Vātes [ˈväːt̪es] – Germanic/Celtic: Seer
The starting point for the concept presented here was the rapid development that cameras equipped with CMOS sensors have undergone in recent years.
Sensors in modern CMOS-Cameras almost always originated from mass-market products. Since the development of smartphones and daylight cameras demanded smaller, more efficient, more sensitive, and at the same time better-resolving sensors, Cameras for Astrophotography had been undergone this evolution too. From these partly substantial changes concerning camera characteristics, consequences arise, which must be addressed in modern telescope designs to fully take advantage of todays and future sensor generations.
While the CCD sensors, with their relatively large pixels, which were superior to the early CMOS sensors for a long time, harmonize better with long focal length telescopes and large fully illuminated image circles, modern CMOS sensors demand significantly different characteristics on the optical system and the supporting mechanics.
The following considerations influenced the idea behind Vātes
Camera pixels became smaller: The pixel dimensions not infrequently became smaller by a factor of 3 and now harmonize better with shorter focal lengths. At the same time, high quality of the optical components is required to keep aberrations below the resolution-limit of such small pixels. Image-scale is one of the decisive factors for a telescope designed for astrophotography - for the appropriate image-scale, the focal length becomes smaller with smaller pixels. Thus the telescope becomes faster when the aperture remains unchanged
Sensors have tended to become smaller: small pixels make it possible to create seeing-limited images with shorter focal lengths. As a result, there is less need to use physically large sensors to achieve an appropriately large field of view despite the long focal length. The requirement on the telescope to provide large image-circles that fit a full-frame or even medium-format sensor becomes less important, as this requirement was more a symptom than a solution. As a side effect, no extraordinary costs for oversize correctors, filters, etc. add up
Quantum efficiency has increased and noise (thermal noise and readout noise) has decreased: Due to the high quantum efficiency and the lower noise-floor, the exposure time for subs can be drastically shorter. While single exposure times with CCD sensors, combined with moderate to slow focal ratios, typically range from 3-10 minutes, a single exposure of a newer generation CMOS sensor using an instrument with a focal ratio of f/3.5 to f/4.5 is exposed background limited in 10-30 seconds - an increase of the exposure time does not yield any new information since the signal already is lifted above the barely existing noise-level
Thanks to the coma-corrector ParaCorr Type II even very fast Newtonian telescopes can be corrected to a high degree: Despite the 15% longer effective focal length, the resulting focal-ratio still can be considered as “fast” as long as the primary system is designed between f/3 to f/4. Field correction and alignment tolerances improve significantly when using this corrector – same time, compact physical dimensions of the telescope mechanics keep maintained
Consequently, the focal ratio becomes faster: Thanks to the faster focal ratio, total exposure times can be shorter - alternatively, deeper results are made possible with the same exposure time. Furthermore...
... the telescope becomes more compact and more lightweight: The shorter focal ratio results in a short tube or truss-structure. Together with a lightweight primary mirror, the compact dimensions allow stiff designs to be achieved even with low weight - the total weight can be reduced considerably compared to a classic design
As another consequence, the mount becomes lighter: the short single exposure times and the reduced tube weight result in a reduced demand on the load capacity of the mount, the quality of tracking over extended periods, and the question of whether active guiding still makes sense
Efficiency increases due to fewer rejects: Inadequacies of the mount, gusty winds, satellites, and moments with stray light have much less influence on the reject-rate due to the short individual exposures than with longer exposure times. Unsuitable single images are recognized by the image processing software and sorted out or weighted less without the overall result losing quality. Seeing influences also affect the final result to a lesser extent
Larger apertures with a short focal ratio become transportable: Due to the lightweight concept, existing mounts can be equipped with larger optics. Or, given the size of the telescope, the mount can be smaller in some cases. The lightweight, compact, portable equipment creates the possibility of photographing under dark skies, increasing the signal-to-noise ratio further.
The idea behind the Vātes-astrographs is to transfer this potential into a useful technical concept.
The standard design corresponds to the classic-design (with some improvements) as it is known from other designs. Primary-mirror cell, center section, and UTA are transported separately, as well as the 16 trusses, which are pairwise connected to 8 pairs. Up to D400mm, the assembly can be done by one person; above that size, it is recommended to assemble with two persons. Each truss-pair is first attached to the center section. Then the center section is placed on the primary-mirror cell and connected to it with eight thumbscrews. In the next step, the center section+primary-mirror cell should be connected to the mount via the dovetail. With the mechanics set approximately vertically, the UTA is placed on top and fastened via eight additional knurled screws.
The standard-version requires about 10-15 minutes of assembly time. From the description, you can already see that some connecting operations must take place to reach the goal; furthermore, larger masses must be lifted onto the mount.
Nevertheless, the standard-version has its justification. On the one hand, this version is more cost-effective than the much more assembly-friendly travel-version presented below. And on the other hand, there are scenarios in which a regular assembly and disassembly do not take place - for observatories, the standard-version is undoubtedly a solution that is entirely sufficient under stationary conditions.
Astronomers who want to enjoy a large telescope under dark skies will appreciate the advantages of the second Vātes variant, the travel-version. The concept behind the Travel-version is designed to allow fast, uncomplicated transport and setup. The instrument is transported in either three or four main components, with the sixteen trusses remaining an integral part of these components. In addition to a set-up time of only 3-5 minutes, it should also be emphasized that (with the same orientation of the components) the basic alignment is almost perfectly preserved each time the instrument is set up again. This shortens the adjustment procedure, which can start directly with fine-tuning.
In addition to the well-thought-out connection of the individual components, the low weight of all Vātes-astrographs also supports transportability. Thus, a D460mm Vātes-Travel can be easily assembled by one person in a few minutes.
Sporty people will also be able to handle the D535mm size alone. The setup beginning from this size is undoubtedly easier with two people. However, this circumstance concerns every telescope of these dimensions if it is used on an EQ-mount.
Due to the described features, it is comfortably possible to do outdoor astrophotography with the travel-version of the Vātes-astrograph - and this even with sizes that were considered pure observatory instruments only a few years ago.
Main dimensions and Weights Standard-/Travel-Version
Due to a consciously chosen combination of materials with different coefficients of thermal expansion supported by calculations, the Vātes-design became quasi-athermal - an inevitable shift of the focus position due to temperature change is reduced to a minimum.
This fact becomes even more important as the "faster" the telescope is designed. The smaller the focal ratio, the narrower the zone in which the sensor must be located to achieve the maximum resolving power.
This circumstance was given special attention during the development because an instrument with an unfavorable choice of material loses its performance quickly.
Primary-mirror and its cell
The optical components and supporting structures are the most notable highlights of all Vātes-astrographs.
The primary-mirror is manufactured individually and with great care, as the high optical quality is a crucial element of the Vātes-concept.
Years of experience manufacturing large, thin optics allows a reliably high optical quality. But the benefit of quality-optics only can be fully exploited when the conditions allow it. Often, atmospheric seeing plays a trick on you – at least, that’s what it seems. Because what is blurring stars usually can hardly be distinguished - homemade problems resulting in close thermals and very distant thermals are therefore often lumped together.
Too often, "seeing" directly in front of the optical surface, constantly sustained by the thermal mass of the primary-mirror, has been accepted as unavoidable, and blurred images of large mirrors have been taken as evidence that large optics are usually unable to exploit their theoretical advantage in resolving power.
However, after numerous remarkable experiences observing and photographing with thin mirrors, one realization has prevailed - seeing has become amazingly good since thin mirrors replaced mirrors designed more classical and conservatively!
Of course, real atmospheric seeing still exists, and it sets every telescope sometimes wider, sometimes narrower limits. But this fundamental problem can be noticeably aggravated when a thick primary-mirror is used, which does not stop to form a convection-layer the entire night caused by constantly falling temperatures. In this typical scenario, the best optical quality is rendered obsolete by its own thermals and cannot perform as its impressive optical laboratory quality on paper does suggest.
A real gain in image sharpness can be obtained using thin optics (and possibly active ventilation). High optical quality and the shortest possible individual exposure times further help preserve resolution too and here, the comprehensive concept behind Vātes does become visible again.
However, the best possible optics can only perform at their full potential when the mechanical support does allow it. Especially with thin primary-mirrors proper support is of particular importance. On the one hand, a mirror-cell with many backside support-points is not necessarily a guarantee for a good axial support – often-seen design errors can cost a lot of performance here. And EQ-mounted telescopes additionally have particular demands on the lateral-support too, which many existing mechanical solutions only partially or even insufficiently fulfill.
A unique feature of the primary-mirror cell being part of the Vātes-astrographs is the central lateral-support which holds the primary mirror in a blind bore precisely in the center of gravity of the mirror and without restriction in other degrees of freedom. Deformation on the glass is minimal with this form of support and limited to the very center of the front surface. When performing an interferometric-measurement, the contact point is only noticeable by a few nanometers "deep" depression which, in practice, when installed in a telescope, is abundantly covered by the shadow of the secondary-mirror and therefore invisible. The well-known potato-chip effect, which is responsible for more or less pronounced astigmatism even with thicker mirrors, does not noticeably occur with this design.
The following example of a mirror with a diameter-to-thickness ratio of 18:1 being measured interferometrically while installed in a telescope shows how effectively the backside lateral-support is performing.
The interferometric evaluation of this primary-mirror using a test bench certifies a high optical quality; the Strehl value was determined to be 0.98. This laboratory measurement was carried out so that test stand-deflections can be reliably averaged out by performing measurements in several positions. (further described here).
The same mirror installed in the telescope again and supported laterally by the backside central-support. The evaluation of 12 interferograms in only this one position reveals that the Strehl has dropped to 0.95, which practically is invisible in practice.
To prove the reproducibility, the telescope was set vertically before the second evaluation and then moved to horizontal-position again. The result: Strehl 0.95
But besides the advantage of equally reliable lateral support in any position when EQ-mounted telescopes, there is another advantage for astrophotographers. Unwanted spikes caused by diffraction effects at holding clamps of classical mirror cells cannot occur - there simply are no holding clamps in this design existing.
The primary-mirror cell used in every Vātes-astrograph is proven numerous times with thin Nauris primary-mirrors. Thus, by using a sophisticated mechanic that is exclusively installed in this form in these telescopes, it is reasonably possible to perform high-resolution photography and visual observation with thin optics. At the same time, the mechanics remain conspicuously inconspicuous during nighttime use - they work and you no longer think about them.
The secondary-mirror does match the quality standard of the Nauris primary-mirror and is held securely in place by bonding points whose positions have been optimized by finite-element simulations.
Special attention must be paid to achieve a precise collimation with Cassegrain- and Ritchey-Chrétien-telescopes and with fast Newtonian systems. Fortunately, with Newtonian telescopes, there are fewer degrees of freedom to restrict. So even very fast Newtonian telescopes can be collimated spot-on, with good repeatability and simple equipment. This makes Newtonian systems the most straightforward optical system to collimate for medium and larger telescopes. Only the primary-mirror and the focal-plane need to be perfectly aligned, the critical positioning of a hyperbolic-convex secondary is not necessary due to the simple nature of a Newtonian secondary.
Thus, given practice and patience, coarse collimation by laser and subsequent fine adjustment by judging star distortions will already lead to success. Using the included TeleVue ParaCorr-II coma corrector, this task turns out to be less challenging than the fast focal ratio might suggest - the corrector reacts well-tempered and is comparatively forgiving compared to other correctors for Newts or even RC-telescopes.
However, collimation by judging star distortions costs valuable time, especially if one does not want to deal with these issues. Therefore, using an autocollimator has proven to be the most precise solution after the star method due to its exceptionally high sensitivity and, thus, the possibility to detect even the most minor deviations from the ideal. Conscientiously executed, an almost ideal correction can be achieved into the corners of an APS-C sensor without further measures.
For enabling collimation by using an autocollimator, each instrument is delivered with a precisely positioned primary-mirror center-spot which allows the adjustment by the CatsEye-Autocollimator. The autocollimator itself is not included in the scope of delivery.
Ventilation of the primary-mirror
The thickness of the primary mirror is the decisive factor on the thermals inside the telescope. The glass body is warmer than the environment and is constantly exchanging with the colder air - slowly rising streaks of warm air are the result. These streaks are located directly in front of the optical surface and develop undesirable effects in the form of distortions of the wavefront.
The most effective way to deal with this problem is to reduce the thermal mass as much as possible - the exchange takes place with less intensity. But even a thin mirror still does not behave ideally, and here the use of fans can help that the boundary layer in front of the mirror is mixed and thus prevent the formation of optically effective temperature gradients.
Ventilation from above is the most effective way to achieve this goal. While venting from behind to the back of the primary accelerates its cooling process but cannot swirl the already formed boundary layer, this is precisely what the arrangement chosen in the Vātes-astrographs allows.
Another purpose of the ventilation is less evident at first sight but real. Together with the light shroud, it is possible to achieve a slight chimney effect - the constant air flow upwards past the secondary mirror can protect it from early dew formation, which is an advantage that should not be underestimated.
The control of the four fans allows speeds between 600-1500rpm - in practice, speeds in the lower third of the speed range are sufficient to achieve the desired effect in terms of stirring the boundary layer.
Secondary Torque- compensation
One of the biggest negative influences on collimation-stability of Newtonian telescopes comes from the Secondary-spider. The one-sided load caused by the weight of the secondary exerts a torque that causes the spider to torsion. However, this can be counteracted by a counterbalance weight calculated to create an opposite moment effect.
The negative influence was identified during the development of the mechanics and reduced to a minimum by this simple measure. The counterweight has since become standard on all Vātes-astrographs - resulting in undistorted stars in the image corners, even after a Meridian flip.
The weight is connected to the secondary in such a way that it can be easily removed for transport or when creating flats with a panel.
The standard-version is designed to use sensors up to APS-C format (corresponds to approximately 30mm image-circle). All necessary measures can be implemented to use sensors in 35mm format or even larger for an additional charge.
In addition to a 3" focuser, the secondary size is appropriate for the larger sensor, the mechanics around the secondary are reinforced and the height of the UTA increases with the larger secondary to safely accommodate it.
Steadiness of collimation
Of course, lightweight-design must not be at the expense of functionality. This was one of the key requirements during development; as a result weight was saved with a sense of proportion.
Whether a fast optic in a very lightweight mechanic is suitable for practical use can best be judged in a worst-case scenario. The two images below show the center and the outer areas of the same field, once before and once after a meridian flip. The focus remained unchanged after the telescope was flipped to the other side of the mount and the correction of the star figures in the peripheral areas reveals that the alignment is still intact.
The following images are a selection of results obtained by Marco Lorenzi from the roof of a skyscraper in Singapore obtained with a D535mm f/4 Vātes mounted on a DIY fork-mount. These images are among the best planetary photographs amateur astronomers have obtained without the aid of telescopes in professional observatories. Even in these first attempts with his new instrument, the resolution achieved in practice is close to the theoretical resolution of an optical system of this size.
More images can be found on Marco Lorenzi's homepage Glittering-Lights.
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