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Old 18-07-2011, 03:56 PM
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gregbradley
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Join Date: Feb 2006
Location: Sydney
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Quote:
Originally Posted by DavidTrap View Post
So can you tell me how to do it in PixInsight and how to use all the tools in clear easy to understand language?



I'll try to digest what you've written later!

DT
I am a beginner when it comes to Pix Insight. So I can't help you there. I intend to learn it soon though.


Quote:
Originally Posted by cventer View Post
Greg,

Good post.

Be great to have this worflow for OSC Cameras as well. Many beginners use OSC be it QHY or DSLR

Also a workflow using the typical free or cheaper tools like DSS and Nebulosity would be awesome.
I have done OSC and DSLR. I used Images Plus which is popular but does cost around US$200 or so.

Here's what I did for that:

1. Create a dark library for one shot colour cameras that are cooled. Pick a couple not 10. Make a standard exposure length and a temperature you can achieve almost all year round or 2 temperatures - one for summer and one for the rest of the year. Shoot darks by simply setting the controlling software to dark and shoot it at night or in a darkened room with the lens cap on or at night.

You can do this with a DSLR as well. Its a bit trickier with DSLRs because they are at air temperatures and of course that varies not only throughout the year but throughout the evening. They will be less noisy on cold winter nights and most noisy in summer.

It is also best to shoot darks with a DSLR and use a DSLR with the power adapter rather than using batteries if you can. Batteries heat up with use and change the noise of the camera. The power adapter does not.

With DSLRs you also need to standardise your ISO as well as the exposure length.

ISO of 400 or 800 is usual. Above that you are only amplifying what is already there so much like boosting it afterwards with software. You didn't really gain anything except stretch what was already there.

DSLRs in light polluted areas will pick up sky glow faster than dedicated astro cameras so work out an ideal exposure length for your area.

3-4 minutes at ISO800 may be close to ideal in a typical urban setting.
You may also need a light pollution filter. Survey shows Hutech LPS-2 or IDAS are the ones to use.

So take about 12 darks or more. Why so many? Because there are random artifacts that the combining process removes so the more the merrier in that regard.

use sigma reject combine for making master darks out of the 12 you took.

Now take a flat field. Put a white t-shirt over the end of your scope and make sure there are no wrinkles. You may need a large elastic band to keep it taut.

At dusk point your scope to a clear area of the dusk sky and take an exposure to about 1/3rd your DSLR histogram and very short exposure.
For one shot colour that is around 20,000ADU. Take about 6.

With flats make sure your scope is in focus and the camera is in the same orientation that you will or did take your images at. If you change that they will not be valid.

Also make sure if you are using a one shot colour camera that is cooled that the camera is cooled to the temperature you will be imaging at.

Also make sure you take fresh flats if you clean your camera window or filters as flats remove the effects of dust shadows in your images.

Take some flat darks (3 to 6) meaning take a dark image of the same duration as your flat exposure.

Now use your astro software to create master darks and master flats.
Images Plus does that. Use median combine and subtract the master flat dark from the flats to make the master flat.

Ok so now you have flats and darks for your camera.

2. Take your exposures with exposures planned as above (exposure time is determined by light pollution, type of camera - DSLR shorter, one shot colour longer so DSLR maybe 3 minutes and one shot colour may be 10 to 15 minutes. Take lots like several hours. Have patience, good images need long exposures - like 3-20 hours worth. Its better to stay on one object for several nights than take 10 short images of several objects none of which will turn out that great (easy to say hard to do).

3. Now you have your images, your matching darks and your flats.

So you callibrate the images. That means removing the dark noise, correcting the vignetting (shadowing in the corners) and the dust shadows (called dust donuts) and uneven illumination of your scope and camera with flats.

With DSLRs and one shot colour for that matter, you can use adaptive darks. This means as long as the dark image is accompanied by using a bias (very short exposure of the read noise at the same temp as imaging, say 6 shots sigma reject combined) then the darks can be applied via software to different temperatures and exposures. This is handy if you did say a 5 minute exposure and only have 10 minute darks.
It is also handy for DSLRs that do not have temperature regulation for their sensors.

4. Now you have subtracted the dark noise and corrected the dust and uneven illumination with your flats.

Your image is probably pretty dull and hard to see. The image needs to be stretched/boosted to be able to see it. In Photoshop that is the levels/curves tool. In Images Plus it is the DDP tool.

Boost the image so you can see the image but do not clip the data. Clipping means to cut off some of the data - usually it is done when trying to make the background dark and you cut off some of the faint background data with it so watch the histogram and make sure the
histogram forms a nice bell curve and you do not see it hard against the left with part of it being a vertical wall on the hard left.

Next step is to align the images. The images are aligned. Images Plus has some great alignment tools for that. Try out a few and see which ones you like best.

Some split the image into luminance and red green and blue here. They do that so they can process the luminance separately and differently to the rgb. RGB is usually processed to enhance colours, adjust colour balance to get rid of a colour bias. Luminance has all the details so it is usually sharpened, increase the contrast etc.
You recombine these at the end. I am not sure if you need Photoshop to do that with its layers ability or if you can do that in Images Plus (you probably can).

You don't have to do that step and it depends on the image. In an image
where you want maximum details you'd do it (like a galaxy or detailed nebula) but on a diffuse nebula it won't gain anything so its not worth doing.

This is a basic overview to help you get started. Ron Wodaski's books are good and Images Plus is the king software for one shot colour or for DSLRs and there is an excellent group that offers advice and tips for it.

I hope this helps.

Greg.
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