Our Top 10 Things That Make For a Good Astrophotography Image

Focus, Focus, Focus! Jk, it takes a lot more than just good focus to create a epic astro-photo. Deep space astro-photos are even more difficult. Below are the top 10 things we look for in a great photo.

1. Focus

Focus is very important to an image. Provided your scope and image train is tuned in then it gives you round pinpoint stars and brings out the details in your image. A good focus can be affected by Seeing, dew, f/ratio, collimation and temperature as well as a proper back focus.

The easiest way to achieve your best focus is to have a motorized micro focuser and run and auto focus routine. Many of today’s integration programs have them built in or you can use secondary software such as FocusMax. These programs create a bell curve or V curve and find the optimal focus point once its over.

Seeing will affect the focus and cause stars that look out of focus or bloated. You can tell seeing is bad if you loop an exposure at a low exposure rate and see the star size and shape changing. This will not allow your auto focuser to get a true reading and your focus FWHM will be higher than what you normally are able to achieve. Dew affects your focus in a similar way until your lens is completely fogged and then you lose your image altogether.

Collimation can affect your focus because your stars may not be round. If your collimation is off then your star is pinched in one direction and you cant achieve proper roundness. This will affect your focus routine as well as the shape of your stars. You can check your collimation by defocusing a star and measuring how centered the dark center disk is in conjunction to the outer white disc.

The f/ratio of your scope can also limit the FWHM you can achieve. If you have an f/10 scope you may only be able to achieve a 1.9-2.0 FWHM on a decent night as opposed to a f/3.6 refractor that can achieve a FWHM of .9. This would be considered when selecting images.

Back focus is another thing to consider for a good focus. In order to achieve critical focus you must be able to reach the correct backfocus between your Camera Chip and the telescope lens. Now this is more critical with a refractor over lets say an SCT. When doing a focus routine its important to be able to go past critical focus on both ends to determine your optimal focus. Most people prefer minimal focus tube outing to reduce any flexure but you need to be able to go out enough to set one end of the bell curve. On an SCT generally the mirror moves enough to achieve focus and if you have the option of a mirror lock you can usually avoid mirror flop, however don’t over tighten your lock screws or you can skew your stars.

Finally there is Temperature. Temperature generally affects your focus during the night as the temperatures drop or in Solar imaging as the sun heats up your scope. The temperature change expands and contracts the metal tubing in the scope as well as the optics. There’s no set rule for focus adjustment when it comes to temperature but I generally find people either refocus every hour or every 1-2 degree drop. Some focusers, when connected to an integration program, can trigger an automatic refocus as the temperature drops due to the attached or built in temperature probes. One such focuser would be the Primaluce Sesto Senso. Primaluce also has a motorized temperature controlled focuser for an SCT Called the ESATTO and it has its own focus control manager.

If you choose to manually focus because you don’t have a motorized focuser, that’s ok, You basically want to expose a sub frame of a star or use a star field and keep reducing exposure times during focus as you max out your well depth. Once you reach your best FWHM your done. Obviously you wont get the precision with your hands over a motorized micro focuser but we all have to work with what we have and you can still get pretty close.

2. Framing

We often spend so much time setting up, getting gear working, keeping warm and the million other things we need to keep in mind imaging we often forget about framing. Taking a few minutes to Frame your shot will often make the different between a good shot and great shot. The rule of thirds is often used when doing terrestrial photography and it can be helpful when used in astrophotography as well.

For those not familiar with the rule of thirds look into any DSLR these days and you will most likely see a grid pattern, this is used too help you frame your shot. We don’t have those grid patterns in our astronomy cameras but you can still break up your shot by thinking about that grid. The general rule is put points of interest (POI’s) on intersecting lines.

So try and stay away from the middle. Of course there are some targets that really call for center framing but many targets have 2 or 3 points of interest, so put a tick-tack-toe grind on your frame and try to align the POI’s to the intersecting lines. Here is great article (Composition in astrophotography: 5 tips for better astro images) that shows other framing techniques that may get your images noticed.

3. Background Noise

Too smooth is not good!
I once asked Bernard Miller (who has a few Nasa APOD’s) what is your technique for get your backgrounds so perfect? He said ”I don’t spend much time on backgrounds, I just gather as much data as possible and they always turn out fine”. That’s really good advice. I remember when I first started this hobby, over 12 years ago now, when my local astronomy club SFAAA would do our monthly member dark site treks, my goal was to get at least 3 “good“ images in one night. Needless to say I don’t follow that philosophy any more. One good image a month is more my speed now.

Backgrounds can’t be forced, you need a good amount of data to get that nice dark (not black, that’s another blog) and natural looking background. But what if that’s not possible and you have all the data that you can get, well software to the rescue. Here are a few good programs/plugins that will get you a smooth background, but remember TOO SMOOTH IS NOT GOOD!

Pixinsights MLT tool, Ron Brecher has a really good workflow for this tool, just look at any of his recent image notes.
Noise Ninja. This a great plugin for Photoshop but they have a stand-alone version as well.
PhotoShop Noise reduction, this has really come a long ways in recent releases.
These tools are great but use with a fine touch a little noise is not bad and is always better than to smooth.

4. Signal to Noise

So how do you beat noise? Add more signal!
When gathering data, we are gathering both signal and noise, that noise is equal to the square root of the signal. So by gathering more photons the signal will accumulate faster than the noise. This is just another reason to accumulate more frames. The more frames you have the more the math works in your favor.

5. Processing

Acquiring the data is only half the battle and then comes processing, which is an art and some pick it up faster than others. The good news is we now have dedicated tools for astro image processing. Photoshop is no longer the only game in town. Pixinsight is fast becoming the tool of choice for many of us, and with new books coming out like Mastering PixInsight and the art of Astroimage Processing by Rogelio Bernal Andreo and multiple websites with video tutorials like Harrys Astro Shed anyone can learn the basics. Remember that sometimes less in more and always try and make your images look realistic and like they weren’t processed at all.

Never use a sledge hammer when a nut cracker will do.

6. Timely and Interest

One thing we look for are images that are event specific. For instance if a lunar eclipse, or a comet is passing by, A solar transit and even a meteor shower. These are events that aren’t in an imagers every day target list and something that doesn’t happen that often. Its great to be able to share these events with other imagers who possibly arent in the path of visibility or just weren’t able to get out and see or image it. We also like to have some fun with the holidays and post images that correlate such as a Rosette image for Valentine’s day or the Christmas Tree for Christmas.

7.learning the RGB’s, Colors, must be correct

RGB or LRGB images are essentially true color images of an object. Depending on how your image gets stacked you may have an image with an off color. Ive processed many images where the combined stack is all green or yellow and the need for a Dynamic Background Extraction is applied to fix it. Even if your image comes out with the proper colors you still have processing options to consider. One option is to use a color calibration tool to correct the color. You can also use a program like Excalibrator to find the RGB weights for the stars in the target region and that will usually correct all the colors of your image.

One thing we’ve notice is a well executed image however there was never a green neutralization applied and all the stars and the image have a green hue. Photoshop, Pixinsight, Maxim all offer a script or plugin to neutralize the green. You can also check your histogram. Possibly your green channel is out of alignment and you can simply adjust your green channel to fix the green overtone.

Another factor to color correctness is color saturation. Sometimes an image might look good with a little saturation applied but saturation is one of those things where less is more. Saturation may help bring out certain aspects of your image but then you can get enlarged stars by over saturating the slight halos around the stars or you add noise and artifacts to your background. Saturation can’t make up for a lack of data. If you really want to saturate your data you may want to use star masks or an inverted star mask so you are only applying saturation to the sections of the image you want to.

One of the best ways to check your color is to search your image and compare them to other images online. You may see some that are narrowband bi colors or others that are over saturated but you should get a general consensus out of them to get a close color representation.

Only Narrowband images have numerous palettes of colors so those would be in different hues of oranges, teals, purples, etc. We welcome artistic representations of images however with RGB we are more likely to select a natural color over an image that has its color hues adjusted.

Lastly keep in mind about over clipping your image. Clipping an image is when you are stretching your image and move the shadows or highlights slider within the bell curve of the image. Over clipping your image can over darken your background or blow out your image and you also get data loss. A clipped image is very noticeable and can ruin an image. If you want to further adjust your your stretching without clipping then use the mid tones slider.

8. NBD or rare targets (Never been done)

Never been done objects or rarely imaged objects are always something that will catch our attention. It doesn’t necessarily have to be a nebula or galaxy. It could be a current Supernova, a video caught of an impact on the moon or a planet and even a passing satellite or the ISS. NBD’s are also great to share so other imagers become aware of these targets and can attempt to image them as well. Also keep in mind if you see a lot of nebulas or galaxies being posted throw in a nice, well focused star cluster. It may not be a NBD or a rare target but it may be a rare type of submission that we are sure to look at.

9. Degree of Difficulty

Just like with any judged competition, degree of difficulty is always a factor. So if a target is very dim of requires a tremendous amount of data that is taken into consideration. With that said a good description can call out those difficulty that my get over look if not submitted.

10. Star roundness all the way to the edge!

Star roundness is one of the things we look at and we do zoom into images. One of the many things that can affect star roundness is poor guiding, flexure in your image train, polar misalignment, focus, coma, poor collimation, stacking in numerous bad images, etc. Sometimes you will have an image with round stars in the center and then oblong stars at the corners. Now if the image was an NBD would that eliminate it from consideration, No, however if we have lets say 5 submissions of the same nebula it becomes more critical. Generally coma or other small corner star aberration can be cropped out so not all is lost if you see some in your image. One way to guarantee the roundest stars in your stacked image is to apply a star quality limit. That way the images that don’t meet a specific FWHM will be rejected.

As you can see there are several factors that can make an image stand out, some are more subjective than others. The team at AAPOD2 will look at every image and try to make a fair assessment and choose images that meet as many of these points as possible. We take the choosing of images very seriously and understand how hard it is, because remember we are imagers ourselves.

Clear skies,

AAPOD2 Team