Your telescope as a super-tele lens: methods, tools, pros & cons of eyepiece projection and prime focus astrophotography

Many a telescope owner, after gazing at the sky for a while, wants more; want to share what they are seeing.
There are several ways to achieve this. The most obvious is shooting through the telescope eyepiece with a small camera or cell phone. Since this is very difficult to do hand-held, you can find many economical cellphone adapters on the web (± USD 20).
The other alternative is using the telescope as if it were a long fixed-focal, fixed-aperture lens, mounting a DSLR or mirrorless (CSC) camera sans lens on the telescope focuser.
In this article we explore the different alternatives, tools, pros & cons of both methods.

Eyepiece projection

The advantage of shooting through the eyepiece, so-called eyepiece projection, is that its magnification allows you to get a high enlargement on sky objects.
The disadvantage is that a cell-phone or a point-and-shoot are not great cameras and, moreover, that image quality heavily depends on the optical quality of one’s eyepieces.
If you still use the eyepieces that came bundled with your telescope, you may want to upgrade at least one before venturing into eyepiece projection. Otherwise you are most likely going to get disappointed.
That said, in my research for this article I came across a DSLR adapter for eyepiece projection (USD 150), which could possibly overcome the low IQ issues of cell-phones and point-and-shoot cameras.
However, I have no idea how well it works. Check it out for yourself clicking here. 

Prime focus

The second method is called prime focus, which is a technique for short exposure photography of the moon and other nearby celestial objects, such as star clusters and nebulas.
Rather than photographing the projection from an eyepiece, the camera captures the incoming light at where the eyepiece would normally be.
The telescope’s focuser is used to achieve focus – hence the term –, in the case of a refractor telescope on the eyepiece lens, and in the case of a Dobsonian (reflector), on the secondary mirror.
Which can be challenging at high magnifications.

Prime focus demands proper alignment and collimation of the optical train of a telescope – because of the wider field of view (FoV) in comparison with most eyepieces – but has the advantage of eliminating the ocular altogether and, with it, its eventual optical defects: lack of sharpness, optical aberrations, CA, etc. 
The disadvantage is, of course, that we have to make do without the magnification of the eyepiece, which, for a standard 25mm ocular, is already in the order of 40x.
The other problem can be that the camera requires considerably more in-travel than the telescope focuser can offer, in which case the use of a Barlow lens is likely the only solution – except on telescopes where the top cage (focuser + secondary mirror) can be moved towards the primary mirror, or v.v.

Prime focus images can be captured with two types of equipment; the first being a so-called eyepiece camera.
As the name aptly suggests, these cameras feature a 1.25” telescope/microscope eyepiece tube, can be plugged straight into the focuser and generally connect to a computer through a USB cable, for convenient viewing and recording.
They come in many shapes and sizes, starting at 1,3 Mp. (± USD 60), through 3, 5 and 10 Mp., up to a 14 Mp. model for about USD 260.

Although the low-resolution models hardly seem to be worth the trouble for serious astro, from 10 Mp. and up they might be interesting, especially for those who own entry-level telescopes incapable of carrying the weight of a DSLR or mirrorless camera.
Even so, it is still good to keep in mind that eyepiece cameras generally feature the same pinky nail sized, quarter inch sensors as smartphones, and that – despite of what the manufacturers would like you to think – one cannot expect miracles from that kind of hardware.
To the point: this sample image is a little hard to believe – to say the least.

Prime focus astro with a DSLR or Mirrorless camera

Unlike eyepiece cameras or cell-phones, with their minute sensors, low resolution and limited capabilities, a modern, digital interchangeable lens camera (DSLR or CSC) is the tool of choice for those on a budget who want to take advantage of their telescope to capture the universe.
On prime focus, they are particularly well suited for short-exposure lunar and general wide-field astrophotography on telescopes that lack a guiding/tracking system, but adecuate for capturing planetary and deep-sky objects on computerized models, only.
Nonetheless, this is a rule that applies to the photography of deep sky- and other faint objects on any platform: with fainter or farther-away objects come longer exposures and/or longer lenses, and in these scenarios compensating for the Earth’s rotation becomes a must.

A typical APS-C camera has a sensor size roughly 15 times that of a typical eyepiece camera, smart-phone or point-and-shoot, and most recent models feature 20 Mp. or better resolution.
This translates in far larger (RAW) images with a much better signal-to-noise (S/N) ratio, better high ISO performance, better depth of field and shorter exposure times, just to mention a few advantages.
Especially on non-guided platforms, the latter is of essence. With a 300mm f/4 lens, exposure times must already be kept under 2,5 seconds to avoid stars “trailing”; i.e., to keep them round. Imagine shooting at 1.200mm, as is the case with a 6” f/8 telescope, for example.

For lunar photography achieving short exposures is not an issue – I typically manage to keep exposure in the order of ISO 1600 1/320 s. f/8 (+/- 2 EV, depending on the moon phase).
But fainter and far-away sky objects are much more challenging, and while the likelihood of success is improved by the ability of the camera to achieve relatively short exposures at low noise levels, it essentially depends on the optical quality, stability and light gathering power of the telescope.
My efforts to shoot Jupiter on prime focus have failed, sofar, but wide-field astro in general is doable.

A word of caution

Before venturing further into the practicalities of prime focus, a word of caution is in place for those who have relatively economical computerized (go-to) telescopes.
If you own what one expert called a department store telescope, i.e., a Newtonian refractor or a Schmidt-Cassegrain, Maksutov (catadioptric), which – including tripod mount and guiding system – costs less than ± USD 750, it is highly unlikely you will achieve satisfying results on prime focus.
Even if the lay-out for a T-adapter + T/T2-ring is minor, why spend money and, more importantly, time, on something that will probably make you unhappy?

Supposing that your platform does resist some additional weight, your best bet would be a light, mirrorless camera, because a typical DSLR will add at least 600 grams, while mirror-slap is guaranteed to send strong vibrations throughout the set-up, which may take tens of seconds – if not minutes –  to die out.
Your camera having mirror lock-up (MUP) might help, but you will still need to wait a considerable amount of time for the platform to completely stop shaking.

Dobsonian telescopes are a different story. While the above still applies to budget computerized reflectors, traditional tube or truss Dobs on Altazimuth mounts are much more likely to make you happy, for three reasons.
One, they are fairly rigid and heavy. Even budget models rarely weigh under 15 kilos (34 lbs) while both tube and truss models from 8 inch and up weigh generally well over 20 kg (44 lbs). Meanwhile, a real bad-ass, USD 3.500, 16 inch truss Dob from a well-known manufacturer weighs as much as 88 kg (195 lbs).

Two, Dobsonian Alt-Az mounts or rocker boxes are more rigid than tripods, have more static mass and can be easily weighted down, if necessary.
Three, a Dob has a much lower center of gravity than other designs, because  – apart of the rocker box –  its heaviest parts, the primary mirror and mirror cell, are located at the bottom of the optical tube, which, in turn, sits at the low end of the Alt-Az mount.

This does not mean to say that traditional Dobsonians are immune to vibrations – they are not – but if you were to venture into prime focus astro, a traditional Dob is by far the best bet, IMHO.
Truss Dobs are probably the most commonly home-built telescopes, today, and their owners can take various measures to make them more vibration resistant; weighing down and stringing appear to be the most effective.

Still, I would strongly suggest testing to confirm that your platform (any platform) is stable enough and can carry the additional weight without problems, before spending money.
One way to do this would be to stick a newspaper page or a focus target on a wall, position your telescope at 5 to 10 meters away, piggyback the camera, focus on the target and use the self-timer or a remote controller to fire it.
If you get images with camera motion blur, forgedabboudit. Your platform is not stable enough for prime focus and you might want to put your money elsewhere. Your best alternatives would be either an eyepiece camera or eyepiece projection.
 

A super-duper telelens for under USD 50

Supposing your set-up is stable enough, mounting a DSLR or mirrorless camera on your telescope focuser requires two pieces of equipment that, combined, will set you back USD 50 at the most.
The first is a universal T-adapter that fits into the telescope focuser, and the second a T-ring – also called T/T2-ring – that mounts specific interchangeable lens cameras on the T-mount adapter.

T-adapters are not much more than a 1.25” or 2” focuser insertion tube with a universal male (external) T-mount thread on the opposite end.
T/T2 is a screw mount originally developed by lens manufacturer Tokina in 1957 as a universal lens adapter; it measures 42mm and has a 0,75mm screw pitch. It must not be confused with the better-known M42 or Praktica-Exacta P-mount, which is also 42mm but has a 1mm screw pitch.
Note that at least one manufacturer offers a T-adapter with M42-mount, which would make it incompatible with T-rings from other sellers. I can confirm this, because tried to mount my T-adapter on an old Praktica Nova and it does NOT fit. See this Wiki for more information.

Independent of thread, T-adapters can be used with almost any focuser from any brand and with any camera, although some telescopes – like Schmidt-Cassegrains – may demand something more specific.
1.25” Models, with only few exceptions, require an additional T-ring to connect to a specific DSLR camera bayonet, be that F-mount (Nikon), A-mount (Sony/Minolta), EF-mount (Canon) or those from other manufacturers.

T-rings feature a male lens bayonet on the outside and a female T-mount thread on the inside, that mates with the T-adapter.
Just like lenses, T-rings can only be used to mount specific interchangeable lens cameras from specific brands. Keep in mind that mirrorless cameras have smaller mounts (like the Nikon1-mount, for example), and that they require specific T-rings.
T-adapters for 2” focusers are often, though not always, camera brand specific and do not require a T-ring. In case they do, their T-mount allows for any T-ring available on the market. Prices for integrated 2” T-adapters start at around USD 75.

Review: Celestron T-Adapter/2x Barlow + Nikon-F T-Ring

I am writing this brief review of the Celestron “tee-set” not because I am endorsing it, but because that is what I have.
T-adapters are decidedly low-tech, and on Amazon there are models from at least 5 different brands, which are mostly produced in south-east Asia, have very similar price-tags, and are most likely equally good (or bad) as the ones discussed here.

Celestron offers two T-adapters: one with a 2x Barlow lens, and one without. In both cases, an additional T-ring is required, which Celestron only provides in Nikon- and Canon DSLR mount. 
Fuji, Olympus, Panasonic, Pentax and Sony T-rings can be found under Gosky, Fotodiox and Ultrapro branding, among others.
I chose the T-adapter with Barlow because it was a cheap way to get a T-adapter and, at the same time, double the focal length of my eyepieces – even though Barlow lenses are not exactly renown for their optical virtues.

Independent of the fact that some reviewers comment that they cannot achieve proper focus without the Barlow, which is likely due to the required additional in-travel, there is another essential reason to go for the Barlow version.
Although I did not consider this when clicking the check-out button, on reflection the sole prospect of connecting an open-ended suction tube to my digicam and then expose it to dusty or humid environments gives me the willy-willies.
Do yourself a favor: if you go for the model without Barlow, at least get a filter – like a moon or variable polarizing, for example – to shut the door on the dreaded, dirty dust devils.

The Celestron T-ring consists of two parts – an outer ring with an internal T-thread and a separate bayonet mount, which is held in place by 3 tiny Allen screws.
Out of the box, these inbus screws needed considerable tightening, because the bayonet part was loose and rotated within the outer ring.
Mounting the T-ring can be a little rough, too, and on a few occasions I have had minor trouble to get it to unmount from the D7100, although not from the D40.
I left the pictures of both mostly unretouched, so you can appreciate the dents on the insertion tube from the focuser tightening screw, as well as some wear on the F-bayonet. These pieces are certainly not made to Nikon standards, but they are relatively cheap and work mostly as expected.

Conclusions

On high-power, computerized telescopes, there is little doubt that prime focus is the method of choice, however, for amateurs on a budget the election is less clear cut.
If you have a department-store telescope, a smart-phone with a good camera and some decent oculars, eyepiece projection is the cheapest way into shooting the planets and the stars, and it might just work for you. If not, a good eyepiece camera could be the ticket.

On sturdy, unguided, medium to low power telescopes, prime focus is great for short-exposure lunar photography and general wide-field, but for planetary and deep-sky astro a properly polar-aligned guiding mount is a must.
Equatorial mounts are generally preferable over Altazimuth mounts, but good ones come at considerable cost – that is, well over USD 1.000.
Al in all, if you are on a budget and own a few good lenses, it might be preferable to forgo the telescope altogether.

If you are even a little bit handy, you could build yourself Dave Trott’s double-arm barn-door tracker, which, even including a 1 RPM stepper motor, would probably still cost less than the 50 bucks you would be spending on a T-set.
Although some say constructing a double-arm is “complicated”, anybody who can hold a power drill can build this thing.
In case you are interested, my 1:1 building plans for the device pictured above can be downloaded here.
The other alternative would be to get a good celestial tracking platform for your camera, such as the Star Adventurer or Skyguider Pro, which, although not cheap by any means, are still considerably more economical than a serious German equatorial telescope mount, let alone a serious telescope.

Summing up, I think I have presented you with the available options as far as my knowledge and research go. The rest is down to what you have currently available, want to achieve and are willing to spend.
I am happy with my modest Dobsonian telescope and with the results I’m getting from it, but would rather invest further in a system for lens-based astro, because even a very good amateur telescope can, in optical terms, not compete with good camera lenses – except in focal length.
Moreover, recommended German Equatorial mounts start at USD 1.300 plus, cash I’d rather put toward another good lens and one of the mentioned star trackers, and still keep money in my pocket.
As usual, the options are many, the money scarce and the choices hard.
Clear Skies!