In recent years, imaging methodology has moved from aftermarket modifications made to conventional telescopes and towards instruments designed from the ground up to serve the needs of astrophotography. The RedCat71 from William Optics is the new kid on the block aiming to serve this need, and courtesy of First Light Optics, I looked forward to seeing if it delivered.
In the box
Quality is evident straight from the start. Eschewing the harsh and heavy aluminium ‘crates’ beloved by many, this telescope comes in a hard-wearing, padded nylon carry-case that usefully falls within the dimensions for airline hand-luggage. Nestling inside its protective foamy womb, the RedCat71 is well-shielded from impact in all directions. I did not throw it encased out of the bedroom window, but I am pretty confident it would not notice.
Optical performance aside, one thing that runs through the William Optics ethos (practically a mission statement) is a very high standard of engineering finish. Decked out in a lustrous satin crimson, the RedCat71 does not disappoint, and its impeccable coachwork simply looks super.
As far as tube furniture goes, attention to detail starts at the top, where the dew cap sits. Cleverly, this incorporates its own matched Bahtinov mask, an essential accessory for achieving tack-sharp focusing, made possible by a large helical focuser barrel (2GT Gear Ring) that takes up 100mm of the 335mm tube length. Its rubberised surface is very tactile and non-slip, with rotational tension intriguingly controlled by the dew tube that acts like a friction clamp.
At the business end, the tube assembly terminates in a 360-degree rotating M54 thread, to which M54 and M48 adaptors are added. These adaptors incorporate a cell for deployment of 48mm filters and (for the most demanding imagers) a tilt mechanism affording microscopic adjustment to match the imaging plane.
I am surprised that the telescope does not come complete with photographic M42, T-mount or C-mount adaptors, bearing in mind that the RedCat is designed to be used with imaging devices.
The tube’s clamp rings offer Arca Swiss and Vixentype dovetail fittings on one side, and to the other a saddle is attached (which also serves as a great toting grip).
Given the aforesaid praise, I must unfortunately move on to the quick-start guide – which is disappointing to say the least. Considering the much appreciated manufacturing cost that is lavished on the telescope itself, the four-page quick-start guide contends itself merely with labelling of the parts and advertising potential accessories. There is no information on how the instrument should be deployed, used, nor even any primary instructions on imaging at all! The assumption here is that a new owner is already up to speed with prior experience and swapping out one telescope for this one. This may be the case, but someone new to imaging will not be in space, but all at sea. It will not have a novice starting quickly – indeed without a set of step-by-step instructions a newcomer will be doing nothing but scratching their head. I do accept that this may not be a beginners’ instrument, but a guide with just a little extra instruction would enhance this kit enormously. It would be short work indeed for a technical writer.
Can it be used for visual astronomy?
The RedCat71 is described by all vendors as ‘designed primarily for astrophotography’. This, I would say, is a masterly understatement. The telescope does not come with any means of using it visually. Indeed, the so-called ‘quick-start’ documentation does not even offer any means of doing so, but that did not stop me trying.
I first went to the trouble of acquiring an M54 adaptor to enable use of a star diagonal, but I found that in this mode eyepieces were too far beyond focus. I was puzzled by this, since short refractors are generally quite happy to work with eyepieces attached in this way. It indicated that visual use would be difficult because of insufficient back-focus. Aiming at a daytime target of a distant television aerial, I measured the back-focus from one extreme of the helical focus travel to the other and found that the focal point at maximum was barely 60mm beyond the M48 adaptor. No wonder the light was not lasting the path through my diagonal to an eyepiece.
Not to be thwarted, I realised that the RedCat’s male M48 adaptor might mate with the 48mm filter thread of a two-inch barrel eyepiece. I was pleased to discover that, without the back-focus consumption of a star diagonal, eyepieces attached in this way could now reach the focus. I am not sure that I (or William Optics) would recommend this mode of rather unconventional attachment, but although the comfort of right-angle viewing was not available, it did afford some spectacular visual astronomy.
In this manner, I used a 3.5mm (110-degreewide field) eyepiece to observe the Moon. Although naturally a high-power eyepiece, used on a telescope with only a 348mm focal length results in a magnification of just 99× – though as a wide-angle eyepiece it still yielded a field of about 40 arcminutes, comfortably encircling the Moon with sky. A gibbous phase revealed Gassendi near the terminator – and even at this relatively low magnification the telescope was showing a riot of detail within the crater rim, including its distinctive rilles and cracks. I would love to have stretched to a higher power (of which the telescope is obviously capable) but I could not deploy a 2.5mm, 1.25-inch eyepiece that would have delivered 139× because I had no means of attaching it.
The rather tardy back-focus may be a concern to those whose imaging train contains more than just a camera, such as filter-wheels, off-axis guiders etc. In fact, I was not initially able to deploy my normal push-fit imaging configuration because my DSLR’s sensor came to rest 30mm distant from the focus. I ended up having to acquire four supplementary imaging adaptors before I could proceed with the review – and that should indicate to potential purchasers that you may not be ready to go upon first receipt.
The optics of the RedCat71 are of a new apochromatic Petzval design (four elements in three groups), comprising a doublet objective, a central floating component and a rear corrector. The claim is that this configuration provides a wide, flat field with a 45mm imaging circle, absent of the need for supplementary flatteners or correctors and suitable for sensors all the way up to DSLR full-frame.
I carried out the review using an Altair GP-CAM3 290c CMOS camera plus three full-frame Canon DSLRs, these being a 5DSr and two 6Ds (one of which is astro-modified). Using its Vixen footpad, I had the RedCat71 riding on my Meade 355mm LX200.
Before I proceeded with testing the telescope’s imaging credentials, I was interested in what the native optics were delivering. This is something that an extra-focal image of a star can clearly provide, the star chosen for this honour being Antares. I was very impressed by the classic, defocused diffraction pattern; the symmetry and evenly distributed light were promising a fine delivery in use.
The focal length of the RedCat is a scant 348mm, which promises very wide imaging fields. I first wanted to check out how ‘flat’its imaging circle was, and for this test the Moon was happy to oblige.
One clear, blue evening in June, a gibbous Moon hung above the southern horizon just begging to be imaged. Compared to a star, its large surface area strewn with detail was just what I needed to detect definition differences between the centre and edge of an imaging circle. Using my full-frame (un-modified) Canon 6D, I carefully focused on my obliging subject – whose 0.5-degree disc swam quite lonely in the huge field that the RedCat71 was supplying. I shot the Moon first in the centre of the field, and then afterwards, without refocusing, I took another shot with the Moon snuggled up as close in the corner as I could manage.
I then sandwiched both images together to afford simultaneous comparison between each lunar disc. The result demonstrated that there was no discernible difference between the two Moons whatsoever. The peripheral Moon was as crisp, sharply defined and equally bright as its doppelgänger at the field centre. In fact, there was no distortion of any kind in the corner-bound image of the Moon, which is very impressive, particularly for those who might want to employ the RedCat71 for astrometric work.
My experiment with the Moon also provided useful information on the field width, for it was possible to work out that I could place about fourteen Moons across the long diagonal of the full-frame field, therefore approximating it as seven degrees in extent. The opportunities of such a large sky net for deep-sky photography are obvious and this is where I turned my attention next.
In early summer the Veil Nebula in Cygnus is a magnet for imagers and rightly so. However, as its two primary arcs span a challenging three degrees, it is commonly imaged in parts, featuring either its eastern or western components. What I had at my disposal was over twice that extent, making capture of the whole object (also known as the Cygnus Loop) not a problem at all.
My modus operandi for nebula imaging is to acquire a firm focus on a nearby star before slewing to my target, and so this seems a suitable juncture to talk of the RedCat’s focusing abilities.
I mentioned previously that focus is achieved via rotation of a large, rubberised ring situated at the telescope’s midpoint. I must confess to some concern that this torsional force on the RedCat’s tube could be transferred to its saddle-plate mounting (particularly as you are having to manhandle the tube to do so) and thus impart movement or vibration during the process of achieving accurate focus, in a way that the normal longitudinal force of a conventional focuser would not.
Fortunately, the RedCat’s tension adjustment on the helical focus mechanism comes to the rescue here. The friction can be adjusted so that even the very lightest touch on the rubberised ring will impart an effect on the image. I was thus able to adjust the tension so that very little adverse energy was evident in the image of the star being focused.
For those wishing to avoid manhandling the tube during a focusing exercise, it is possible to retrofit a drive-belt EAF (electronically assisted focusing) unit to a toothed ring on the RedCat’s focuser to control the process digitally, thus reducing those microscopic ‘tremors’ to a vanishing level.
The RedCat’s Bahtinov mask is cleverly installed in the dew cap and it is only necessary to unscrew the end cap to make it available. The mask is made from a transparent disc, which (it is claimed) makes the resulting diffraction pattern brighter. It served perfectly throughout the review – the resulting three-axis starburst pinioning the point of focus with uncanny accuracy,
As with all Bahtinov masks, the ‘crossed-swords’ diffraction pattern necessarily spreads the light of a star. The ultimate limit of the light grasp of the RedCat’s 71mm aperture means that a reasonably bright star is required for this procedure, or a means of increasing a star’s intensity through raising the gain of the sensor. This focusing fidelity was very much appreciated, since a consequence of such a short focal length delivering a flat-field focal plane is that the depth of true focus is very shallow indeed.
This time, using my astro-modified Canon 6D and a dual-band filter, I captured the Veil Nebula during a total exposure time of a little over an hour. I took care to position the whole Veil to one side of the field to better illustrate the camera’s yawning capture. The result showed the RedCat’s potential admirably. Almost lost in the profusion of faint stars that the telescope pulled in, all of the Veil’s components were captured easily within just one half of the field. It demonstrated to me once again that while ‘up-close’ images of nebulae are compelling, seeing objects in the context of their surroundings is also valuable. The profusion of field-wide concise stellar specks also confirmed that stars remained stars throughout the field, wherever they were.
Cygnus, of course, offers that other gem of the summer sky, the North America Nebula. It too is often imaged piecemeal via its major components, rather than in its entirety. The RedCat71 handled this vast celestial object with ease. The astromodified 6D, in concert with a hydrogen-alpha filter, drew in not just the North America Nebula, but its neighbouring Pelican Nebula and the surrounding HII regions.
I turned next to M13 in Hercules. I am used to imaging this object with my 355mm Meade ACF, for with the amplification of the Meade’s 3.5-metre focal length the globular cluster comfortably fills the frame. Its startling capture in the cavernous field of the RedCat showed it almost lost in a sea of stars, giving it the wonderful sense of being a forlorn stellar community adrift in the immensity of space surrounding our Galaxy.
The RedCat71 comfortably demonstrated its abilities as an astrograph, and in the hands of those more able than I it is clearly capable of incredible astrophotography, especially widefield imaging to which its focal length is suited. The large tactile helical focuser is a nod towards making it useful as a terrestrial telephoto too, so those who love to capture candid shots of wildlife get an extra bonus. I thoroughly enjoyed my time using this excellent instrument.
Steve Ringwood is a regular contributor to Astronomy Now.
At a glance
Focal length: 348mm
Focal ratio: f/4.9
Design: apochromat, four-element Petzval (quadruplet)
Focus: Helical (2GT Gear Ring)
Adaptor fixings: M54/M48