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Re: M42 chronicles

Posted: Mon Dec 17, 2018 6:41 pm
by turfpit
Thanks Joe.
  • The first image is a good example of poor focus - stars like golf balls.
  • The second image is a good example of poor polar and/or mount alignment - stars like hyphens.
These are items that cannot be fixed the next day.


Re: M42 chronicles

Posted: Wed Jan 02, 2019 11:44 am
by turfpit
Captured 2019-01-01, 30x90s. A test image in preparation for an assault on LRGB captures.

2019-01-01.jpg (47.99 KiB) Viewed 843 times

Re: M42 chronicles

Posted: Wed Jan 02, 2019 3:50 pm
by oopfan
Great work, Dave, and thanks for emailing me screenshots of the calibration frames.

Having this info is revealing. What stands out is how much sky glow you are capturing with a 90s exposure but notice, as expected, that it doesn't adversely affect the final processed image, it actually helps! As you know long exposures are preferable to short ones.

I know for a fact that the Brain would never recommend a 90s exposure. It prefers 10s-15s exposures. Somewhere I read, perhaps AAVSO docs, that it is all about relative differences not absolute values. In other words, the contrast between sky glow and faint nebulosity is constant regardless of exposure but your camera's ability to differentiate between them depends on the noise. We know that we can increase the signal-to-noise ratio (SNR) by increasing the exposure. For example the difference in SNR might be 1.6 using a 90s exposure but it might fall to 0.5 for a 15s exposure. In other words, in a single 90s frame you can see more nebulosity than a 15s frame because the SNR of the nebulosity grew more rapidly than the sky glow. The only downside with increasing exposure is that it tends to bang bright objects up against the right-hand-side of the histogram.

The counterargument is: "stacking 6x15s is equivalent to 1x90s," but you and I know its not the same. A good comeback line would be: "wait a minute, you just invested 15 seconds to get to the point of recording the target, then you stop and waste another 15s on the next frame? Think of that 15s as the friction you needed to overcome to get the ball rolling. It makes more sense to keep the ball rolling than to start over." Of course you need to be careful who you use that argument on.

A better name for the Brain is the Minimum Exposure Calculator. This satisfies the needs of EAA but for someone interested in AP they should regard the Brain's recommended exposure as an absolute minimum. Longer is better.


Re: M42 chronicles

Posted: Wed Jan 02, 2019 4:25 pm
by oopfan
If I might add:

Part of the problem that people have is with their experience with conventional cameras and the concepts of "underexposure", "overexposure", and "proper exposure". We can all relate to that but most people don't do post-processing with software so there is no recourse but to get the exposure right when they click the shutter button. It is different with AP. Here, underexposure is bad but overexposure is actually good.


Re: M42 chronicles

Posted: Wed Jan 02, 2019 5:24 pm
by turfpit

The stack was created from 30x90s, 10 darks, 60 bias and 60 flats.

The only processing the stack had was:
  • crop
  • single stretch (curves)
  • adjust black point
  • apply contrast
Minimal processing after stacking - suggests to me decent data was acquired. What I have noticed with longer exposures is that
less processing (minimal) is required. I feel I am no longer using software to try to create data when there is none.

The 90s exposure data sets me up for 2 things:
  • A baseline to use with the LRGB filters, they will tone things down.
  • A starting point for some HDR experimentation. I can capture sets of data with lesser and lesser exposure to see I can retrieve the Trapezium.

Re: M42 chronicles

Posted: Sat Jan 05, 2019 5:12 am
by donstim
turfpit wrote:
Mon Dec 17, 2018 5:43 pm
This thread is more about the journey and to try to get over to beginners that my improvement is about mount alignment, polar alignment, focusing, checking the histogram of a single frame in FITs Liberator rather than equipment.
But Dave, at the risk of coming off argumentative, which I am really not trying to be, some of that sounds very equipment-oriented to me. For example, polar alignment appears to require a good equatorial mount and your focusing improvement you indicate is due to use of a Bahtinov mask and a motorized focuser. Maybe the largest portion of your improvement was due to gaining knowledge and experience, but that improvements in equipment was not negligible?

Also, I am trying to better understand the exposure for the light frames in your last set. You described them as 90 second exposures, which I take to mean a single capture with the shutter open for 90 seconds. Is that correct, and what type of camera was used? The RI2 that Joe and I have is capable of exposures of 5 seconds max although we can get a 90 second video capture (not the same thing) with Sharpcap.


Re: M42 chronicles

Posted: Sat Jan 05, 2019 1:44 pm
by MrAstroBen

I see many folks having similar issues. Unfortunately cameras with a 5s max exposure get sold as astro cams. If sensitive enough 5s is plenty for use as a guide camera.

A camera with a 5s max exposure though will give you a hint of what is up there. It will never get you a nice DSO image. Very bright objects need around 30 to 60s. Most objects nearer 2mins, some 5mins. If wish to use Ha then 5mins plus is typical.


A DSLR is better than most budget cmos cameras for DSO
Something like a ZWO 178mc is a good camera for widefield though.

Do not believe much of the hype. A cooled CCD is still better than most of the 'wonder cameras' .
Used they can be picked up for less than many cmos cameras that will just cause headaches and disappointment.
Search Astrobins, see images you like. Look at the cameras used along with the exposure.

People will have other opinions, let the images talk to you.

Use the correct process as described by Dave, get an appropriate camera. Good images will follow.


Re: M42 chronicles

Posted: Sat Jan 05, 2019 9:16 pm
by MrAstroBen
This will help in choosing a camera that meets requirements in terms of FOV and resolution.

The resolution is a guide, very widefield images will be over 2 and still good. The FOV is your first concern.

20meg cameras sound great on paper or if you are used to daytime photography. Forget megapixels.

Look at FOV first and then resolution.

Also watch

Re: M42 chronicles

Posted: Sun Jan 06, 2019 12:56 am
by donstim
Thanks Rob. I'm not looking for a camera, but maybe this will help others. The "camera" (Revolution Imager 2) I have is not billed as an astrocamera. It is an imager primarily meant for beginning EAA that came bundled with my telescope purchase (i.e., it was essentially free). It meets and exceeds its stated (and my) objectives for imaging. It may not be capable of true astrophotography, but the real time images as well as processed photos are good enough for me (at least at this time when I am VERY hesitant to commit the time and resources to real astrophotography).

An example processed image (with much hep from Dave!) is attached. Admittedly, this is an easy object and maybe this would not be considered a good image by astrophotographers, but obtaining an image like this from the RI2 exceeded my expectations. It is particularly incredible to me compared to the capabilities that existed 45 years ago when I last "practised" astronomy.


Re: M42 chronicles

Posted: Sun Jan 06, 2019 3:46 pm
by MrAstroBen

Yes you have done well to get an image like this given the limits of the camera.

My comments are to assist anyone reading this.

While its possible to overcome difficulties, the points I would like people to note are.

Whatever the equipment, follow good practice. It is not as complex as it seems to become from reading many of the posts.

Get the basics right and a lot of other issues disappear.

Spending 1k on a camera isn't necessarily going to help.
Some of these cameras create more problems than they solve.

Many people are getting held back. Pick the right equipment and it simply works, the process of getting an image becomes much easier.

I have been there, read the sales blurb, spent the hard cash, found the drivers cause problems, the image is plastered with amp glow, messed with flats, darks etc to overcome problems. USBs that struggle to cope.

Then did my homework, picked gear that astrobins images prove works. Looked at the capture details other successful people use. Progress then happened.

Snakeoil and reinventing the wheel is all too common, people lose money and time working through a lot of this. People end up confused and frustrated.