A Photographer Perspective

As a photographer you probably own some interchangeable lenses. Depending on the focal length and the optical quality of that lenses, they most probably can be used for astrophotography with a good results. In astrophotography, optical quality requirements on a lens grow with the focal length and aperture much faster than in the regular photography. Still, as a photographer, you can use your start in astrophotography to align your set of lenses.

Especially, for the wide-field nightscape astrophotography you can even start with using an inexpensive 20-50mm kit lens, whereas prime lenses are certainly a much better choice. It definitely makes sense to invest in a good, stable tripod. The tripod should have load capacity of around 10kg or more even if you do not have gear of that weight. Another small accessory which you will need is a wired (yes, not wireless to less things which can go wrong) remote shutter with intervalometer and timer. Both are needed for daytime photography as well.

Deep Sky regions, for example Orion or Sadr, will require a 100-135mm lens on APS-C or a longer one on fullframe. This focal length is broadly used in regular photography as well. The Samyang (also know as Walimex and Rokinon) 135mm f/2.0 ED UMC lens is inexpensive and available for most cameras brands. It is very fast and according feedback on the Internet it has a very good optical quality. The drawback for daytime photography is the lacks of autofocus and image stabilization. A good alternative are lenses from the “Art” line by Sigma. They available for all systems and support autofocus. Beware, that in most cases, the focal length of 100-135mm already requires a star tracker to keep stars round. The famous rule of 500 does not produce good results.

Deep Sky objects, for example the Veil Nebula will require a 200-400mm lens. 200mm is actually the shortest focal length for telescopes. Starting from this focal length, it is hard to find inexpensive lenses with an optical quality producing very good astrophotography results. The only exception which I’m aware of is the Canon EF 200mm f/2.8L II USM. This is a very old (1996) lens which is still available second hand for a very reasonable price of around 500 Euro. A mid-price range telephoto lens, even some good zoom lenses, will produce at least the same or even better results than a telescope with a similar focal length for the same money. Fou you, as a photographer, it makes much more sense to invest in a good telephoto lens which can be used for daylight photography than buy a cheap telescope which you probably will not use very long.

Even Deep Sky objects can be captured with ordinary kit lens. Still, basic knowledge and good image processing skills are required. Watch these videos made by Trevor and Nico. There are two videos from each of them, see the links. They use a simple DSLR, a budget kit lens and just a star tracker to make impressive photographs of an object where one would expect a serious telescope setup.

Fast lenses (f/6 and faster) with a focal length longer than 400mm are very expensive and very heavy. In this range, a telescope is the only way to do. Do not expect and do not get blind by bargains. A telescope with a good optical quality and a large aperture (f/6 and faster) will still start at around 1000 Euro and quickly move to 2500 depending on the optical design.

It is very easy to get captivated by telescopes with a long focal length. You think, you can dive so much deeper into the stars! Be careful! I learned it on my own experience, you can often read this on Internet and in the books: challenges grow exponentially with the focal length in astrophotography and then multiplies by the speed the optics. This is a very important fact for any beginner. It is very tempting to buy a used and inexpensive Newtonian telescope with, say, 800mm focal length. You might think you will get all the stars closer to you. DO NOT DO THIS as a photographer in the beginning, if you want to get some good results soon without getting completely frustrated! Make some pictures with photo gear and later consider to upgrade to somethings what you photo gear cannot do.

Limitations of Interchangeable Lenses for Astrophotography

Why not making all lenses good for astrophotography as well? Astrophotography has different requirements than the regular photography:

• Lower chromatic aberration than expected in regular photography
• Very good collimation, low astigmatism
• As fast as possible, f/6 and faster
• The is no diaphragm, the aperture is fixed and even has a different meaning than in regular photography
• Auto-focus is still appealing, but totally optional
• The focus always sits at infinity, other distances are not relevant
• No zoom is needed
• No image stabilization is needed
• Size and weight are not of a hight priority
• Hight expectations on the mechanical rigidity and thermal stability of the design and used materials

Consumer expectations on modern interchangeable lenses on the mass market are almost ridiculously opposite to the above:

• As much zoom as possible
• Prime lenses are known to have far better optics, but are also seen as pro-gear and so automatically more expensive
• “Normal” chromatic aberration are OK, since its negative influence on the image quality is not significant in most scenes in regular photography
• Fast and versatile auto-focus, possibly with scene detection
• Short minimal focus distance
• Image stabilization
• Low weight

Not All Telescopes are Suitable for Astrophotography

Not disputing that a telescope can bring better optical performance, it is also important to clearly realize that there are several types of telescopes. They are different in their optical designs and various features. By far not all of them are good for astrophotography. Due to this, all designs are also split in two groups: for astrophotography and for visual observations. This split is relatively new. Just 15-20 years ago, any amateur telescope was a visual one.

Generally, there are two major kinds of optical designs: refractors and reflectors. Reflectors are based on a system of mirrors, and only partially use lenses. Refractors are similar to interchangeable lenses. Their optical train mainly consists of lenses. See this article in Wikipedia for a more detailed list. It seems like most people use refractors for astrophotography.

Interestingly, almost each of the above kinds and their subtypes can be rated as being suitable for astrophotography or not. In general, this classification is driven by fundamentally different ways of how a human eye and a camera receive and perceive faint lights from the nightly sky.

The main difference comes from the fact that astrophotography is the art of collecting photons, composing an image and processing this image to an appealing presentation. A camera connected to an astrophotographic telescope basically accumulates the light signal in a sequence of images. Due to this, a single image in this sequence is called “sub” in the astrophotography slang. A sequence of subs is stacked and combined with various calibration images in order to increase signal-to-noise ratio and reduce various distortions. This combined image is then manipulated with image processing tools.

Even though human eyes have unique visual capabilities, they cannot collect photons and make a use of multiple visual frames. Thus, the visual observation needs an own range of telescopes to appeal human eyes. Since all objects in the sky are very small and faint, a stronger total magnification is an important. An observer focuses the attention on the center of the image. Due to this, the topical quality further away from the center is less important. Imaging planets and the Sun often uses video recording and special post processing techniques. Video is closer to the way how eyes work. Due to this, and due to required strong magnification, telescopes for visual observations are often used for planetary imaging.

The best and more detailed summary about different types of telescopes is in my opinion on this page from Jerry Lodriguss.

Sometimes some products are advertised as “suitable for astrophotography”. Reading posts in forums and various other sources on the Internet, I got an impression that just “suitable” is by far not sufficient.

Your First Telescope Should Beat Your Best Telephoto Lens

One important fact upfront. There is no telescope in the focal length shorter than 200mm. 100-200mm is a very important range with a lot of nice objects in the sky. It definitely makes sense to improve your selection of lenses in that range and to make some pictures with them. The following sources provide a good overview of lens choices:

• Lonely Specks Ultimate List of Best Astrophotography Lenses
• Camera Lenses for Astrophotography by Jerry Lodriguss
• Recommended Digital Cameras and Lenses for Nightscape and Astro Photography by Roger N. Clark

If you own a mid-range or even a high-end telephoto lens, it does not make sense to buy an average telescope even if this telescope is said to be very good for astrophotography. Such a telescope might not perform much better than your lenses. An objective judgment if and how far a given telescope is better than your good lenses requires a lot of time, long nights or possibly an expert-level test with very expensive optical measurement equipment. Is it worth it?

I have a privilege to own a few high-end Canon telephoto lenses. Most of time, I use these two for Deep Sky astrophotography:

• Canon EF 100-400mm f/4.5-5.6L IS II USM
• Canon EF 200mm f/2.8L II USM

After around 10 months in use, I concluded that I will keep using them for targets fitting into their FOV on an APS-C camera. Over these months, I still was curious if there is an alternative which would provide a much better results at 400-600mm for a much lower price than the next best photo lens.

So, what are the choices?

Keep on Using a Refractor

In the focal length of 400mm longer and considering a large aperture, a telescope is the only affordable way to go even from the price prespective. For example, the Canon EF 400mm f/2.8 IS III USM or Canon EF 600mm f/4 IS III USM cost almost a fortune or at least as much as a small observatory would cost in total. Plus, they are very heavy. Good telescopes at this focal length will most probably provide better results in astrophotography, since their are designed specifically for this.

What would be the essential attributes of a telescope which could become an upgrade for a telephoto lens?

• Pre-sorted, and calibrated at the factory or, much better, at by competent dealer
• Fast, at least f/6
• 80mm and more aperture
• At least 500mm, better 600mm, but not more than 900mm focal length
• Classified as being designed for astrophotography, for example as an astrograph
• A triplet apochromat
• Has a flat-field corrector integrated or you can get one which is made specifically for this model
• Full-frame compatible, meaning at least 44mm image circle
• A bayonet adapter for DSLR is available. And it is onfirmed that it is possible to achieve focus with a DSLR
• Focusing system should allow installation of a motorized focuser, e.g. ZWO EAF

Suitable Refractors Choices I Found so For

The choices on the market is large. Most choices claiming good quality are in the price range of 1700-3000 Euro which is not cheap, but still much less expensive than DSLR lenses with a comparable focal length, aperture and optical quality. All choices below are refractors which are said to be good on Internet. I’m currently more considering to get a reflector to my gear, most probably a Newtonian or a Ritchey-Chrétien telescope.

It looks like one of the most popular refractors are from the Sky-Watcher Esprit series, especially the Sky-Watcher 100ED (550mm, f/5.5). It has a good reputation and seems to be very popular even though it is not a very modern design: it is on the market since 2012. It is currently offered for around 2000 Euro. An optional flat-field corrector (TS Optics 2,5″ PHOTOLINE Corrector – TSFlat25) is available for around 300 Euro. According reviews and tests a flat-field corrector is even required to achieve the best possible optical performance.

TS-Optics Imaging Star 80mm (352mm, f/4.4) seems to be a good choice too. It goes for around 1200 Euro. This is a triplet apochromat (also known as full apochromat) with integrated flat-field corrector. This kind of apochromats care are also called “quadruplet apochromat”. There is a long focal length version in the same series: TS-Optics Imaging Star 130 mm f/7 FPL53 Triplet Apo (910mm). Its price is almost as much higher: around 2400 Euro. It provides a powerful 3.7″ focuser, but does not include a flat-field corrector which is can be bought separately. There are quite some excellent images on Astrobin made with this telescope!

TS-Optics 94 EDPH (423mm, f/4.5) is a new model, looks right in all aspects and costs around 1700 Euro. A flat-field corrector is integrated. It seems to be the same as SharpStar 94 EDPH, but with flat-field corrector included.

There are also some newer choices with smaller apertures. The ASKAR FRA400 72mm (400mm f/5.6) Quintuplet with integrated flat-field corrector offered for around 1100 Euro. The FRA600 version has 600mm focal length abd 108mm aperture and cost around 2600 Euro. Another quadruplet apochromat which is popular and a good quality product is William Optics RedCat 51 APO (250mm, f/4.5) for around 900 Euro.

On the other side, going into a longer focal length but also with a smaller aperture, the EXPLORE SCIENTIFIC ED APO 127mm (952 mm, f/7,5) FCD-100 CF HEX for around 2500 Euro seems to be a popular choice. Its price is around the same as for ASKAR FRA600 but I’m not sure how the optical quality compares.

It seems like 1500 Euro or more must be invested beat the quality of a Canon EF 100-400mm f/4.5-5.6L IS II USM. Considering that this lens costs roughly the same as used, the question comes: Is this really worth the money spent? I meanwhile think it makes more sense to go another way, if you own a good telefoto lens. Keep the DSLR setup for wide-field and get a good reflector for a longer focal length.

A Reflector in Addition

Newtonian telescopes are widely known and used. This is probably the second most used category after classical refractors. A good Newtonian telescope provides the best value for money at focal lengths of more than 400mm and wide, 200mm and more, apertures. Notably, there are other kinds of telescopes based on optical designs with mirrors. The most prominent in astrophotography are the Ritchey–Chrétien telescope. Compared to Newtonians, a Ritchey–Chrétien telescope has two parabolic mirror. Due to this is possible to have longer focal length at roughly the half tune length than it would be in a Newtonian. For long time making a good Ritchey–Chrétien telescope was very expensive. Since a few years, GSO offers a selection of Ritchey–Chrétien telescopes with in same design, but in different sizes for a reasonable price. Especially the RC8 is very popular in astrophotography .

From the optical design point of view, it is always a big dispute in the community if a reflector generally performs better than a refractor. The argument against reflectors that the a portion of light is lost by the nature of reflection. Plus, reflectors are difficult to adjust well. This reoccurring need for adjustment is getting annoying for many people aver time. On the opposite side, a refractor introduces optical distortions by nature of refraction. Its design is far more complex, and is hard to manufacture in good and, most importantly, constant quality.

One strong argument remains for a Newtonian. It is close to impossible to get a good telescope of any other optical design with, say, 8″ (200mm) aperture and a 800mm focal length for around 1300 Euro. The UNC and especially ONTC series from Teleskop Service seem to be very popular at least here, in Germany, and provide a good value for reasonable price. My personal preference would be either an ONTC808 or NC2008, both are in the range of 1600 Euro. With ONTC, you need to select and buy a focuser, and both series cab be used with a corrector. The total still goes to 2400 Euro, around the same as for a good refractor, but provides much better aperture at the same focal length. Telescope Austria developed a new, “2021”, version of their “Newton ohne Namen”. It is more expensive than UNC and ONTC models, but seem to be worth money.

Getting Something Really Different? Getting RASA?

For a while I was also considering to extend my setup with the Celestron’s RASA series and get its smallest 8-inch model.

RASA 8 provides currently unbeatable combination of 400mm focal length and 200mm aperture, resulting in f/2. f/2 means 8 times shorter exposure time than with f/5.6. And it is almost 12-16 times shorter exposure time than the f/6.3 on Canon EF 100-400mm f/4.5-5.6L IS II USM at 400mm being stopped from f/5.6 for a better quality. If you already had made some astrophotography images, you should understand what “10 times shorter” actually means… It means an that a 5h sequence can be made in 30min. This means more sleep, a larger buffer for bad images spoiled by planes and satellites, and, at the end, it is far more fun. This comes with its price and drawbacks…

RASA 8 is currently offered for around 2200 Euro. This is the same price as for Sky-Watcher Esprit 100ED, but it is also very different in handling. There are a few drawbacks which should be considered when thinking of RASA:

• Quite some people still report collimation problems even though Celestron claims that the collimation is adjusted at the factory. Maybe newer samples do not have this problem
• Doing a collimation on your own is risky and should not be done by beginners
• Quite some people report problems caused by a tilt of the camera. There are meanwhile some special adapters with tilt adjustment screws
• f/2 does not come for free. Even in daylight photography, this f-step is tricky to handle. In astrophotography, it much harder. The RASA design makes it more fragile
• It is not possible to use DSLRs or larger mirrorless cameras with the 8-inch version, since they obscure a too large part of the view
• It is not possible to use a filter-wheel with the 8-inch version
• Like any other reflector, RASA 8 is more sensitive for wind
• It is not possible to use RASA 8 for visual observations, if you would want to