No.4 - Getting Started in EAA, Part III

 In the last issue, we discussed what sort of cameras and what types of software can be used to do EAA. For anyone coming from a visual astronomy background, cameras and EAA software can be unfamiliar territory. However, the other key pieces of equipment needed for EAA - optics (telescopes or lenses) and mounts - are items that many visual astronomers already use. One of the most common questions that comes up is, "Can I use my existing scope and mount to do EAA?"  As is the case for many things in astronomy, the answer is .... it depends.



Telescopes

The good news is that many scopes used for visual astronomy can be used for EAA.  As was mentioned in the last issue of this blog, one important consideration when looking at various scope and camera combinations is the resulting field of view: A long focal length scope may be usable for EAA with a large sensor camera if the FOV is not too small. Conversely, a short focal length scope may yield a wide field of view, but one should look to see if the camera pixel size is small enough to give a good image scale.

Another consideration is the speed, or f/ratio, of the scope.  For faint extended DSOs, a live stacked image will build more quickly with a  faster scope. So what do you do if you have a relatively slow scope, such as an ubiquitous f/10 Schmidt Cassegrain? Many budding EAAers may have a 8 inch Meade or Celestron  SCT that they would like to use for EAA, but worry that the scope may too slow  at f/10 on all but the brightest DSOs. The recommendation is to use a Meade or Celestron f/6.3 reducer with the scope - this has the advantage of both making acquisition  faster, and widens the FOV, which is especially important with a small sensor camera.   

My own suggestion for a good scope to start EAA with, is a small relatively fast short focus refractor. The light weight and short focal length will put limited demands on the mount, and give a relatively large field of view. If you like to view in color, a small  ED refractor, e.g. a 70mm-80mm f/6 scope coupled with a color camera is a good place to start. In addition, even if you own only an achromat that suffers from chromatic aberration, there are software tools in SharpCap that can mitigate chromatic aberration (CA) when used with the appropriate filters.

 
Orion Short Tube 80
As an illustration that achromats can be used for EAA, my trusty 1990s vintage Orion ST80, a 80mm f/5 achromat, coupled with a 7nm narrowband H-alpha filter and mono camera does well for wide-field views of emission nebulae in H-alpha. I've used this scope/filter combination to capture a number of emission nebulae. Chromatic aberration is greatly mitigated when the bandpass is only 7nm, as shown in this 15 minute capture of the Crescent Nebula taken with the Orion ST80 at its native f/5 (using the 7nm H-alpha filter) and an ASI290MM mini at 300 gain.

Crescent Nebula (H-alpha); 36 x 25 s



Mounts

 The tracking demands on a mount that is used for EAA are less than if it is used for traditional AP, primarily because the duration of EAA sub-exposures  in a live stack are shorter than those used in AP.  As a result, many doing EAA will utilize mounts that might struggle with longer AP sub-exposures. For example, some mounts may not guide well over longer sub-exposures or may have noticeable periodic error "spikes" if used unguided. Short exposure EAA subs can be fairly forgiving, and EAA software, such as SharpCap, can automatically reject subs that have distorted star shapes, using software filters that measure the size of a star's image, e.g. FWHM (full width half maximum) or similar rejection filters.


Although it is true that lesser mounts can be used for EAA, it certainly makes life easier when the mount is of high quality, as this may result in fewer rejected subs. Some may favor a high quality mount that performs reliably, as this results in one fewer set of problems during an EAA session. In any case, it is pretty straightforward to experiment with any given mount to see if its tracking (with or without guiding) is sufficient to avoid egg-shaped stars or star trails for a given image scale and sub-exposure length.

 Another question that often comes up is whether alt-az mounts can be effectively used for EAA. The attraction of alt-az mounts for some users is the potentially quicker set-up time, as there is no need for polar alignment with an alt-az mount. However, those who favor equatorial mounts will point out that accurate polar alignment can take less than five minutes with modern astronomical software tools like SharpCap's polar alignment routine, or NINA's 3 point polar alignment plugin, and so there really is very little time saved from avoiding polar alignment if you can use these modern software tools. In addition, a well polar aligned equatorial mount will not exhibit field rotation over long exposures. If using an alt-az mount, there will be a small amount of field rotation between sub-exposures, which will result in a cumulatively increasing and unusable fraction of the field at the edges when stacking sub-exposures over a sizable period of time; part of the edge regions of the FOV early on in a live stack will have drifted out of the FOV later in the stack, due to field rotation. In addition, field rotation for an alt-az mount puts a time limit on the length of a sub-exposure before the effect of field rotation is noticeable within a sub-exposure. This manifests itself as stars appearing as circular arcs, centered on the middle of the frame. The effects of field rotation also depend on where the telescope is pointing and the observer's latitude, and so the maximum duration of a sub-exposure before field rotation is apparent will depend on these factors. There are parts of the sky where field rotation may be apparent in just a few seconds, e.g. at the zenith, and other parts of the sky where you may be able to expose  for up to 20 or 30 seconds.  A helpful discussion of this topic is presented here.

The decision as to whether to use an alt-az or equatorial mount for EAA, or whether to "future proof" the mount by buying a high quality mount capable of handling the weight of a potentially bigger (future) scope, or instead start with a cheaper mount, is a difficult one. There is no one-size-fits-all solution, and many EAAers make use of more than one mount. For example, some EAAers use a light portable rig for travelling, and a heavier, less mobile set-up in the back yard that can handle heavier equipment. My own recommendation for those starting EAA is to make use of your existing mount, if possible.  Once you have dipped your toes into EAA and decide you want to continue, it may then make sense to invest in a bigger or better mount if you run into limitations arising from your current mount. Usually that takes a while if you start your EAA adventures with a simple wide field short focal length set-up. 


 


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