Pitfalls and opportunities

In this article we want to show the different ways to enlarge the image or to take pictures through the eyepiece quickly and easily with a camera. These techniques are interesting for photographing the moon and planets as well as for anyone who wants to photograph through an eyepiece - for example, for nature photography through a spotting scope. "Getting closer" is the goal of many photographers. In other words, the longest possible focal length to image the objects as large as possible on the sensor. A practical rule of thumb for astronomers is that the sun and moon appear on the sensor approximately with a diameter of just under one centimeter per meter of focal length. Since long focal length telescopes quickly become very unwieldy or at least require a very heavy mount, it is difficult - especially as an amateur - to reach for the largest possible telescope. A 14" Schmidt-Cassegrain with 3900 mm focal length, as it can be found on many observatories, is for most people the longest focal length telescope, which they can access at least through a club membership. So how do you get closer? The answer is simple: a magnifying lens. This can be done either with barlow lenses (check out our blog post: Barlow lenses, their magnification factors and working distances) or, for larger extension factors, by photographing through an eyepiece - which is popularly known as digiscoping or digiscoping and a common way to - to photograph through a spotting scope and thus save the money for in expensive telephoto lens. Choosing another camera with a crop factor or smaller sensor, on the other hand, is pointless if you really want more magnification.

Crop factor – only a myth

Especially beginners are always confused by the term crop factor. The 100 mm lens of a full-frame camera is supposed to change magically to a focal length of 160 mm on an APS-C camera and even 200 mm on a Micro Fourthirds (MFT) camera! Of course, this is nonsense: A camera lens or a telescope always has the same focal length, no matter what kind of camera is behind it. Just because the sensor of an MFT camera is only half the size of that of a full-frame DSLR, neither the focal length nor the aperture ratio (speed) of the lens change. Only the field of view is smaller. If both cameras have the same resolution (pixel size) and the image is displayed in full resolution, the full-frame camera will only show a larger image field at 100% display and the smaller camera a part of it because it has less pixels - but both images will be identical in resolution or scale. It also does not help to enlarge the smaller image to that of the full-frame camera: This does not reveal any new details. The crop factor therefore does not indicate how large the focal length really is, but only how large the image section ("crop") of a smaller sensor is in comparison when the lens is used on a full-frame camera. Since photography through a telescope or even an eyepiece uses very long focal lengths, the comparison with a full-frame camera is of little use - here it makes more sense to specify the field of view in degrees, or the magnification. When photographing through an eyepiece, the effective focal length (i.e. the total focal length of the system) also depends on the distance of the eyepiece. More about the calculation of the resulting effective focal length can be found below. By the way, you should not magnify arbitrarily high: At some point, the resolving power of the telescope, together with the pixel size of the camera, will set limits on the magnification that can be achieved. For many planetary cameras, the optimal resolution is achieved at a focal ratio of about f/30; higher magnification only inflates the image - just as the maximum magnification depends on the aperture of the telescope when observing with an eyepiece.br]

Eyepiece projection - using a the system camera at the eyepiece

In visual observation, you can achieve higher magnification by using a Barlow lens on a fixed focal length eyepiece. In principle, you do nothing different with eyepiece projection than you do when observing with a Barlow lens: you insert an optical system in between that increases the effective focal length of the objective (or eyepiece). Barlow lenses usually extend the focal length by a factor of two to three. They are usually designed for use with eyepieces, and the factor also depends on the distance from the field stop of the eyepiece or, in the case of a camera, the image plane to the barlow lens. If you increase the distance, you also increase the effective focal length or image scale. Within certain limits, a Barlow lens can thus also be used with different magnification factors. This is consistently implemented in the modular Barlow Lens VIP, modular , visual and photographic (2x) (#2406101 , € 228,- ) : Photographically, the barlow element can be placed directly in front of the camera, thus providing the desired twofold magnification factor; the distance can be varied visually, in particular, with additional T-2 extension rings.

The magnification is calculated from the distance of the barlow element to the field stop of the eyepiece with the following formula:

1 + (distance [mm] / 64 [mm]) = magnification [x].

The field stop of a 1.25" eyepiece is usually located at about the transition from the eyepiece body to the nose piece or at the outer end of the eyepiece clamp. Of course, a Barlow lens works best when used close to the specifications for which it was calculated. If you deviate too much from this, the image quality will eventually decay, usually starting at the edge. Specially developed for photography with medium format cameras with 90 mm image circle, the FFC ( FFC – Fluorite Flatfield Converter (3x-8x Barlow Lens) (#2458200 , € 730,- ) ) provides three to eight times focal length extension. Because it was designed for photographic use, it provides the necessary flat field of view - even for very large sensors! The lenses also provide the indispensable high resolution to prevent the image from becoming muddy. The "world's best Barlow lens" has its price, of course - but for that you also get ten times the line resolution of a Zeiss Abbe eyepiece, for example. Especially in amateur circles, where the budget for an FFC is not immediately available or where the technology is to be experimented with first, eyepiece projection has been established for decades. Instead of a barlow lens, an eyepiece is simply used for higher magnifications. The camera should not be placed directly behind the eyepiece, but at some distance. Ultimately, an eyepiece is calculated to project a sharp image onto our curved retina in interaction with our pupil, and is not calculated for the flat camera sensor. Therefore, if the camera is too close to the eyepiece, the image will be heavily distorted at the edges. The larger the sensor, the further away it must be from the eyepiece. This additionally magnifies the image - even an eyepiece with a medium focal length thus quickly brings you into the range of maximum magnification.

As a guideline, a full-frame DSLR needs a distance of around 95 mm, an APS-C camera 85 mm, and for MFT 70 mm deliver good results. On a DSLR with a standard T-ring, the sensor sits 55 mm deep in the housing (the "flange focal distance"), so you are already on the safe side with a 40 mm T-2 extension. With mirrorless system cameras, further extension sleeves are necessary to get to the minimum distance. Alternatively, you can of course just use the sharp part in the center of the image. The effective focal length is calculated from the magnification of the eyepiece (which results from its focal length and that of the objective) and the distance a between the eyepiece and the sensor as follows:

f_Equivalence = f_{Spotting scope} × ((a/f_Oku)-1)

To mount the camera body behind the eyepiece, variable projection adapters were used for years. Here, the eyepiece is inserted into a sleeve system, at the end of which is a T-thread. On the telescope side, there is either a thread or a standard nose piece. The distance to the camera (and thus the projection distance) is adjusted either by moving two sleeves (which can lead to tilting depending on the weight of the camera and the manufacturing precision of the adapter) or by additional extension sleeves. The OPFA - Eyepiece Projection adapter (I - VII) (various versions available) from Baader Planetarium relies on stable screw connections instead and can even be screwed to telescopes that have a thread on the focuser. Thus, the system is secured against tilting as well as against lateral stray light. These adapters have only one limitation: Their inner diameter restricts the diameter of the eyepieces that can be used. What works wonderfully with classic orthoscopic or Plössl eyepieces literally comes up against its limits with modern wide-angle eyepieces. In the OPFA, for example, eyepieces with an outer diameter of up to 38 mm fit. Therefore, the Baader Planetarium Hyperion- and Morpheus wide-angle eyepieces have a connection thread on the eye side. The M43 thread leaves room for an extra large eye lens and can be converted to a T-2 thread with the Adapter M43i to T-2a – for Hyperion® / Morpheus eyepieces (#2958080 , € 19,- ) . Other eyepieces like the zoom eyepieces of some Celestron scopes already provide a T-thread (with a correspondingly smaller eye lens). Thus, the camera is attached to the eyepiece in a stable and tilt-proof way; the 2" nose piece of the eyepieces provides a firm hold in the focuser. Focusing is then quite simple via the camera's viewfinder or display, preferably with digital zoom. In order for you to see a picture, you will probably have to turn off the autofocus and select manual mode or automatic exposure. Otherwise, since there are no lens electronics for the camera to communicate with, it may not trigger at all. Use a remote shutter release, or self-timer or interval function and mirror lock-up, if necessary, avoid unnecessary shaking. When choosing a projection eyepiece, there are a few things to consider - not only obvious things like the best possible optical quality and, if necessary, diameter or connection thread. The focal length is particularly important. Take a quick look at your calculator and estimate the effective focal length. Extremely large focal lengths are useless, since you are limited by both air turbulence and the resolving power of the telescope. And due to the necessary distance, you will quickly achieve high effective focal lengths anyway. Especially for the beginning, you should work with longer eyepiece focal lengths or not quite so extreme effective focal lengths. Do not forget that the focal ratio and exposure time also increase with the effective focal length. With modern, increasingly low-noise cameras, short exposure times can be achieved even with eyepiece projection. Many planetary cameras make optimum use of the resolving power of a telescope at about f/30; with smaller pixels the optimum is already reached at faster focal ratios, while the large pixels of a DSLR can also tolerate an even larger focal length extension.

Digiscopy / Afocal Photography - Compact Camera and Mobile Phone at the Eyepiece

What was born out of a stopgap - holding an affordable compact camera with lens behind the eyepiece instead of an expensive DSLR - actually does exactly what we do when looking through an eyepiece: Holding an optic (the camera lens or our pupil) behind the eyepiece and projecting the image onto the camera sensor or retina. This technique is known as afocal photography and has experienced a renaissance in recent years, especially among nature photographers and birdwatchers. Even if this adds even more lenses and thus possible image errors into the light path, very beautiful images are nevertheless also possible - and above all with very little effort. Basically, all you have to do is hold the camera lens close to the eyepiece (about the same distance as the eye relief of the eyepiece) without the lenses touching and scratching each other, align the optical axes (i.e., hold the camera centered and straight on the eyepiece), and pull the trigger. Once the eyepiece is in focus for the eye, the camera's autofocus will do the rest. Sounds simple? It is with a little practice. Freehand photography is easier if you hold the camera lens at the front. For a smartphone, you can also use the eyecup of the eyepiece as a support. To ensure that the image fills the format, you still need to zoom in a little or use a light telephoto lens, otherwise the camera lens will look past the eyepiece and the subject will be surrounded by a round, black border, as in ancient photographs. Therefore, wide-angle eyepieces with a large eye lens are also a good choice. However, the black border has one advantage: It shows you whether you are holding the camera crooked. If it is sharp all around, the camera is correctly aligned. More convenient than freehand is a digiscoping adapter. Lighter compact cameras can be positioned exactly behind the eyepiece with the Microstage II Digiscoping Adapter (#2450330 , € 55,- ) and folded to the side to quickly look through the eyepiece. For most smartphones, the Celestron NexYZ Smartphone holder offers the optimal adjustment possibilities; not only the position above the eyepiece can be adjusted via the set screws, but - unlike cheaper models - also the distance to the eyepiece to achieve the optimal illumination. With an adapter, self-triggers or remote triggers can also be used. Particularly convenient today is the option of some cameras to transmit the image via WiFi - so the camera can be conveniently controlled from the shade. The first camera apps even support live stacking, i.e. the addition of several shots in real time. This means that cell phones can now be used for recording techniques that are only known from large cameras with post-processing on a PC!
Not only for daytime observations, stray light protection is interesting. The SP54 thread of the Hyperion eyepieces can also be used on the Morpheus eyepieces with the Adapter M43i to SP54a – for Morpheus® eyepieces (#2954251 , € 31,- ) . From the SP54 thread, in turn, the eyepiece can be adapted to the filter thread of a camera lens by means of the Hyperion DT rings (cameras with M43 thread can even be adapted directly; the M43/M43 distance ring is additionally recommended). This allows for a light-tight, stable and perfectly centered camera adaptation. Just don't forget that the entire weight of the camera then hangs on the filter thread, which the lens and thread must be able to withstand. Not every camera or focus motor can tolerate this setup. Robust, lightweight cameras have an advantage here, and video cameras/camcorders are also often connected in this way - be it to record a planetary occultation by the moon or the feeding time in a bird's nest.

Conclusion

Although a telescope is the equivalent to a fixed focal length lens when used for photography, you are far from being locked into one focal length. With afocal photography or digiscoping, it is possible to photograph through the eyepiece with little effort. With a digi-clamp or a smartphone adapter, longer exposure times can also be realized. This makes this method ideal for snapshots that capture the view through the eyepiece - but also provide images of very good quality with a little practice. If you want to get the highest magnifications out of your telescope or if you have a system camera, you can achieve higher magnifications with fewer lenses using eyepiece projection. With the camera body stably screwed to the eyepiece, stray light and tilt are eliminated, and you can push the limits of your system when photographing the moon and planets. You can learn more about this topic in the book Digiscopy, published by the author on Amazon.