Telescope Reviews: AP Images Taken with Original NexStar

Some great stuff going on in here guys. I hope you take my comments as helpful/constructive, since I really love to see what you are doing with these mounts, and I think you are off to a great start so far!

Jerry I like where you are going with your imaging. I notice that you have just moved to a GEM, which provided you get a sound polar alignment will give you an excellent platform to work with. You can pull more out of both your M74 and M82. From a cursory glance, the M82 image has a significantly higher SNR, which is a definite step forward (stands to reason when you consider the relative surface brightnesses). Go with the longest exposure your mount (and conditions) can provide but total exposure rules. 60 minutes at 45 seconds is better than 45 minutes at 60 seconds provided the target in question is of an appropriate brightness. I will explain the drawbacks of this later. You can also be a bit more aggressive in your processing since it appears you captured quite a bit of data.

Maverick - love the Orion shot (one of my favorite targets). As I mention below, calibration is of great importance. You can also look into the shadow/highlight options in photoshop, or learn how to mask in photoshop to control the brightness in the trapezium and keep the dynamic range of the image. The background looks a little red on my monitor, but that may be thermal noise (removed by darks) or just a weird colour scheme on my end. Flats would control the light fall-off allowing you to bring out some of the fainter elements that are a bit hard to spot (but present) in your current rendition. Very nice capture, and a great job with keeping the stars round. This isn't always easy to do in alt/az, but you did very will.

Calibration is the most important aspect of astrophotography. I have had a few opportunities to discuss the topic with a local operator who has 60 years of experience in photography at the Dominion Astrophysical Observatory and those are his words. Take your calibration frames every time, and even short exposures can reveal lots of information. Since the image has many/most defects removed, you are left with just the signal to work with, therefore garnering the best image possible with the available data.

As I mentioned above total exposure rules. I said that 60 minutes of 45 second exposures would generally trump 45 minutes of 60 second exposures. Lets get into this a bit with an example of an imagined galaxy. Say the core of said galaxy had a nice amount of detail that was relatively bright compared to the outer areas of the galaxy and it delivered a photon to the sensor every 30 seconds. The 60 second shots and the 45 second shots would both contain the data (1.5 photons on average, and 2 photons on average). In this case the image with 60 minutes of total exposure will likely have a higher SNR when compared to the 45 minute image since each stack consists of frames containing information from the area in question. For most objects in the Messier catalog this is the case; Alex Post produced a series of remarkable images with just 10s exposures, but a large total exposure #.

On the same imagined galaxy an outer arm is much fainter than the core, and delivers a photon every minute and a half. Now two out of every three one minute frames contain a piece of information from that outer region (while still containing 2 photons from the core region). The 45 second frames will only have the piece every second frame. Theoretically this discrepancy will not create an issue if you combine the frames through an averaging or median technique, but clipping methods are often used to create higher SNR's on the imaging target, while clipping out useless data (bad tracking, planes flying through, satellites, clouds, etc). If the clipping method needs the information to be present in<60% of frames to be maintained, then both images will capture the bright regions of the galaxy (in fact the 45 second frames will capture it better), but the outer faint regions may only appear in the 45 minute image of one minute frames despite the lower total exposure. This is important when hunting faint galaxies and hydrogen alpha data, but as mentioned above most objects do not fall into this category.

*Disclaimer-In my two scenarios the numbers for both sub frames and total exposures were just to illustrate the point and not actually representative of a more realistic example. Generally it's more on the order of a 5 minute exposure vs a 1 minute exposure.

Hope some of that made sense (I'm tired and have been doing history readings for 4 hours). If you want to take a look at some of my imaging there are a few examples in the "graduates" imaging thread.

Great job guys,

Dan