Astrophotography Equipment: Mounts

Introduction

Our equipment introduction for Astrophotography has now discussed telescopes and cameras. The mount holding this equipment is the final, and most important, of the equipment basics. The field is a little simpler this time. While there are a wide variety of mounts available for visual use, only a few types are suitable for astrophotography.

This article is a discussion of telescope mounts specifically for astrophotography purposes. For a general description of mounts for normal visual observation, see this article.

It’s all about the mount

The mount is where most beginners run into trouble with astrophotography. If you have an entry-level telescope that came bundled with a mount, the telescope is probably adequate to begin astrophotography. If you own a DSLR or “mirrorless” camera, that’s good enough too, with the right connectors and adaptors.

However, the mount that came with your entry-level telescope is almost certainly not good enough. It will be adequate to photograph the moon and, with some accessories, the bright planets. It will not do the job if you are planning to do long-exposure photography of deep-sky objects such as nebulae and galaxies.

Get started by doing lunar and planetary photography. If you want to pursue deep-sky, plan to buy a mount separately and, as a rule of thumb, plan to spend more on your mount than you spent on camera and telescope combined.

Objectives

With apologies for repeating this important message:

For very basic “snapshot” photography of the moon, and for basic webcam imaging of bright planets, any good mount suited to visual use will do.

However, if you want to explore long-exposure imaging to capture dim deep space objects, your mount is your most important piece of equipment – more important than the telescope or the camera. If you bought a low- or moderately- priced telescope/mount combination, it is probably serving you very well to begin visual observing, but the chances are it is not up to the job of long-exposure astrophotography. Again, even a simple mount can be pressed into service and will produce results that will impress you; but if your objective is high-quality astrophotographs, good rule of thumb is that you should invest more in your mount than you did in your telescope and camera. Yes, combined.

Why is the mount so critical? Because it has a very difficult job to do that requires very high-precision machining. Even for visual use, the stability of your mount is important, but the magnification of your telescope combined with the long exposure of your camera will amplify the effect of any imperfections, ruining astrophotographs in situations where you may not have noticed anything wrong using your eye and an eyepiece.

For astrophotography, there are several requirements for your mount:

Tracking Objects

The most important feature of a mount is motorized tracking, to keep a target object in the field of view for a long period of time.

This is even important for visual use (with an eyepiece); however, for visual use it is only a convenience – your eye and mind will automatically compensate for minor movement of your observing target. As long as it is kept in approximately the same part of the field of view, everything will be fine.

For astrophotography this approximation is not good enough: the mount must track so accurately that the target is kept in exactly the same spot in the field with no motion whatsoever. The target cannot even wander away from the centre and then wander back – even though such wandering would yield a target that is perfectly centred “on average”, that would spoil an astrophoto. The target must be tracked accurately enough that it does not move at all.

(As we’ll see later, even high-end astrophotography mounts find this a challenge, and are usually assisted by a feedback technique called auto-guiding. Auto-guiding can help a less capable mount work well enough for astrophotography too, so don’t give up on a moderate-quality mount until you read that section.)

We will also see, below, that astrophotography requires the tracking be done on a circular path that matches how the sky appears to move, to avoid “field rotation”.

Stability

Your mount must not only track accurately, it must also be stable against external disturbances. It will be outdoors in a world where things are happening and must resist gusts of wind and the rumble of passing traffic without jiggling the target being photographed. This generally means the mount must be of heavy-duty construction, and all the parts must be machined to close tolerances to eliminate sources of vibration and wobble.

Stability, by the way, is also affected by your choice of location. You need to be on solid ground – don’t try to do astrophotography with a tripod sitting on a roof or deck. Every time a Dachshund wanders by your image will be ruined.

Finding Objects

One of the reasons for doing astrophotography is to capture faint objects that you can’t see with your unaided eye. Your camera can capture objects many times too faint to see. But if you can’t see the target, how do you know you are pointed at it?

If you like to locate challenging targets by star hopping, your mount can help you by having good slow motion controls with little backlash.

For even more help, many astrophotographers like to skip this step by using a go-to mount that can locate objects automatically. Opinions vary on this feature – some astronomers regard it is somehow “cheating”. I don’t. You get to define the parameters of your hobby, not someone else; if your primary interest is in astrophotography and not in finding targets, you may consider this feature an important time saver. We won’t be discussing this feature any further in this article, however. (A separate article discusses Go-To.)

Review of Mount Types

With the above objectives in mind, let’s review the various types of mounts that a beginner to Astronomy might encounter, and comment on their suitability for astrophotography. We’ll go from least to most suitable for astrophotography.

First, let’s have one more reminder: visual observing and astrophotography have different requirements for mounts. It is possible – even likely – that a mount ill-suited for long-exposure astrophotography would function extremely well for visual use, and might be an excellent choice for a beginner. This is one of the reasons why I recommend that beginners avoid long-exposure astrophotography at first – you have enough to do learning the basics, and should start with equipment that makes your initial experiences easier; you can always upgrade your mount later.

The following section is going to read like I am trying to talk you out of one type of mount and into another type of mount, and I guess I am. However, let me emphasize that what I am really doing is explaining the reasoning for my own personal preferences. I encourage you to read more and, especially, visit some skilled users of both mount types, and make your own informed decision.

Non-motorized Mounts

This section applies to any style of telescope mount that does not have an electric tracking motor, including non-motorized equatorials, alt-az, and Dobsonians.

Non-motorized mounts have limited use for astrophotography since they cannot hold the target motionless in the field for any amount of time. Because they can’t hold the target motionless, they cannot be used for long-exposure astrophotography and, therefore, cannot be used to image dim deep sky objects.

You can keep a target approximately centred with a non-motorized mount by skilfully using the manual slow-motion controls or nudging the telescope to track the object. This will allow you to attempt two kinds of astrophotography:

First, you can probably photograph the moon because it is so bright that you will be able to use a fast shutter speed to freeze the motion. The camera’s auto-exposure circuits will probably work, and with a shutter speed of 1/60 of a second or faster, the motion should be imperceptible. Only the moon is bright enough to photograph this way.

Second, with practice at smoothly tracking, you can keep a bright target such as the moon or the bright planets in the field of view long enough to use a webcam to capture short video sequences, which can then be stacked together to form single images. (The target will bounce around in the field, but the software you use to combine the multiple frames of the video segment can locate and align the target in each frame.) This is not easy, but once you can track smoothly enough to keep a target approximately centred in the field for several minutes, you can achieve reasonably good planetary images this way.

Motorized Fork Mounts

Motorized fork mounts holding catadioptric telescopes are very popular – probably the most-purchased telescopes by moderate-level beginners. For visual use they have many advantages, which are discussed in another article.

These mounts are always motorized, and are usually go-to mounts. Because they are motorized, they do a good job of tracking targets. Even the most basic can keep the target in the field of view well enough to do bright moon snapshots and webcam planetary imaging, as was discussed above for non-motorized mounts.

The higher-end mounts of this type are very stable, and can hold a target quite motionless. Does this mean they can be used for long-exposure astrophotography of deep sky objects?

No.

Well, Yes. But not without help, because there is another problem to deal with: field rotation. This concept will be a little hard to grasp at first, but consider the following points:

  • The sky appears to rotate around the Celestial North Pole, so stars and other objects must appear to follow circular paths across the sky.
  • A fork-mounted telescope can point (almost) anywhere in the sky, but the tube itself never changes its rotational position relative to the ground. The “top” of the optical tube is always the top, and the bottom, left side, and right side are always in those roles.

This point is really important: as time passes, the sky is rotating but the telescope tube is not. So, if you track an object for several minutes, your target will rotate slightly as it follows its curved path across the sky, but the telescope tube does not rotate. The result is that all the stars in your target will appear as small arcs in the image. The longer the exposure, the more pronounced the arcs will appear.

Here is a separate article that discusses this phenomenon in a little more detail.

To do long-exposure astrophotography with a fork mount, you need a way to cancel the field rotation. There are three methods to achieve this. In order of complexity they are:

Stacked Short Exposures

One simple solution to seeing arcs in long exposures is not to take long exposures. If you work out the longest exposure you can take before you see stars arcing (perhaps 30 seconds), then instead of taking a multi-minute exposure, you can take a large number of short exposures (perhaps 20 seconds each), then combine these in software.

This process, called stacking, can produce results that approach the quality of longer-exposure astrophotography. And the stacking software usually has some capability to notice the field rotation between the multiple frames and de-rotate the images before adding them together.

For the ultimate in rich deep-sky images, however, you need to be able to take longer exposures – at least several minutes in duration, and you will see field rotation in such exposures unless you cancel it mechanically. (You cannot use software to “de-arc” an image that has suffered from field rotation, because information is lost.)

Equatorial Wedge

An Equatorial Wedge for a Fork Mount; image from Celestron.The simplest and most popular mechanical way to tackle field rotation with a fork mount is the use of an Equatorial Wedge. This is, as the name suggests, an adjustable wedge of metal that is inserted between your mount and the tripod, so the base of the alt-az mount is tilted away from the horizontal. You

  • remove the fork mount from its tripod,
  • bolt the wedge to the tripod where the fork mount used to be,
  • bolt the fork mount to the wedge,
  • tilt the wedge to an appropriate angle, and
  • tell the mount’s control software that you are operating in equatorial mode.

This effectively converts the fork mount to an equatorial mount. The direction of movement that used to be Azimuth (left-right) is now Right Ascension, and the direction that used to be Altitude (up-down) is now Declination. You then must polar align this new-born equatorial mount, and use it like an equatorial.

As we’ll discuss below, properly-aligned equatorial mounts don’t suffer from field rotation, so a wedge-mounted fork, properly aligned, doesn’t either. See the “Motorized Equatorial” section below for other attributes, most of which apply to a fork mount on an equatorial wedge.

Some personal opinion: I see skilled astrophotographers getting excellent results with wedge-mounted fork mounts, so clearly they can work very well. However, I, personally, don’t care for them. My reasons (and these are personal – make your own assessment) are:

  • Although a wedge converts a fork mount to an equatorial mount, it doesn’t convert it to a very good equatorial mount.
    • It is harder to polar align because the polar axis is vague (it’s a line perpendicular to the tilted base of the fork)
    • The centre of gravity is thrown off centre but, unlike an equatorial mount, there are no counterweights to help maintain balance on the RA axis, and no way to shift the weight to balance the Dec axis;
    • It lacks a traditional equatorial mount’s ability to manage backlash in the RA drive by moving the counterweight to slightly imbalance the mount toward the East (a technique we’ll discuss later).
  • While bringing you the disadvantages of an equatorial mount (non-intuitive directions of motion and need to polar align) it also retains the disadvantages of the fork (e.g. poor clearance for the camera when the fork is pointed high in the sky). You get the worst of both worlds.
  • It seems, to me, to be a bit of a contradiction. You buy a fork mount because they are simpler and more intuitive than an equatorial mount, then you immediately convert it to an equatorial. Why not buy an equatorial to start with, and get a better one? I have witnessed many beginners struggling with the additional complexity added by a wedge – extra fittings to be loose, misaligned, unbalanced, etc.

Anyway, as I said, there are many astrophotographers far better than me, and producing results I can only dream of, who use wedge-mounted fork mounts. I encourage you to visit someone who works this way and, by all means, use that approach if it feels right for you.

Field De-Rotator

A third solution to the problem of field rotation with a fork mount is an accessory called a field de-rotator. This is a motorized collar that mounts between the telescope and camera, and slowly rotates the camera at a rate that exactly cancels the rotation of the field. The result is that the camera rotates while tracking, exactly like an equatorial mount.

A field de-rotator retains the advantages of a fork mount while allowing long exposure imaging. Some disadvantages, though, include:

  • They add length to the optical chain, moving the camera back another inch or two. This may make more of the sky inaccessible if the camera won’t fit between the optical tube and mount base when pointing high in the sky, and it may move the camera out too far for the combination to reach focus (although SCTs usually have plenty of focus room to spare).
  • They require power and control wires, meaning a bit more complexity in your setup.
  • They are fairly expensive. The Mead model shown here is the lowest-priced one I am aware of, and is still in the $450 range – others tend to be priced as exotic, high-end gear.
  • It’s another motor, turning at a slow and precise rate, so another source of tracking error.

It’s probably worth mentioning that most modern professional, multi-million-dollar observatories on mountaintops now work this way: alt-az mounts with mechanical field de-rotators.

Under-Scope Clearance

You should be aware of one other potential problem when using a fork-mounted SCT for astrophotography. When the telescope is pointed straight up, the eyepiece is directly underneath it, between the optical tube and the flat base of the fork mount.

Chances are your camera will not fit in that space. This means that some part of the sky will be inaccessible to you, depending on how bulky your camera is. (This is not a permanent problem, of course. You can always wait until the desired area of the sky rotates lower toward the horizon.)

Motorized German Equatorial Mounts

A small German Equatorial Mount - too lightweight for astrophotography

A small German Equatorial Mount – too lightweight for astrophotography

The motorized German Equatorial Mount (GEM) is the most commonly recommended mount for long-exposure astrophotography. In this section we’ll discuss why this is so, listing its advantages for this purpose, and acknowledging some disadvantages.

A technical note: any mount organized so it rotates around a polar axis is an equatorial mount. What we’re talking about here is the “traditional” equatorial mount that you see for sale in Astronomy shops – the mount where the telescope is held at an angle and a shaft, with counterweights, extends off the mount opposite the telescope. This design is properly called a German Equatorial Mount.

Some people argue that an equatorial mount is not an ideal mount for a beginner to visual astronomy, because it requires more set-up time and has a steeper learning curve than a fork mount. I don’t agree that the mount is unsuitable for a beginner, but it is certainly true that it requires a little more learning. However, for long-exposure astrophotography, the equatorial mount has several distinct advantages:

  • The motion of the mount mimics the motion of the sky. The mount follows a curved path when tracking objects, so there is no field rotation during long exposures.
  • The camera is offset from the mount so there is less of a problem with lack of room. Equatorial mounts have no problem pointing straight up. (There is a problem where a telescope on a mount may have trouble pointing to a particular part of the sky because the telescope or camera impacts the tripod. However, with an equatorial mount, that part of the sky will always be accessible by doing a meridian flip, moving the telescope to the other side of the mount.)
  • The adjustable counterweight system gives fine control over the balance, accommodating a range of telescope and camera weights.
  • The polar axis is well defined, and there is usually a system to assist with precise polar alignment.

Sturdy equatorial mount, suitable for astrophotography.

There are, of course, also some disadvantages.

  • The biggest problem is that there is a large range of quality of equatorial mounts. Good ones for astrophotography – sturdy and stable – can be expensive. Plan to spend from one to many thousands of dollars just on the mount.
  • Small and light equatorial mounts (like the one in the first photo in this section, just above) have the same mechanics, and offer the convenience of long-term tracking for visual use, but they are not suitable for long-exposure astrophotography.
    • Although they track, they don’t track smoothly enough due to low-precision gears and loose tolerances.
    • They aren’t sturdy enough to handle the weight of a telescope and camera with no vibration or shifting.
  • Equatorial mounts require polar alignment, which must be very precisely done. For long-exposure astrophotography you must master drift alignment, which is straight forward but time consuming.
  • Equatorial mounts can experience a problem if an object crosses the Meridian (the line from North to South directly overhead) during an exposure – the balance shifts from one side of the mount to the other, and backlash in the gears can allow an image shift at this point, spoiling an image.

The disadvantages listed here are not considered “show stoppers” but, rather, additional challenges to be worked out. The advantages are so important that most experienced astrophotographers are using high-end GEMs for their work.

Error Correction Features

All motorized mounts have moving parts, gears and pieces that fit together with close tolerances. As a result, all motorized mounts have some amount of error in their tracking. Mounts designed for astrophotography are built to minimize this error within a price range, but also have features designed to correct any remaining error.

These features are covered in separate articles, but we mention them briefly here as an overview of critical mount features for astrophotography. The two most common and affordable methods for correcting mount tracking errors are:

Imperfections in the gears that drive a mount can cause minor fluctuations in tracking accuracy that repeat on regular intervals (as the same imperfections in the gear rotate around again and again). Because such Periodic Error repeats consistently and reliably, many mounts have the ability to record the error and then alter the Right Ascension drive speed slightly in a manner that exactly cancels it. This Periodic Error Correction (PEC) feature requires some effort to initially “train” the mount, but then improves tracking substantially.

Mounts with PEC will have a button on the control panel or an section in the control pad menu for programming and enabling the correction.

Even with PEC, non-periodic errors in the mechanics of a mount will still result in targets “wandering” very slightly in the field over long periods. Mounts designed for astrophotography will usually have an input connector labelled “Guiding”. This connector is used to send direction change signals to the mount that result in very small motions that serve as a “fine tuning” control, allowing the mount to be re-pointed with a precision far exceeding that provided by the normal motion controls.

The original use of such a system was to manually correct mount errors by watching a star, in a separate “guide telescope” on the same mount, and using a hand control to keep it exactly centred on the cross hairs in a high-powered eyepiece containing an etched reticle. Modern amateurs have access to a much improved system: connecting a second camera to the guide telescope and having software watch for the drift of a reference star and send correction signals to the mount’s guide port. This technique is called autoguiding.

Mounts with guiding capability will have a connector labelled “Guiding” or “Autoguiding” on the control panel.

Mounts intended for astrophotography will have one or both of these features – usually both. Absence of these features is a pretty good sign that a mount was intended for visual use, not astrophotography.

A Rule of Thumb

Most manufacturers supply mounts in a range of sturdiness. For example, the EQ series of mounts are rated EQ1, EQ2, through EQ8, with each being larger, heavier, and sturdier the one before.

For astrophotography, your mount should be at least one grade heavier than what would be considered a good sturdy mount for visual use. For example, a telescope that was comfortable for visual use on an EQ4 mount should be on an EQ5, or heavier, for imaging. When you are using your telescope and mount visually, if there is even the slightest problem with mount vibration and jiggling, don’t try to use that combination for long-exposure astrophotography.

Next

The next topic in our introduction to astrophotography equipment will be a discussion of autoguiding, an optional technique but one that is almost essential if you hope to do long-exposure imaging of deep sky targets.

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