General Info Some beginners basics of astrophotography that I have learned for my equipment

Introduction

This article/guide is a work in progress. I'll be adding to it as time goes along and I pick up more useful information..

I will try to detail, in various pages of this article, some of the issues that cropped up for me as a beginner, and some of the terminology that was somewhat confusing to me.
It will hopefully allow a centralized area that any other beginner can refer to to maybe get some guidance in manner that is somewhat easy to understand, as many of the terms and processes were (and still are) confusing to me as those that explain them tend to get into using terminology that a beginner is not necessarily familiar with.

My equipment started with an EQ35 Pro mount (William Optics Rebadged), a Zenithstar 103mm refractor, a NexStar 8se (and Celstron mount for it) and a Nikon D7200 DSLR using Backyard Nikon. I had very little luck with the DSLR and the telescope, and after putting the equipment aside for a few years due to that frustration, ended up jumping in with both feet, getting a Pro grade ASI cooled camera (mono) a filter wheel and eventually an automated rotator when I came back to it.

Many people have great luck with a mount and a simple DSLR and lens, but as I said, this is MY walk down the path. Some of the stuff I found that I would not repeat if I had it to do over again are detailed in this article. This article is NOT for those that are interested in pursuing this avenue... as I said, it's MY walk down the path using my equipment.

My equipment in use at time of this guide:
William Optics ZenithStar 103mm (with WO 0.8 flattener/reducer)
Pegasus Falcon rotator
ZWO ASI533MM Pro mono camera
ZWO 8 position filter wheel
ZWO filters for LRGB and Ha/SII/OIII
RPi 4 8GB running Stellarmate OS

Equipment

I will preface this by with the warning... don't jump into the deep end of the pool. If you have just gotten all your neat toys at once (scope, camera, mount, auto focuser, filter wheel, etc) let me give you a bit of advice. Start out with simply live viewing and become familiar with the way your mount and telescope work. This will also have the added benefit of you actually learning the sky instead of simply depending on a computer to point your scope at it.
Now, let me give you a little more advice... research HEAVILY.

The #1 rule I have learned when buying your equipment... DO NOT skimp on a mount.
My William Optics ZenithStar 103mm kit came with a rebadged SynScan EQ35-Pro. It's a decent mount for live viewing..... but once you start adding on a camera, a filter wheel, a power box and a RPi/NUC you will quickly exceed the mounts capacity.
If I had it to do again, I would have simply purchased the scope I wanted along with a flattener, and then purchased a separate mount for it that would allow for expansion.
There is probably a reason that William Optics no longer offers a kit like the ZenithStar 103MM that I purchased... as once you start doing what they advertise you can, the warts of the EQ-35 Pro fully display themself in all their glory. As you can see on the site, I'm getting some decent captures, but it takes a LOT more work than it should as I frequently have to pause the capture, rebalance and then restart the guiding and the captures.
This is an example of M31 (Andromeda Galaxy) and you can see some tracking issues present (as you can see by slight elongation of stars), and this was BEFORE I added more to my image train.


These are some more recent captures, processed using PixInSight, which I am still a neophyte with.


When it comes to telescopes, in my opinion, the easiest for doing DSO in order are:
Refractor
Reflector
Ritchey-Chretien
SCT
Dobsonian (mainly due to wonky tracking but they do GREAT for live viewing DSO objects and even getting short exposure images)

Too many people think they have to jump into a large scope for astrophotography... and the SCT/Ritchey-Chretien/Reflector seems to be the way many tend to go for some reason. With the SCT (not so much the RC scopes), you have to be careful because of certain quirks with the format of the telescope. Mirror flop can be a big issue with them. And then you usually really need to use off-axis guiding (OAG) with them, which tends to be more expensive than a guide scope/camera setup for the Reflector/Refractor telescopes.
I have a NexStar 8se that is only getting used for planetary viewing and capture right now, as the requirements for getting it up to DSO ability are more than I want to invest when I can get a decent refractor for what it would take.
My recommendation would be look at either the refractor or reflectors. For refractors, something in the 60-80mm range makes for a nice beginners scope and will get you most any target you want. You may have to do mosaics on certain ones, but there are not that many (M 31 being one).
For a reflector, I'd probably look in the 130mm-150mm range, but I have no current experience with a reflector other than a really cheap one we bought our son about 3 decades ago. I am looking at getting a GSO imaging Newtonian (relector) F4 in the 6"-8" range, but that will be a little further down the line as I really want another decent mount.

Do NOT go overboard on your camera. There are many that use (successfully) DSLR cameras. To get the best results from them, you will eventually need to mod them with filters, which tends to ruin them for everyday use. I tried at first with a Nikon D7200, and the results were simply so frustrating that I decided to get an actual astrophotography camera for use. This avenue is a good done for someone that want's to "get in cheap" and actually capture decent images. You can usually find a nicely modified Canon T3i and with a good lens, you can do wonders (but not to the level you will be able to with a dedicated telescope).

A decent intro level ASI Pro camera will do everything you generally need it to, with the understanding that if you get something like the ASI533MM/MC (which is has a 1x1 aspect ratio) you will have to learn to do the occasional mosaic (reference my M31 photo above needs to be done in one since it was captured with a 533MM Pro). But this can happen even with the cameras with a wider FOV. The benefit of most of the newer ASI cameras that you have is that you usually have NO amp glow. You really need to decide what primary targets you are looking to capture... and if you are willing to learn mosaic captures are if you have lots of money to invest in one of the newer capture cameras (like the ZWOASI2600MC/MM or ASI461MM or the ASI990MM Pro). I simply don't have $3K-15K to invest into a camera, so am more than willing to work with what I can afford.
This is a more recent capture (before I found an issue with the telescope/focuser connection) using the ASI533MM Pro.

Be SURE you have a decent guide scope and camera. For any lengthy captures, it will help greatly. Historically, the recommendation is that your guide scope be 1/3 the focal length of your primary telescope. I use a 50x200mm William Optics on the ZenithStar (710mm) and it does great. Now, for my NexStar 8se, I'm going to have to investigate further on as since it's a SC scope, it has issues with flexure and also mirror flop. I'm probably going to go with OAG (off axis guiding) on it honestly.

A decent polar scope will be of help also. Yes, with an RPi running Stellarmate or any of the Indi/EKOS stuff, you can do a polar alignment... but the closer you are when you first start, the better it is. Then that assumes that you have an adequate FOV to do a polar alignment with it having to slew to various stars. For me, using the polar scope on the mount I've been able to get decent tracking for 15-20 minute exposures since my FOV is very limited by trees surrounding the house.

One Item I'd strongly suggest if you are going to do astrophotography using something like an ASIAir Plus, a Raspberry PI 4 (8GB) running Stellarmate or Astroberry or an Intel NUC running Ekos or N.I.N.A. (Windows only). Also a nice camera rotator is HIGHLY recommended.
If you frame each of your captures for the best positioning, when you go back to it later to grab more captures using (in my example) StellarMate OS you can do a Load & Skew to the object.... and it will then tell you which way to rotate the camera to match the existing image. When you do this you have a choice of rotating the entire telescope manually, or if having installed a manual rotator simply rotating the ring, or if you are blessed with the funds to get one, the Pegasus Astro Falcon automated rotator (which I currently have and dearly love) which will do it automatically.

Darks? Lights? Flats? I just want to see images

First, we will address the much dreaded flats/bias/darks. It can be rather confusing... but in a nutshell this is what each consists of as far as settings required:

  • DARKS must match LIGHTS on: gain, offset, temperature, exposure time
  • BIAS must match LIGHTS on: gain, offset
  • FLATS must match LIGHTS on: gain, offset
  • DARK FLATS must match FLATS on: gain, offset, exposure time
Some say that temperature is not important for Flats, dark flats and bias (only Darks)... but with a cooled camera, it's not that hard to keep consistency. I prefer to try to keep as minimal variables in my calibration frames.
If you are not using a cooled camera, you can disregard the temp for those 3, but for Darks the same temperature is important.

Darks are just what they sound like... they are captures utilizing a cover over the end of the telescope or camera that allows no light in. They need to be of the same gain setting, offset and exposure time as the light images. They ideally need to be at the same temperature.. this is even more important when using a cooled camera like an ASI Pro series.
Some ASI cameras do better without using dark frames. The ASI533MM/MC Pro's seem to be two of them.

Bias images must match both the gain setting, offset and temperatures of the lights... the difference is that they are ran at a high shutter speed/exposure setting. For some astrophotography specific cameras, bias frames are not needed and some cameras cannot produce a reliable bias frame (ZWO ASI294MC Pro being one). Each camera used will need to be researched to see if bias frames are needed, or if it can take them. Most DLSR's have no issues, and in fact are needed. It's the devoted astrophotography cameras that you will need to research.

The method of taking bias frames consists of:
  1. Use the fastest shutter speed possible (often 1/8000″ on DLSR, I use 0.05 on my dedicated astrophotography cameras that need it).
  2. Keep the lens cap on your camera or telescope (and cover the viewfinder on a DSLR, many DSLR cameras come with a viewfinder cover)
  3. Use the same ISO (DSLR) or gain/offset as your light frames.
  4. Capture about 20-40 bias frame

Flats
Flat frames will help to magically remove any dust spots that were on your camera sensor. Flats are targeted to correct for minor vignette, sensor mottling (if any), as well as dust and other stray particles that get into your imaging train (on reducers, the sensor window, or filters).
There are several ways of taking them, but the most popular seems to be the "white t-shirt" process.
Stretch a plain white t-shirt over the telescope objective, you create an even flat field when pointed at a bright light source. I personally have a heavier white t-shirt (not an undershirt) that I use, and only need 1 layer of it. If using a lighter undershirt style t-shirt you should double the thickness up. Then take the t-shirt and put it over the end of your telescope, making sure there are NO wrinkles in it. I use 3 rubber bands to hold mine in place, spaced out evenly on the scope body with one right at the top to minimize any wrinkling.
Once you have your t-shirt on, you can then point the scope at the early morning blue sky if you are still up. Using a flashlight at night and pointing it at the t-shirt is not going to work as you want an evenly distributed light source. If you have a larger iPad Pro you can use this and an app to make the iPad screen show white light, but you don't want it on the brightest white. A more subdued will work better. You can point the telescope upward and then place the iPad Pro over the t-shirt and use it as your light source and not have to hold it in place. If you want to get "really fancy", Artesky makes a flat field panel that you can purchase (about $380). You will have to use a computer to run the software that it requires, so if going to remote sites and your capture computer is not Windows based, it's just another computer you will have to take to run their software. There have been reports that there are issues with uneven light exposure (wires interfering in the light panel) with the units... personally, I use the iPad Pro/t-shirt if I don't want to stay up until daybreak or if capturing Bode's Galaxy, since I have to stay up until almost daybreak to do it the t-shirt/blue sky method.

You will want to examine the histogram of the flats you have taken. If the data clips the right side of the histogram, that’s an immediate indicator that your light source is too bright. The solution here is to dim your light source by adding another layer of t-shirt or to point somewhere less intense. If using the iPad Pro method, lower the intensity of the app.

  1. Shoot at the same ISO/Gain as your light frames.
  2. Keep the camera connected to the scope/lens.
  3. Maintain the same focus as light frames.
  4. Shoot a minimum of 15-20 flat frames.
  5. Temperature is not important for most cameras, but if using a cooled camera, i would keep it at the same temperature your lights were taken at
Generally around 20 flat frames will be adequate to create a good master flat image.


Dark flats
Taking dark flats is very simple. After you are finished taking your flat frames, cover your aperture with your dust cap if DSLR or telescope with lens cover and proceed to take your dark flats with the same exposure length and settings as your flats.

Not all cameras (specifically astrophotography specific ones) will require some (if any) of the above. You will need to research your camera train and ascertain what you need to do.
The same way if you use filters... darks don't care which filter is used,
Flats on the other hand will should be done with each filter you used in your capture as part of the think it does is help offset dust particles that can be present during each capture, and each lens will be different. There are many that will only take them with Ha filter or Luminance filter.. which is fine, but you will get your best results utilizing each filter you captured with.


Notes on my equipment:

The ASI533MM Pro that I have doesn't really need darks, but does benefit from bias frames. I have not tested the ASI585MC yet as it's primarily used for planetary capture on the NetStar 8se.

Cameras

Now we enter into the realm of cameras.

First thing you will have to determine is WHAT are you wanting to capture. A camera that is great for planetary captures may not be up to the task for DSO's. Then you have to decide do you want a dedicated astrophotography camera or do you have a DSLR already that you want to use.

I currently have a ZWO ASI585MC that I use for planetary captures with the NexStar 8se. It does great for that. But when I tried to use it for some DSO objects, I found it was lacking in clarity for the captures with the NexStar 8SE. I have an Apertura 72mm refractor on order currently from HighPoint Scientific (twin to a Williams Optics 73mm and several other similar scopes) that I will be trying this camera with to see if it does well for DSO objects also.

I also have a ZWO ASI533MM Pro that I use for my DSO captures. It is a cooled camera, which allows you to obtain longer exposure times.
I am just getting into the image processing aspect.
This was my first attempt at the HorseHead nebula with the ASI533MM Pro and is a mono only SHO image.



This is the same data after I learned a little more of PixInsight and was able to do some colorization.


And this is one of the more recent after I have figured out a little more PI processing.


This is a more recent capture of M81/M82 (not very many captures) that I processed with PixInSight from the ASI533MM Pro.

I also recently obtained a ASI533 MC Pro... and am playing with it. I have NO filters yet for it, so the below is pure camera/telescope (an Apertura 72mm APO refractor).


You will need to decide if you want a OSC (one shot camera, basically a color camera that captures RGB in one photo) or if you want a mono camera and then use filters with it. If the latter, I would suggest investing in a filter wheel instead of using a filter drawer that you place and remove filters one at a time.
Even if you go with a OSC, you will still want certain filters for it that will enhance the target you are capturing (something that I am still researching for nebula captures).
These filters can be screwed on the front of the camera in the OSC train if it is the ONLY filter you will be using, but if you plan on using several different filters with your OSC, you will want a filter drawer to use.

The old school philosophy was that mono gave you a better image at the cost of having to take more exposures... and that is still somewhat true, as with a mono you use every pixel on the camera for one band (whatever is coming through the filter), but with a OSC you usually get 1/4 of the available pixels red, 1/4 pixels available blue and 1/2 of the available pixels green.
They typically capture with a CFA (color filter array) that referred to as a Bayer matrix, which looks like the below image layout.

bayer_2.png

Basically what this pattern consists of is in a 4 block area, each square corresponds to 1 pixel. So in the lower left you see a B/G and then above it a G/B, resulting in 2 green, 1 red and 1 blue.

There is some loss of "resolution" when using a OSC, but it is not so drastic that you should allow it to turn you away from them. As you can see in several of our members photos here, they are getting some great data with OSC cameras.

In fact,, I'm currently various ones to purchase for my use, specifically with my NexStar 8se when I want to start trying DSO with it.

What filters do I use?

Type
Ideal Targets
Best paired with
Multi-BroadbandGalaxies
Star Clusters
Reflection Nebulae
Dark Nebulae
Any color camera, including stock DSLRs
Multi-NarrowbandEmission Nebulae
Planetary Nebulae
Supernova Remnants
Modified DSLRs & Dedicated Astronomy Cameras

Some examples of multi-broadband filters are the IDAS LPS, Optolong L-Pro, Baader Moon & Skyglow,

Examples of a narrowband filter are the Ha, Hb, SII and OIII standalone filters from many manufacturers. Then you have multi-narrowband filters like the Optolong L-eXtreme, Optolong L-eNhance, Radian Triad, ZWO Duo Band, IDAS NB1,

Then you have your "luminance" filters for mono cameras, which consists of luminance, red, green and blue filters (known as LRGB). These are used frequently with galaxies and planetary captures that the narrow band filters will not work with for, usually any target that is light emitting. You will NOT typically need these type of filters for a OSC (one shot color) camera unless you want to enhance certain colors.

One major issue I had, since i have a mono camera, is I thought I had to use EVERY filter EVERY time on each object... which is not correct. Choose your filter based upon your capture target. If you are capturing a nebulae that is mainly in the Ha/OIII range (called a gaseous nebula), trying to use LRGB isn't going to do much other than maybe for the background stars, and is the reason that sometimes I will still use LRGB depending on what else in the target area I want to capture. LRGB are used for reflective DSO's, like galaxies, reflective nebula and star clusters.

In addition to the above filters, you have a wide range of filters that are targeted at planetary captures.
Most of these filters will deal with IR and UV light.
The IR filters are typically used when seeing is bad as they help filter out some of the interference.
The UV filters are especially useful when capturing Venus, as it allows the amateur astronomer to capture cloud structures on the planet.

Processing software

This is where one of the big "issues" crops up. Even if you have a color camera, simply capturing a few frames aren't going to give you the detail you see from others photos.

First, you need to determine what software you are going to use to capture your images. There are paid and free versions that do this, as well as systems that can be installed on Raspberry Pi's or a NUC (for attachment to the scope assembly itself) or even laptops/desktops to assist in capturing images. Currently I use 2 Raspberry Pi 4's and have installed StellarMate OS on them. You can do the same thing using Astroberry, which is free, but you do not get the nice IOS/Android app that allows you to connect to the RPi.

Some free examples of capture software are
SharpCap
FireCapture (targeted at planetary)

A free software for the RPi to allow it to do not only capture, but tracking and selecting your targets is
AstroBerry
Free package to use on an Intel based system is
N.I.N.A.
KStars/EKOS


A paid package (and what I currently use) for the RPi is
Stellarmate OS

The very first step (after capture of course) is stacking.
There are a ton of stacking programs available (many are listed in the Resources area).
A few examples of free stacking software are:

ASTAP
Siril
AutoStakkert!
Registax
DeepSkyStacker

Then you get into the paid softwares (listed in the Resource area also).
I currently have Advanced Pixel Processor (APP) and now that I'm learning it I like it, even at the cost.


Next, we will enter the area of processing stacked images.
When you first see your captured data, you aren't going to be greatly impressed. VERY little shows up until those images have been stretched.

The pretty much acknowledged "giant" is PixInSight. It's a great package, but it does SO much that it's hard to learn and will take you awhile.
This is an area that I am still learning and cannot speak much upon yet. I do like PixInSight, but it is somewhat intimidating.
A very base process is for my narrow band captures is:

Script -> Batch Processing -> WBP (Weighted Batch Processing) - this processes all your images into several different states, the main you will be interested in using are the Masters
Masked Stretch (ran on each image, click to highlight the image then the apply option - square box)
Linear fit (use the Ha channel as your base channel for this then click on the SII and OIII and use the little arrow in the bottom left of the option to apply to that image)

Screen Shot 2022-11-26 at 12.31.03 AM.png


The triangle in the lower left is what you should select after you have highlighted your SII or OII image.

Screen Shot 2022-11-26 at 12.31.18 AM.png


Channel Combination (I use the Hubble Palette Red->SII Green-Ha Blue->OIII)
SCNR and choose Green and remove it from the photo you get from the above combined.

Then if you want to, you can play around with the Curves & Transformation process to fine tune the colors.

The process is basically the same with it for LRGB, with the exception you have to add the Luminance channel in. Learning PixInSight is WAY to detailed to go into here, but the above was the base first-line process I learned to actually be able to get images out of the camera.

This image was captured in Ha/SII,OIII.

Handy mobile apps

A few mobile apps that I constantly use. I have an iPhone and an iPad Pro that I use quite a bit (since I also use StellarMat OS currently).

First on the handy list is PS Align Pro (link is to the IOS version). A similar program can be found on Google Play here. These programs really help with your polar alignment if you have a manual alignment scope on the mount. The IOS version also has a balance scale built in, but since I don't have an Android device I'm unsure if that application does. These apps will help with your polar alignment tremendously as they give you a visual aspect that most GOTO mounts do not on the hand controller. Where this is handy is on the 2 SkyWatcher EQ mounts I have that had the polar alignment scopes in place, NONE of the clock faces were correct, requiring you to tilt your telescope RA frequently to get it to "look right"... which is simply more mass movement that can knock it off somewhat when returning the scope to where it needs to be to start the 1-3 point alignment process if using the hand controller.

Next up, and definitely one of my favorites on the iPhone is SkyVIew. This is one I use every night that I'm out to do captures with to familiarize myself with what I can see and locate what I want to capture since my FOV is limited from where I normally do captures. You can get a lite version, or the paid, which is what I use. The Android Play Store version of the lite install is available here.

Another program that I'm beginning to gain trust in and like is Good to StarGaze (IOS version). Generally it has been pretty accurate. There is a lite version that gives you 1 day forecasts. If you want extended forecasts, it is about $30 USD to subscribe per year. And Android version of this software is available here.
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About author
Tracy
I've been interested in astronomy since I was in Jr. High (graduated in the early 80's).
It wasn't until I retired that I was able to really get into it due to monetary and time constraints.

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