How to Choose A Telescope For Astrophotography

While choosing lenses for photography we look for the right optics, focal length, focal ratio, optical errors in the lens, performance of the lens in different lighting situations, and as well as with different sensor sizes. The same goes with Astrophotography – glass quality, focal length and focal ratio are something I look into before getting a lens. There are also other aspects to look into, like aperture, compatibility with different sensor sizes/cameras, the weight of the scope (this will help you in choosing a suitable amount). And then there is framing. Let us discuss each of these by one.

Type of targets you like to shoot – Focal Length

Deciding the type of objects you like to shoot will drastically narrow down your search for the right equipment. Once you have decided on the targets, you can find out the suitable focal length and type of design of your scope – Cassegrain, SCTs, or Refractors. Focal lengths mentioned are effective, i.e., full-frame sensor size is assumed at the end of the setup.

I like to divide targets into two categories based on their apparent size in the sky – Deep Sky Targets and Ultra Deep Sky Targets.

Deep Sky Targets:

Some of the well-known targets are – Moon, Orion nebula, Andromeda, and other large nebulae. All these targets will need an effective focal length from 500mm to 800mm. There is no one scope perfect for all targets, so you have to choose which suits you best. Usually, wider is better.

Ultra Deep Sky Targets

In this category, galaxies like (Cigar and Bode), smaller nebulae, globular clusters. You can also shoot planets with a scope that can shoot targets in this category. Effective focal length should be around 1500mm to 2500mm. At ~2500mm you can shoot individual galaxies in Leo Triplet and at ~1500mm Leo Triplet will just fit in your frame.

Above the Ultra Deep Sky category, I’m not considering those focal lengths in this post. Scopes capable of shooting those targets are usually SCTs like Celestron EdgeHD or Meade LX series.

Here are some example frames with different scopes:

Focal length stated is after taking crop factor into account.

Deep Sky Target: Orion Nebula, Camera: Nikon D5600 (crop factor – 1.5).

Above images are only to show framing (red rectangle) of targets with different scopes.

Ultra Deep Sky Target: Leo Triplet, Camera: Nikon D5600.

Note: After comparing both types of targets, one can say that 840mm is a sweet spot for choosing the focal length. But again that’s my opinion.

Here, I’ve only taken two targets as an example. There are many more. You can’t just simply divide all the objects into two categories. You can also try in Stellarium with different combination of Camera sensor and astrographs – see what looks best to you.

If you are a beginner, start with something around 500mm. For a mount you can get a star tracker (like Skyguider Pro). 500mm is around the upper limit that star trackers like SGP could handle. For a telescope, you can get a small refractor (William Optics Zenithstar series) or a good quality telephoto camera lens (Sigma 150-600mm). You would not need a guiding at this focal length (at least for small sub-exposures like around 1 minute).

Fast Optics – Focal Ratio

For shooting widefield MilkyWay shots, an f-ratio of f/2.8 or less is recommended. The lower the f-ratio, the faster the lens is said to be. Why fast? because with f/1.4 you will need less time (~ half the time) to collect the same amount of light as with f/2.8. However, in the world of telescopes, f/4.9 or f/5.9 is considered fast enough.

See Cory Schmitz’s article on – Why Aperture is so Important in Astrophotography

Design and Glass used – Optics

This section is purely for refractors, as SCTs, Cassegrains or Reflectors doesn’t suffer with chromatic aberration.

• Design
  1. Doublet: Two lenses are used.
  2. Triplet: Three lenses are used for correcting CA.
  3. Quadruplet: Four lenses are used. Expensive and heavy.
  4. 5-element: Five lenses. Typically, a doublet objective lens and triplet corrector group.
  5. 6-element: Six lenses.
• Glass used, according to their Abbe number.
  1. FCD100: 95.10
  2. Calcium Fluoride: 94.99
  3. FPL-53: 94.96
  4. FPL-52: 90.32
  5. FPL-51: 81.61
  6. FCD1: 81.61. Purple and cyan fringing. The main issue is with stars. Only when you zoom in.

Abbe Number:

In optics and lens design, the Abbe number is a measure of the material’s dispersion, with high values indicating low dispersion.

Note:

1. Doublets compensate remaining chromatic aberration by using better quality of glass – like FPL53 in most cases. This results in virtually no chromatic aberration.
2. FCD100 and FPL53 are similar with almost same Abbe Number. Only difference is – FCD100 is made by Hoya and FPL53 is by Ohara. Both are Japanese glass manufacturers.

Point Spread Function for some refractors

Point Spread Function (PSF) describes the response of an imaging system to a point object. A refractor with a PSF value of 1 will generate a perfect image. You can see ’60’ and ’40’ are perfect refractors of 4″ and 6″.

On studying the above graph, we can conclude two points:

1. A doublet FPL53 performs better than a triplet FPL51. Although under night-sky these refractors will perform differently. But this graph shows us that there is not much difference in a doublet and triplet in most cases. So, for your first refractor, you can go with a doublet with FPL53.
2. Fast doublets with small apertures can compete with triplets with same aperture. While same can’t be stated for large aperture ones.

Weight of the scope

The weight of the scope is also something you have to keep in mind. The weight of your rig shouldn’t go over 60% of the maximum payload capacity of your mount – to get the best performance. Portability is also something to consider. If you have to take down your setup after every session – you would need something like a small refractor and a light-weighted mount. Something like a compact Doublet and a Center balanced equatorial mount (known for their lightweight) will be best for that.

Conclusion

Getting your first telescope is never easy. Most of us wait for years to get our first. There is always more to learn about telescopes and sometimes you may get overwhelmed by all the information. I’ve covered refractors here, other types of scopes like – SCTs, Cassegrain that I will cover in the next article.

~AK