AT8IN Newtonian vs c8 SCT vs SV 102mm APO EAA Comparison
By: Vlad Fedosov
I would consider myself as a visual observer. I have tried to get into astrophotography on several occasions, but ever time I have concluded that it's just not really for me(at least for now). I was at a star party about a year ago(see: Moro) where one of our club members had a basic electronically assisted astronomy(EAA) setup going with his 16” GoTo Dob. I was blown away by how quickly he was getting an image of the Horsehead nebula in color! I thought that this EAA is something I definitely need to look into.
Over the next year in my search for a perfect EAA scope I have gone through many OTA’s. I’d like to share my experiences with the most popular scope designs. The scopes that we will be comparing are: c8 with f/6.3 reducer, Astro-Tech AT8IN, and SV 102mm APO. I chose these scopes as they are some of the most common entry to mid-level scopes that are affordable to most common folks like us. If you are just getting into EAA, or perhaps looking for another scope to use for EAA I’m sure that this info will help you narrow down your search!
I carried out this comparison of scopes in my mag 4 light polluted back yard on the same night, using the same mount and camera. I used the ASI 178MC camera for this comparison as it has a relatively small sensor that most people starting out in EAA will be using. This sensor is similar in size to the popular 224MC(although the 224 is even smaller then the 178MC so all the scopes would frame even less of the object if using that cam). The seeing on this particular night was not very good, but since all the images where taken within about a 30min time frame its still a very fair comparison. All of the images where captured using SharpCap, saved “as viewed”, and with zero post-processing. I did a 3-minute stack of 5-second subs at 400 grain for all of the images. I chose 3 targets that range from very easy to difficult including: m42(nebula), m37(open cluster), NGC 1275(Perseus galaxy cluster). For the most part, I think the images speak for themselves, but I will add what I noticed about the images.
The 8" Newtonian is advertised by Astro-Tech as an imaging instrument. I had high hopes for this scope and for the most part I was not disappointed. It does produce nice images and does have a decently large filed of view(FOV). As you can see, if you are looking to image larger objects like m42 you will have a tough time framing the entire object. You can technically add a focal reducer to the scope to increase the FOV but this would likely exaggerate the coma that is already quite evident even at the native f/4. Similarly when you are ready to upgrade to a larger sensor like the 294MC you would really have some issues with the performance of the scope at the edges of the image.
The most obvious difference with the SCT is that the FOV is quite narrow. I think that this is the biggest downside of the scope. You can reduce further down to f/3.3 fairly easily by ether stacking two f/6.3 reducers or just buying a Meade f/3.3 reducer. I used to own a Meade f/3.3 reducer and it does work quite well with s small sensor like the 224MC.
I think that the APO frames all of these objects much better then the other two scopes. Stars are also nice and tight little points of light. I did want to point out that the smaller aperture does not produce an image of the Perseus galaxy cluster that is not quite as bright and detailed as the larger scopes. I do find that the refractor is just fun to use!
Overall if you can swing the cost of a triplet APO I think that is the way to go for a majority of deep sky EAA images with a smaller sensor camera. You get a FOV that is wide enough to capture the majority of objects and nice crisp stars all across the FOV. You also do not have to worry about collimation! If you cannot spend the money on a triplet the decision gets a bit harder as secondary color is a bother with less expensive achromatic and ED doublet lenses(see: Revolution 80mm). An 8” SCT is probably my second choice, especially if you are interested in doing some planetary imaging. It is fairly easy to reduce down an SCT to f/3.3 with a Meade reducer and you can go up in focal length as high as your seeing will allow for planetary work. If you are into small galaxies or planetary nebula then the SCT might be a better choice as well. The Newtonian is perhaps the least expensive scope to get into EAA and is certainly capable of some decent images but it does display some coma even with a small sensor like the 178MC. Good luck on your EAA adventures!