December BSIG -- Intro to Astro Imaging - Sat. Dec. 9th - 2:30pm

[Sureshks]

Hi all,

The December B-SIG Presentation will be on Astronomical Imaging for Beginners.

Dave Venne, one of our club's best imagers, will take us through different imaging techniques, and how to process and finish them. We'll also go through the needed equipment and software. Astro Imaging is a very hot topic right now, and we're lucky that Dave has agreed to show us how to get started. He's taken some of the best deep sky images I've ever seen...

The presentation will be held on Saturday Dec. 9th from 2:30pm-4:30pm at Southdale Public Library in Edina.

Southdale Public Library
Southdale Full Meeting Room
Address: 7001 York Ave. S. Edina, MN 55435
Phone: 612-543-5900

Feel free to bring along friends/family who may be interested as well.

We hope you can make it!

Suresh Sreenivasan
MAS Beginners Group Coordinator

[ddn]

The 19th is a Tuesday not a Saturday. Did you mean the 9th?

[Dave Venne]

Yep, it's the 9th.

[Sureshks]

Fixed it... thanks guys.

Yes, it is this Saturday, the 9th.

Suresh

[BradNasset]

Really dissappointed I can't make it this Saturday, but have a question about imaging that I will ask here in case it might spark someone else's thinking. I have a Celestron C8. Really new to this. I have been fiddling with this using a 2 inch back and 2 inch tube going to the Canon 40D camera. Got a great image of the moon, although all of the moon didn't fit in the frame. Seems the Celestron and most imaging cameras are 1 1/4 inch. The Barlows and a reducer/corrector and all that are lots lower in cost with 1 1/4 inch hardware also (instead of the 2 inch), so will likely want to go to the 1 1/4 inch. Would I "lose light" going to the 1 1/4 inch, or is all pretty much ok? Do I purchase the same length 1 1/4 inch extension tube that goes between the camera and scope as the length I have for the 2 inch tube? (what I have is approx 44 mm extension tube) Can I keep the 2 inch visual back on the scope and use a 2-to-1 1/4 inch adapter that fits into the 2 inch hole? (hope so) Thanks for your expertise, Brad Nasset

[Sureshks]

Hi Brad,

There are two main types of DSLR sensors -- full-frame sensor and crop (APS-C). I see that the Canon 40D has a APS-C sensor.

Crop sensors essentially shrink the field of view by about 1/3. So, let's say that you're using a 24mm lense on a full-frame camera... and the stated field of view is 60x40 degrees. If you were to use that same lense with your Canon 40D (APS-C), it would give you a smaller roughly 40x27 degree FOV.

Using the below field of view calculator, I see that the APS-C field of view thru a C-8 is 38'x25' (this explains why you aren't quite fitting in the ~ 30' Moon)

Using a full frame camera on the C-8 would net you about a 60'x41' (enough to fit in the entire Moon)

http://www.skyatnightmagazine.com/astro ... calculator

Crop sensors are good and bad. The bad is that, for a given telescope or camera lense, you'll have a smaller field of view than if you're using a full-frame camera (and is the reason you cannot fit the Moon in your C-8 with the 40D). Back in the film days, I used to fit the entire Moon in my C-8... but that's because film cameras were the equivalent of modern day full-frame sensor DSLRs.

The good is that, because of this sensor limitation, you might actually be okay (in terms of lost light) using your 1.25" visual back (since the APS-C chip is smaller in real size than a full frame sensor). You could still have some vignetting from lost light around the edges, even when using your 1.25" visual back instead of 2". But it wouldn't be as bad as it would be if you were using a full-frame camara with your 1.25" visual back.

I found this online that might help explain a bit further: https://www.photo.net/discuss/threads/s ... 0d.426685/

I hope this helps!

Suresh

[BradNasset]

Thank you, Suresh. I like the chase as much as time at the scope. Lots to learn, so this helps. Appreciate all your time, and the links. Brad

[Dave Venne]

Brad,

Sorry you can't make it to the talk! Here's what Suresh is talking about in terms of relative sensor size.

sensors.jpg

The APS-C is quite a bit smaller than a full frame sensor. This is what the moon probably looks like in it with your scope (the orange box is what the camera sees if you're operating at f/10):

moon.jpg

For a relatively slow scope like your C8 you probably won't see much difference between a 1.25 and 2 inch back, but if you eventually go to something faster like an f/5 refractor you'll want 2" accessories.

[Sureshks]

Hello everyone,

We had great turnout for Dave Venne's presentation Imaging for Beginners last Saturday. A total of 41 people signed the attendance sheet... I suspect that there might've been a few more than that present...

For those who weren't able to make it, I am attaching Dave's Imaging for Beginners deck. Below is a summary of his presentation:

Dave did a fantastic job describing the various aspects of imaging -- from the different types of imaging targets, and the equipment needed for each, as well as providing a primer for understanding image processing basics. Thanks Dave!!!

He started off by explaining that imagers today use two basic types of cameras: DSLRs and dedicated CCD. Today's presentation concentrated on DSLRs, as this is what most beginning imagers will be using. Modern DSLRs comes with either of two major sensor types: ASP-C (crop sensors) and full-frame sensors. Crop sensors are the more common type, as their smaller chip sizes allow camera manufacturers to lower production costs. These sensors essentially shrink the field of view by about 1/3 vs. full-frame cameras. So, if you have a 35mm lense, and use it with a crop camera, it will give you a more narrow field of view, comparable to a 50mm lense on a full-frame camera (or an old film camera).

He then described the imaging histogram, where pixel brightness is shown on the x-axis (black is 0 and white/full saturation is on the right), and pixel count is shown on the y-axis. He explained that by keeping the image spike centered about 1/2 way from the left of the histogram to the center, the histogram can be used to ensure that the proper exposure has been taken. This way, there is enough data just to the left and right of the spike, that can be used when processing. This is where the detail resides in fainter objects such as nebulae and galaxies.

Next Dave took us through the different types of imaging:

- Using a basic DSLR /camera lense on a regular tripod: Though we're limited by the Earth's rotation to taking exposures up to only a few minutes, (depending on the lense being used), this method is enough to capture star trails, basic Milky Way shots, and interesting night scenes like planetary conjunctions and the Northern Lights. This is the easiest and cheapest way to get started in astro-imaging...

- Tracked Wide-Angle Shots: In order to take longer exposures with your DSLR and camera lense, you'll need a tracking device. There are two main methods of tracking for wide-angle astro-photography: adding a small tracking platform under the camera, or by piggybacking the camera on top of a telescope with tracking capability, and then adjusting the telescope's tracking to fix periodic errors. If you add tracking to compensate for the Earth's rotation, then longer exposures can be taken, enabling much more detailed Milky Way and night shots.

- Imaging at Prime Focus: This essentially turns your telescope into a high powered camera lense. So, if your telescope is an "f/10" optical system, think of it as a camera lense set at a focal ratio of f/10. The higher the focal ratio, the longer the exposure will need to be. Also, the size of the field of view is determined by the focal length of the telescope. The longer the focal length, the more magnified the image, the smaller the field of view and more accurate the tracking may need to be.

Imagers shoot at prime focus to capture some bright objects, such as the Sun and Moon, but they mostly use it for much dimmer objects, such as comets and deep sky objects (such as galaxies, star clusters and nebulae). As these objects are usually quite dim, they necessitate taking exposures taken over many minutes, or even hours! In order to do this, you will need a mount with very precise tracking. A very stable (and expensive) German Equatorial Mount (GEM) is most common type of mount used in prime focus imaging. These aren't the easiest mounts to learn how to use. They can also be a pain when transporting, setting up and polar aligning... but once set up properly, they can allow for very accurate tracking over long periods of time.

Another way that imagers try to minimize tracking errors is to use an intervalometer or laptop to control the exposure sequence. They can then take many shorter exposures, say 3 minutes in length, which lessens the propensity for tracking errors. They then use imaging software packages, such as RegiStax, Deep Sky Stacker, Maxim and PixInsight, to electronically stack the images on top of each other. This adds more "data" to the combined image, allowing more brightness and detail, which can then be further processed in packages such as PhotoShop, Maxim and PixInsight to reduce noise and enhance certain colors or features.

- Planetary Imaging: Finally, we reviewed what's involved in imaging planets. Here, you really need to boost the magnification in order to reveal detail on objects located many millions of miles away. We accomplish this by using an eyepiece to project the image (ie. making it larger). Depending on the eyepiece, this can turn a telescope with a focal length of 1,000mm into 10,000mm or more! Increasing the effective focal length not only makes the image larger, but it also makes it dimmer. So planet imagers often stack many frames that, in total, add up to several seconds of exposure.

Another big difference between projection (planets) and prime focus (deep sky/comets) imaging is that, instead of taking many single exposures of many seconds or minutes, and then stacking them... planet imagers use what are essentially specialized video cameras to capture many hundreds or thousands of frames over several minutes. They then utilize special software to select and splice together the very best frames, those that are free of atmospheric distortion, to produce a single very detailed image. Or, they'll select the best single frames and create a movie sequence.

Finally, Dave left us with some To-Do's when imaging:

- Focus, focus, and then focus again! Nothing can fix an unfocused image.

There are many ways that imagers achieve proper focus:
- Take many test imagers for review on your laptop to fine tune the focus
- Using Bahtinov Masks -- this is placed on the front of the telescope and produces a grid like pattern. Once the left-right spikes are centered between the upper and lower spikes, you are extremely close to being 100% focused
- Increasing the magnification on their camera's viewfinder will assist in nailing the focus point

- Shoot in RAW (instead of in JPEG or another format), as it allows for the most post-image processing. JPEG and other formats compress data, which limits the ability to unpack and then process the image.

- Do not shoot at ISOs under 400 (too dark), or over 1,600 (too much added noise)

All of the above imaging set ups and techniques are light-years ahead what we were doing with film back when I started in the 1980s. Instead of waiting days or weeks for our results, we can see results virtually in real time. Image stacking and processing techniques enable astro-imagers to select and enhance particular features that they want to highlight. There's certainly a lot of art involved in modern imaging!

DSLR technology and enhanced imaging techniques are enabling amateur imagers to get results today that were unheard of previously... so many deep sky objects now have names that weren't even known to amateurs 10 or 20 years ago!

We had quite a few experienced imagers present today, and it was neat to have then participate in the discussion. Special thanks to Mark Connolly, Robert Miller, Nick Hartman and Doug Neverman for lending their imaging expertise!

Though a lot of imaging questions were asked and answered today, I know that there's still plenty more out there...

I hope that we continue this discussion, and can help each other learn the basics. Sometimes, beginners learn most effectively from other beginners. More experienced imagers are also CCed here, and I'm hoping that they can chime in when necessary to answer questions. Once it warms up again, we can look at holding beginner imaging sessions at ELO, or at other locations around the metro to better facilitate the learning.

Suresh Sreenivasan
MAS B-SIG Coordinator