thinking was to collect the darks at as close to the same temperatures as other frames.

Sorry, it was a joke. Sometimes I'm just so dryly literal I needed to pull your legs.
Gottcha!

But when collecting Darks, the other night, the thought occurred to me.

Marc G Loc: East Grinstead, West Sussex, England

Sonny, you bad boy :)

If you had a TEC (temperature controlled) cam, you can build a dark frame library within your home during the day.
My library consists 30s 60s 90s 180s 240s 300s @ -5 x ten frames & the same @ -10 thus saving hours under the stars

Ballard Loc: Grass Valley, California

Yep. I do something similar to get darks also. I use the dew shield for the 16inch scope as a dark chamber an put the camera face down inside and take shots once the camera is down to temperature. Sometimes I have had to put a few plastic covered Ice blocks with it to get down to the lower temperature setting like -20C when taking darks in the warm house. I also keep a library of darks with temperature and time so I don't need to take the same darks over again (I expect they may need to be refreshed in a year or so).

robertjerl Loc: Corona, California

OK, confession I had no idea what you meant by "darks", I assumed it was photos at night.

I looked it up and found: darks, lights etc. and that they are terms for astro photography among others and designed to get a cleaner noise free image doing what as far as I can tell is just a form of exposure bracketing. But turned into an "in crowd" jargon.*

I don't do astro photography, too much light pollution in the Greater Los Angeles Metro Area. Seeing a few stars is almost reason for a party. And years ago I did moons for a couple of months and got bored and haven't done them since.

*PS, I hate jargon, though some is colorful and interesting-ONE TIME. I am a advocate for "Plain English" and that is how I taught. Any jargon in a history, government or geography textbook, I translated to Plain English for my students and told them in their future careers they should stick with plain english, not try to impress people with how great they are by using jargon. Technical and Scientific terms and words are just fine, in fact necessary.

Ballard Loc: Grass Valley, California

Hi robertjerl

Here is a short description of the frames type used for deep sky image calibration and is actually quite different than exposure bracketing.

Dark frames (Darks) are images taken with the camera with all light blocked at the same temperature and exposure time as the light frames. These are averaged together to create a master dark frame that is then used to subtract thermal noise that can become large with long exposures and is very temperature dependent.

Flat frames (Flats) are images taken with the same optical train used for lights but with a diffuse light source in front of the optical train (like a white tee shirt of pillow case over the front of the scope taken during the daytime). The exposure time for the flats is adjusted towards the middle of the light histogram for the image. These also averaged together to make a Master Flat frame and is used to compensate for any vignette in the optics and to subtract out dust donuts that can sneak in. A separate set of Flats are used for each filter used.

Bias frames like darks are also taken with all light blocked but the exposure time is a short as the camera allows and although it is recommended to take these at the same temperature as lights I haven't really seen any difference. These are averaged together to create a master bias frame that is used to subtract read noise from the light frames.

With deep sky images we try to get as much clean data as possible out of these dim objects and the calibration frames can really help (In order of importance are Dark frames, Flat frames and Bias frames).

robertjerl Loc: Corona, California

You mean totally dark, like with the lens cap on? If the idea is to have a black/dark frame, that is the easiest way I can think of. Black is black, unless what you really mean is a VERY DARK gray.
If the frame is to be black, the lens cap on is easier than changing the settings, so a shot of the scene comes out black.

This site explains it as just a complicated manually done version of bracketing, only you take each type separately, blend them in your software, then blend the results of each type.
A long convoluted version of bracketing.

https://practicalastrophotography.com/a-brief-guide-to-calibration-frames/

Then you use those frames to remove noise from your sky shots. Yes, No???????

Hell, why am I worried about this, besides trying to keep busy. I don't do night sky photography, where I live the light pollution is so bad that all I can really see in the night sky is the Moon, a very few of the brightest stars (sometimes), aircraft lights* and fire works at certain times.
Now where I grew up in Western Kentucky - back in the 50s and early 60s before their light pollution got worse (still nothing compared to LA) my youngest aunt who was only about 8 years older would put a blanket in the farmhouse front yard and flop down with her high school science book open to the Astronomy section and look at gazillions of stars. Milky Way, moon, meteors and even migrating ducks and geese in the moonlight. And the whole world below, all in silver, grays all the way to jet black. I have a memory of a V of geese against the full moon while a Great Horned Owl in a dark silver tone glided past and down into the valley to disappear into the trees along the creek. If I had the money, and it was possible, I would pay a lot to have pictures and video of that and could probably make ton of money off it if I had any business sense.

*A police helicopter with spot light is just not as enchanting as stars.

Ballard Loc: Grass Valley, California

The lens cap on will do as long as there is no light leakage for both darks and bias frames but since these frames are for calibration you do want the darks to be the same and bias to be at the same (gain or ISO with a DSLR) and the same exposure length and temperature as the light frames for darks. Once you have calibration frame you can typically pass the stacking software, the light frames, and the master calibration frames and it will do the combination to create the calibrated images. You then need to align the calibrated images and then stack them. Up to this point most stacking software you find will allow you to make a script do these steps. A this point this image normally needs to be stretched (bring up the darker areas) so you can see the image details.
There is a lot of jargon and processing steps to learn for this type of deep sky imaging.
Although light pollution is a problem if you can still take photos using narrow band filters to get decent images even in fairly heavily light polluted areas. These type of filters only allow very small range of frequencies through that many nebula contain but normal light pollution doesn't have. This filter do cost an arm and a leg and usually not appropriate for a standard DSLR. Once you go down this rabbit hole you end up spending a lot of money and effort.

robertjerl Loc: Corona, California

OK, now I have a better picture of what you are doing.
And why I probably will never do it, my birds, flowers, bugs and trains keep my cameras busy enough.
I will just remember my Aunt and her textbook on warm summer nights teaching me the names of stars and constellations.

SonnyE Loc: Communist California, USA

I did, I did do dat just yesterday.

Hi Marc!
Yep, just to catch you up because I have been a very bad boy and stayed away way too long.
I'm using a ZWO ASI 1600MM Pro camera now. My Atik died on me, and I haven't resurrected it. So, I suspect a serious internal failure. I probably killed it with my bad image taking.

So I wanted to go Mono. And this was a package deal, camera, electronic filter wheel (8 position), and a set of 7 ZWO filters. Huge expenditure for me, but I bit the bullet and clicked the order. About a year ago?

And since, I got a new ZWO ASI 290 MM guide camera. I was thinking I could do double duty with it since it had higher specs on the sensor. My thinking was the better sensor should give tighter guiding. And it seems to.

Lately I added a ZWO ASI EAF, electronic focuser. And it's been a game changer for me. My old Arduino focuser died, so I replaced it with the ZWO, except I installed it according to the directions. Novel for me.

Anyway, yesterday I was wondering about Darks, Biases, and Flats and found out I can add them by making some up in the kitchen sink. I just set my telescope on the floor, connected it, set the temperature and made some files to help learn how to process the Light Files I've been collecting while learning how to do this next step for me with a Mono camera.
Mono makes sense, but it is sure a lot more difficult. Never the less, I'm forcing myself to figure it out.
Using a color camera was always my easy way out. I'm as lazy as an old dog. (And probably about as smart, sometimes.)
So I built some files to keep learning with some of the images I've collected.

JimH123 gave me a link to a program called Siril that looks to be just what I've needed. In trying to learn it, I've been gathering files (Piles) of images to have data to work with.
And yesterday I made some files to try.
In fact my first run just finished, so off to see if I succeeded in using my Darks, Flats, Bias, and Lights. Total 122 files.

I have gotten much of the back spacing fixed, but not all of it.
Making baby steps.
I'll try making more dark frame files at the higher time counts.
The Blue files came out good for me. But it took 42 minutes to be looking at blue stars.

Fumble, bumble; bumble, fumble....

SonnyE Loc: Communist California, USA

Hi Ballard!
I was able to do some sets of darks yesterday so I would have something to try today.
I also made Flats and Bias frames.

I'm learning a program called Siril. And it appears to do everything I need.
Fits files have always been as hard for me to deal with as focusing was. But Siril is made for fits files.
And my 1600MM only makes fits files.
This is going to be a long haul for me. But I'm finally moving.
Toot-Toot!