Let's Photograph the Night Sky

...with Minimum Gear and Budget

From a Novice to Novices

The most important gear is a 'German Equatorial Mount' which follows the apparent motion of the sky as a result of Earth's rotation. You can keep the budget for a mount low by using a light-weight telescope. The price for a mount increases with sturdiness, payload capacity, tracking accuracy, as well as extras, such as GPS or WiFi. If you keep the payload low and choose short exposure times, say, under a minute, then accuracy is not a serious issue while you will get a sturdy mount under USD 1,000 new or find a second-hand deal for less.

The difference between tracking and guiding being is, the mount motors track the sky motion to keep an object within the eyepiece field of view, while additional guiding is performed by means of a computer controlled CMOS camera which follows a bright guide star in its view and regularly feeds its position to the mount for compensation in case of drift in order to increase tracking accuracy for longer exposures. Guiding is not essential for the first steps or very short focal length lenses.

Select a mount with sufficient payload capacity. Your OTA plus gear should not weigh more than 70% of the specified load capacity in order to have plenty of buffer for assuring tracking accuracy. As a rule of thumb, refractors up to 80mm aperture weigh up to 2.5kg, 80mm refractors weigh up to 3.5kg. A 5-inch (Ø130mm) Newtonian weighs about 3.5kg, a 6-inch (Ø150mm) brings about 6kg onto the scale.

Optical polar alignment of an equatorial mount with a built-in polar scope requires an unobstructed line of sight towards a celestial pole, while modern electronic software algorithms are capable of performing this alignment without view at a pole. This requires a dedicated CMOS camera, such as a 'planetary camera' and PC capture software for controlling the alignment process.

Another type of mount is the altitude-azimuth mount which compensates first in one direction, then in the other. It is aligned to the horizon, not to the pole. This will generate blurred objects (field rotation). However, if you limit the exposure time to, say, 30 seconds then you might be happy with what you get. If you do not yet own a mount by all means pick a German Equatorial design for photography. An altitude-azimuth mount if fine for visual observation through eyepieces.

Telescope

The more aperture a telescope has the more detail you can capture. The difference to smaller scopes is often like that of a $100 bottle of wine and a $4.99 bottle of wine. Since a wide field and fast optics (provided by short focal lengths) are preferable for imaging deepsky objects, such as nebulae and star clusters, a refracting telescope or a reflecting Newtonian are the primary candidates. Fully multi-coated lenses or optically treated mirrors of good quality provided, small apertures starting from Ø60mm for lenses and Ø130mm for mirrors will be quite sufficient, also considering the capabilities of modern DSLRs and image processing software. Prices increase 'exponentially' with growing aperture and with optical quality as production becomes more demanding. Also, telescopes are not manufactured in lots of millions, not even in China, reflecting into sales prices. Telescopes often require the preparedness to compromise - in more ways than one. In December 2018, a Japanese astronomer discovered the faint comet C/2018 Y1 (Iwamoto) with only a Ø100mm lens.

Low cost, short focal length (fast) refractors typically sport a doublet lens (aka Fraunhofer achromat) which produce notable color and comatic aberration resulting in blue and red halos around bright objects and elongated stars in the edges of an image, respectively. Color fringes rob contrast and valuable detail that a large lens could theoretically provide. Refractors which correct or minimize such optical errors (ED doublet or triplet lens apochromats) easily cost 3-4 times more than an achromat. You can add a set of filters to a low cost achromat but possibly ending up spending a little fortune for filters that could buy an ED doublet APO. Large Ø120-150mm aperture F5 achromats are designed for visual tasks. No optical correctors exist for such scopes as not ideal for imaging. An option is a long focal length refractor, say about 1000mm which is way less prone to aberration but trades this merit against extended exposure time, narrower field of view and need for more accurate tracking. As for aperture, Ø50-80mm should work just fine, otherwise the cost increase per inch of aperture quickly brakes a bank. Watch out inacceptable optical quality with ultra-cheap refractors; they are, sorry to say, crap.

A few high quality products excepted, achromats are basically not recommended for astrophotography, but can help make you familiar with imaging without spending a fortune.

Apochromatic refractors (APOs) use (two to four, sometimes five or six) extra low dispersion (ED) lenses with high permeability to minimize color aberration. There are basically four different types of apochromatic refractors in sequence of increasing optical performance:

1. Doublet with two lenses
2. Triplet with three lenses
3. Quadruplet with four lenses (typically a triplet with an integrated lens for flattening the field of view)
4. Petzval designs with 5 or more lenses

While in most cases doublets and triplets require an optional 'flattener' when imaging with large image sensors, such as full frame and APS-C, a quadruplet or Petval eliminates need for this extra purchase. Depending of course on manufacturer and model it is often more economical than a doublet or a triplet with an optional flattener. Also, most flatteners are designed for a specific focal ratio and telescope model only and are useless for other refractors.

APOs are designed with imaging in mind. They feature rigid 2" to 3" focusers achieving an image circle of up to Ø44mm which fully illuminates DSLR sensors. APOs priced under 1000 USD sport up to Ø80mm aperture with 350mm to 480mm focal length (F5 to F7). Since quality lens glasses are employed, this category is excellent for starting with affordable astrophotography. Recently, small Petval APOs around Ø55mm aperture for wide field captures became popular for their high optical precision, wide image circle and cost performance, such as currently offered by Askar, Svbony, Vixen, William Optics, etc.

Ultra-portable APOs come with small apertures and can not provide high resolution, yet they do offer sharp images rich of contrast, especially when they employ quality lens glasses, such as 'FPL-53'. Their low weight and short focal lengths permit the use of portable tracking mounts (with autoguide control for imaging).

The waning moon captured in detail with a low-cost 1/3-inch CMOS camera through a Ø71mm APO.

The most common reflector achitecture is a Newtonian which is way lower in cost compared with refractors of same aperture as it employs parabolic or spherical mirrors which are cheaper to produce since there is only one surface that needs to be ground and polished. It has a secondary mirror held by a 'spider' which obstructs the aperture by some 30% (10% by area) or so in that a Ø130mm to 150mm mirror should work well for imaging.

As a major advantage for photography, a Newtonian is free of color aberration because light is reflected, not refracted. Due to mirror curvature reflectors are susceptible to coma at the edges of a view. For optimal results a fast Newtonian, say an F5, may require a 'coma corrector' in order to produce nicely rounded stars in image corners, especially on 'full frame' wide camera sensors. On 'cropped' sensors the worst of the coma will be off of the edges in that a corrector may not be worth while spending some USD 200, yet basically a reasonable investment in view of the lower prices of Newtonians. Newtonians produce diffraction spikes around bright stars which do not look astronomical but often very beautiful.

The back focus and focus travel of a reflector is typically shorter than that of a refractor in that it is wise to make sure that your camera will reach focus preferably without additional lenses or extension tubes. Unlike refractors, the optical axis of a Newtonian tends to shift by usage time resulting in distorted images. Accurate adjustment requires a tool known as 'laser collimator' or 'collimation eyepiece' which is an extra purchase. After all, Newtonians offer the best value per inch of aperture and are most satisfactory instruments for astrophotography. Absence of color aberration makes a Newtonian comparable with a quality apochromat refractor, but at a third of the cost. In plain English, "most bang for the buck".

The advantage of larger aperture: Moon craters taken with a low-cost 1/3-inch CMOS camera through a 150mm reflector and 3x barlow lens.

CMOS cameras have rapidly evolved within merely a few years. Click to enlarge an image taken with a PC webcam back in May 2019.

The larger the aperture of the telescope the more details can be resolved. This was possible in November 2023 with a dedicted planetary/lunar camera and an 8-inch (Ø200mm) Schmidt-Cassegrain telescope:

Catadioptric (or compound, combination of mirror and lens) Schmidt-Cassegrain (SCT) or Maksutov-Cassegrain (MCT) telescopes typically come with a mirror of long focal length, mostly 10 times the aperture in millimeters, which equals an F10 ratio, yet they are physically the shortest tubes. The resulting narrow field and longer exposure times disqualify this type for wide field deepsky imaging but are optimal for lunar and planetary work with astrocams. With precision tracking and guiding on sturdy mounts, SCTs with large aperture starting at 200mm (8 inches) are great for closeup imaging of deepsky objects, galaxies in particular. For instance a large 12 inch (305mm) F10 SCT has a focal length of 3050mm which results in a 0.44° field of view on a APS-C camera sensor which is a little less than the width of a full moon disk. Since focus is adjusted by moving the primary mirror the focal length slightly changes with mirror position.

Like a Newtonian, a SCT eliminates chromatic aberration but has coma and field curvature, too. The image circle (over 60% equal light intensity) of an SCT is hardly Ø30mm wide, resulting in notable vignetting, calling for a focal reducer/flattener when using a large CMOS camera or a DSLR.

Most common are DSLRs which offer selectable exposure times up to 30 seconds and bulb exposure mode. Since astro-imaging merely requires manual camera mode and infinite focus, without need for CPU control of lenses, a used or partly defect DSLR would do the job. Most common entry-level DSLRs sport an APS-C sensor chip which measures about 24 x 16mm with up to 6000 x 4000 pixels, which is more than sufficient for the hobby. Special features, such as noise reduction and in-camera image processing are dispensable as this can be done much better with image post-processing software. A vari-angle (tilt and swivel) LCD monitor would be great for comfortable working. A touch screen is a nice touch, though not essential for our goal, but the reduced mechanical wear on buttons and rotary switches, etc. can extend the camera's lifetime. Anyway, if you do not already own a DSLR, try and find a used camera body.

Camera Sensors Say, we have a telescope with a Ø44mm image circle, wide enough to fully illuminate a full frame DSLR sensor (36mm wide) without edge vignetting and a focal length of 750mm. This scope will also fully illuminate a smaller sensor, such as the more common APS-C (23.5mm wide). The 'crop factor' 36mm/23.5mm = 1.5 has the effect of apparently extending the focal length to 750mm x 1.5 = 1125mm because the smaller sensor is like a smaller field of view. Equally, one can say that the smaller sensor magnifies 1.5x. The field of view (fov) as a function of sensor dimension (sw) and focal length (fl) is: fov = 2 * atan(sw / (2 * fl)) Example APS-C: 2 * atan(23.5mm / (2 * 750mm)) = 1.8° Example Full Frame: 2 * atan(35.9mm / (2 * 750mm)) = 2.74°

Starscape & Constellation Photography

You can take beautiful photos of the constellations and the Milky Way with a standard camera lens and a tripod, without telescope and tracking. Note, that a lens does not need to support autofocus and other control functions. As long as the iris can be changed and focus set to infinity, a used or partly defect lens will do. If you are living in a brightly illuminated area, consider a light pollution filter for your lens. For wide field starscape and constellation photos a lens focal length between 24 and 50mm will be fine. The aperture should be at least 1:4 or wider. A 100-200mm telelens can capture large deepsky objects, such as the Andromeda galaxy and the Rosette, North America and Orion nebulae. However...

...for telelenses and exposure times exceeding a few seconds, a mobile tracker typically seated on tripods helps avoid star trailing with telelenses. Another fine choice is Sky-Watcher's Star Adventurer series as are trackers from iOptron, Fornax, etc.

Heart & Soul nebulae and the Double Cluster. Nikon D5500 with 50mm F2.8 lens. 195 exposures of 60 sec each at ISO800.

If you own a DSLR bundled with a lens, a tripod and a tracker, then you are ready to go for wide field imaging. Prime lenses are preferable over zoom lenses as they typically provide crisper star images.

• Telelens

Example: With its 135mm focal length, the Samyang/Rokinon all-manual telephoto prime provides a desirable 10° horizontal field of view with a crop sensor DSLR (APS-C) and 15° with a full frame sensor for wide field deep space photography. At maximum aperture of f2.0 short exposure times are possible yet pin-point sharp in the corners without coma or aberration.

One of its lenses (11 elements, 7 groups) is an extra-low dispersion (ED) element to reduce the impact of chromatic aberrations (color fringes) while full Ultra Multi-Coating (UMC) considerably helps reduce flare and ghosting. The lens comes with bajonet lock for major camera brands. With a long 200° of rotation the focusing ring has a good touch and feel, smooth and without play while holding its focus well. The lens aperture is Ø67mm, the front filter thread size is the popular Ø77mm, say, for a light pollution filter. In other words, a fine astrophoto lens which easily rivals, often outperforms, a fully corrected short-focal-length apochromatic refractor.

• Standard Lens

Example: The Nikkor 50mm, f1.4 is a "fast", low-cost standard lens designed for manual mode and is often bundled. The field of view on an APS-C sensor is about 27° x 15° which is well suited for many constellations, though a wider 35mm, 28mm or even a 14mm lens would be favorable for large constellations. The lens threads Ø52mm filters. With its iris wide open at f1.4 the stars in the image corners get strongly elongated, in that it is recommended to stop the lens at f2.8 or f4.

The "faster" the lens (the lower the f-value), the more light a lens can collect during a given exposure time. With either lens, a filter to reduce the impact of light pollution (LPR) threaded to the lens is warmly recommended in light flooded places even though the filter drops (darkens) by around +1/2 stop (Example).

You will need an adaptor which connects your camera to a telescope focuser sleeve, a so-called T-Ring (Ø42mm or Ø48mm) which threads to the telescope on one side and on the other with a bayonet lock for your camera model, Canon, Nikon, Pentax, Sony, Fujifilm, you name them. Depending on the type and make of the telescope you may need an extension tube or a converter to reach focus. Alternatively, the camera can be inserted into the focuser in place of an eyepiece by means of a nose piece that fits the T-ring thread diameter. A nose piece is typically threaded for 1.25" filters.