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CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.
The TAL 200 K is a modified Cassegrain system composed of a primary spherical mirror and a secondary optical
train consisting of a Mangin lens and a meniscus assembled together. The combination of the Mangin lens and the
meniscus is intended not only to correct the spherical aberration of the primary but also to contain color aberrations
to a level above the color correction expected from a well made Apochromat. It is also meant to correct for coma
and field curvature (or at least partially) so to result in an aplantic telescope. In terms of class of telescopes
the TAL 200 K therefore qualifies as a member of the catadioptric family.
The TAL complete system was bought in the first days of April and was the third of a first batch to reach Italy. The serial number of the telescope, 0029, shows it as being an early specimen. At the time of the purchase the TAL 200K (in future referenced as TAL2K) was only sold as a complete system, OT, accessories and mount. The whole system was shipped in a huge wooden case (about 50cm x 70cm x 25 cm) as shown in Figure 1. The whole lot weighted in at about 45 kg and using it as a carrying case was beyond questions (or at least this is how I felt after hand carrying it for about 10 m). While the case looks pretty sturdy it isn't so as it managed to crack along one corner twice while moving it around the house.
Figure 1 - The TAL2K as delivered in its shipping case
Once opened the case the whole arrangement looked impressive and well laid out. Only one support for the pier yielded during transport without major damages to the pier itself or the surrounding bits. The content of the case included the following:
- Optical Tube Assembly of 200 mm with a declared focal ratio of f/8.7 inclusive of a rack and pinion focuser and a support plate for the included mount. The OTA had a plastic (half cracked on a side) lid to protect the mirror from the dust.
- A pier with three legs.
- An equatorial head incorporating a synchronous 12 V clock drive and the counterweight bar.
- 220V - 12 V transformer with 5 m of electrical cord.
- Two 3.5 kg counterweights
- An 8x50 Finder with a plastic dewshield
- A 2-ring finder support.
- A mirror diagonal for 1.25" accessories.
- One 25 mm Ploessl one 10 mm Ploessl EP
- A 2x Barlow
- One solar and one lunar filter of 1.25" size
- A crosshair screw-on for EP (similar in shape to a standard 1.25" filter)
- A brush
- A screwdriver
- A ring adaptor for prime focus photography M42x1.5
- A ring adaptor from M42 to T2
- 2 fuses
- A user's manual in English
All accessories (EPs, finder, filters etc) were wrapped into white packing paper while the EPs were also bagged into protective plastic bags. As can be seen from the list above the TAL2K is provided with everything needed to start observing from zero.
The user's manual declares a fully assembled weight of 30 kg (about 60 lb). The OTA together with diagonal and finder weighs in at 10.5 kg (21 lb), that is about 2 times the weight of a recent C8 OTA (with finder and diagonal too).
Figure 2 - TAL2K on its mount
The OTA, as shown in Figure 2, is painted white and has a bulky ring built into the outer edge of the tube itself for the evident aim of supporting the massive secondary optical train (Mangin lens and meniscus). The length of the OTA, from end to end, was measured at 430 mm (as long as a C8). The inner diameter of the OTA is 210 mm (give or take one mm), barely sufficient as spacing between the mirror and the tube walls. The external diameter of the secondary, inclusive of the external metal cap, measures at 72.3 mm therefore the geometrical central obstruction can be quantified at 36.1%. If the external metal cap is removed the mean central obstruction (geometrical, that is) can be set at 34.7%.
Figure 3 - Inside views of the TAL2K OTA
The secondary optical train support, as shown in Figure 3, is made with three rectangular section arc spiders
having a 5 mm thick section (the total area of which constitutes a significant part of the primary mirror obscuration)
which are welded on one side to the tube ring and on the other side on the secondary plate itself. From what I
gather, all parts of the secondary optical train including spiders and the ring on the OTA are made of steel. The
spiraling shape of the spiders has been chosen to eliminate the diffraction spikes so common in newtons and cassegrains.
The choice of steel as the base material for the secondary optical train may be due not only to the higher stiffness
of the steel but also to the lower thermal expansion coefficient of it, allowing for a better coupling (less variations
of spacings during thermal transients) of the optical parts composing the secondary train. Overall, the mechanical
arrangement seems poor when compared to those of other Russian manufactures, notably Intes and Intes-Micro. The
surface finish of those parts appears to be poor with a black paint rather glossy.
On the secondary support plate there are 6 bolts which should be used to collimate the instrument. However, when and how to do it is not mentioned in the user's manual nor there is any reference to the word "collimation". The bolt heads together with the supporting plate were found with a thick layer of primer (lacquer) applied on top, clear indication that the manufacturer does not intend the scope to be serviced by the end user.
The interior wall of the OTA appears to be grossly threaded and it is painted back with a rather glossy varnish. The effectiveness of the paint seems (apart form the glossy finish) to be dubious as is not completely uniform all around, with zones having somewhat less paint than others (they appear "shiner" than others). The tube itself is made by a seamless aluminium tube of some thickness supporting the secondary train and ring (to which is attached via six stainless screws, see Figure 3) and fixed to the scope rear end (presumably by the six screws found on the outer rim). On the rear end of the scope one can find the conical baffle tube common in all cassegrains design. The inner wall of which is threaded (much better than the OTA external wall) and painted opaque black with much better finish the OTA tube wall. The primary mirror support cannot be seen from the open end of the OTA and I haven't dismantled the OTA itself to find it out.
To the rear end attaches the rack-and-pinion focuser (see Figure 4). The total focuser travel is 25 mm. On the drawtube there are 2 screws at 90° from each other to hold the diagonal in place. The supplied diagonal (pretty short) is specific to the TAL2K as it has a V-flared end to provide clamping place of the 2 screws on the focuser. While this design allows the diagonal to be rotated freely while not risking dropping it off from the focuser it takes a while to get accustomed to (sometimes I didn't insert in properly and this caused some EPs not to reach focus). The focusing knobs not only allow for focusing motion but also, by turning each in the opposite way of the other, allow for controlling the "stiffness" of the focusing motion.
Figure 4 -TAL2K focuser fully drawn out
The 8x50 finder attaches directly to the OTA (as shown in Figure 2). The OTA is furnished with a suitable holder
so that removing, inserting and adjusting the finder is as is as it can be. The holder is provided with a blocking
screw. The finder itself is a well built unit having two well-though V-flared rings on its body to provide for
a clamping place for the retainer screws of the finder support. With such a design is considerably more comfortable
to adjust collimation of the finder without risking it falling off.
The OTA mounting plate (shown in Figure 5) is a dovetail unit built entirely out of stainless steel and slides inside a holder mounted on the OTA itself. The position of the mounting plate with respect to the OTA can be adjusted (by sliding it) and fixed in position by tightening the big knob shown in Figure 5. The dovetail has two end plates (fixed by two screws each) to avoid sliding it off. On the mating surface to the equatorial head there are two big pegs to be inserted on the mating holes on the mount attachment and two threaded holes for the securing bolts on the mount side. These two pegs can be removed (they are screwed onto the dovetail) so that modifications of the mounting plate can be made to add any additional adapter to a different mount. Such a mod can be made with proper tooling as the adaptor plate is a pretty thick unit (a press drill should suffice).
Figure 5 - Mounting Plate
The Equatorial Mount:
The equatorial mount unit is apparently identical to equivalent units from the same manufacturer. As Figure 6 and 7 show it is quite different from the "normal" German equatorial mounts so common today. The clock drive is concealed in the "black box" seen in the pictures.
Figure 6 - Overall view of the Equatorial Mount fully assembled
Figure 7 - Close-up view of the equatorial head
On both sides of the head there are two large micrometer knobs to allow adjusting the R.A. axis. On the head itself there are two other knobs to allow securing the OTA to the mount itself. The adjustments in Dec. are made via a tangent drive arrangement, which allows for +/- 4 degrees of freedom. The tangent arm is driven by a large plastic knob on the other side of the head (therefore not shown clearly in the pictures). On one side as well as on the back side of the clock drive housing there are two small portholes to allow accessing the innards of the clock drive itself and more specifically to allow tightening and releasing the screws controlling the tightness of the clutches (2 for the R.A. axis). Once the rear panel is removed the clock drive unit appears as shown in Figure 8.
Figure 8 - Clock drive units
Apart from the synchronous drive there is the primary gear reduction train end in a clutch which acts on the
worm (stainless steel unit). The worm gear is a brass-made 180-teeth unit mounted on the primary R.A. clutch (the
large black wheel in Figure 8). The micrometer movement knob acts directly on the worm itself hence the need of
a double clutch arrangement (which turned out to be the weakest mechanical link of the drive unit). On side opposite
to the first clutch there is the on/off switch, a red led and the socket for the power cord (from the transformer
The pier unit is quite heavy and substantial. The three legs attach to the pier using a single knob for each leg. These legs do not allow for any adjustment in height. The equatorial head inserts in the flange of the pier and is kept fixed by three knobs as well. There is no provision (again) for any micrometric control in azimuth on the mount. The altitude fine control of the equatorial head is non-existent as well. The equatorial head swings on a hinge at the base. The relative movement is controlled by a large knob which stiffens or release the movement of the head acting on a sort of graduated lever and once the position is (roughly) achieved is kept in position by a clamping lever. There is no polar finder nor it is possible to add one.
Using a TAL2K.
Assembling and Polar Alignment:
After having removed all the grease covering whatever was endowed with threads mounting the TAL2K was a piece
of cake as the mount is basically on single piece. One problem I've found was that in certain positions the equatorial
head covers one of the knobbed screws holding the same head in position. This obviously worsen with increasing
observer's latitude (mine varied from 45.5 to 52.5 N). At the end I left one out and fixed the position of the
head with respect to the pier with the remaining two.
The exact balancing of the OTA is strongly dependant on the position of the finder and, eventually, heavy oculars. Doing it is pretty tricky as I discovered that the knob fixing the position of the dovetail plate needs something more strong than my bare hands to tighten up to the right amount the whole assembly. In the end I resorted to use a pair of pliers to tighten it properly. Tightening it with just hand pressure just didn't work and the OTA after a while started to slide off its intended position. To properly balance the OTA in R.A. the counterweights need to stay at the extreme end of the supporting bar, increasing the lever arm and hence the amplitude of the vibrations.
As previously noted the mount is devoid of any way to control the azimuth and the latitude position of the equatorial head therefore aligning using the star drift method requires a lot of patience, a gentle rapping to move it azimuth and a perfect balancing in declination. The knobbed screws to fix the position of the head with respect to the pier do not help here has tightening them causes a slight change of the position of the head itself. In the end the best I could manage to get was a slow drift so that a centered star at 220x stayed within the field of view for about 40 minutes (on average).
In one sentence this the worst mount I've ever used and absolutely not up to the task of supporting (even if only for visual use) a heavy tube like that of the TAL2K. In addition to the problems described before the R.A. axle suffers from a large mechanical play (about a couple of degrees) in the support bearing, which could not be removed or reduced with any settings of the clutch stiffness or by adjust all other mechanical plays in other sections of the clock drive. The R.A. axle is of considerably small dimensions and the bearing (or whatever is used) is of no consistence. The large mechanical play in this axis causes large oscillations when focusing, so bad in fact that focusing at high power is really a pain in the neck. Apart from this defect the R.A. worm/gear assembly suffers also from a large backlash (several minutes) and in some positions (notably when the tube points near the zenith) the adjustment knob turn around without causing any movement. The tangent arm drive of the Dec. axis suffers from a considerable play as well, although not as bad as for the R.A. axis.
Using the mount is painful and nearly impossible in any comfortable way when the scope points to the zenith.
At my latitude of 52.5° N the focuser is just 80 cm above the ground and to observe (not to mention pointing)
I have to sit on the bare ground (which isn't going to be anywhere near to something that can be described as "comfortable"
when the average outside temperature is in the -10 °C range, no matter how well dressed up you are). The setting
circles are of acceptable dimensions and have a 2 degrees graduation in R.A. and 20 minutes graduation in Dec.
Optics and Accessories:
The first trials with the TAL2K under real sky revealed an in-focus comatic image with some astigmatism thrown
in for good, clearly due to misalignment. I therefore set forth to collimate the TAL2K which resulted in a daunting
task. After having removed the cap sitting on the secondary holder I had to face the presence of not less than
6 bolts (bolts!!) for which I didn't have at first not even the right wrench. The first trials were done with narrow-nose
pliers and were quite unsuccessful. After I bought the right wrench I went through a week-long set of trials before
I got it right. In the end I discovered that the best way around is to unfasten three of the six bolts and collimate
the secondary using just the remaining three than and only then hand fasten the other three (do not force them
into being tight fastened or you're going to loose collimation again). Due to the underlying mechanics of the secondary
collimation has never been textbook perfect and all I could manage to get is nearly no coma and some residual astigmatism.
The EPs quality was a mixed bag. The 25 mm Ploessl was pretty bad (astigmatic) from 2/3 of the field onward while the 10 mm one was rather good with a good enough eye relief to be used with spectacles. The 2x barlow was indeed one of the best achromatic barlow I've ever used with no lateral color to speak of nor coma and made a nice high resolution combo with the 10 mm. The images of Mars or Jupiter using this barlow were virtually identical to those seen while using an Ultima 2x or a Meade 140. All trials were done using the TAL2K and the MK67DL for comparison. Either alone or with the TAL barlow I've never seen any significant ghosting using the supplied EPs neither on the Moon nor on Jupiter or Mars. Actually, I haven't seen any with any EP I've tried the TAL2K on. All my EPs of 1.25" size (ranging from a 32mm Ploessl to a 4 mm Ortho) failed in reaching focus although the Pentax SMC 6 mm Ortho made it just by a tad. I haven't many wide field multi element EPs of 1.25" apart from a Pentax XL 21, so I cannot testify on their suitability for use on the TAL2K. To the ergonomics side of the TAL2K (or lack thereof) I can say that using short focal length EPs (high power), which are usually quite small in height, one risk banging the head against the rear of the scope if he/she uses preferentially the left eye as I do. It was quite annoying to go around to find the best observing position, also because the space in my balcony is quite limited.
The crosshair was never in-focus when mounted on the 10 mm EP (or the 25 mm, by the matter) and therefore of limited utility. The prime focus adapter was of no use either as I could not reach focus neither with my Olympus OM-1 nor with the Canon AE-1.
Thermal equalization of the TAL2K has never been a problem as was fogging. In one case, however, I did find that the secondary to have fogged up during a long observing section but I must add that the external humidity was about 99% and all other telescopes had fogged up their correctors much earlier. One thing that I've noted is how big is the focus variation with temperature in the TAL2K. After having brought the OTA from inside home to outside (very mild temperature) the star originally brought in focus was found after half a hour quite inside focus. After the scope had acclimated itself no further variation in focus were found.
All my scopes usually spend their life outside, appropriately protected and covered by suitable (i.e. waterproof) coverings for the season. This is much more so in winter when the temperature difference from inside to outside can go up to 40 degrees C and thermal equalization is going to be a problem. After 8 months of usage the TAL2K developed a some rust in all metal part exposed including part of the spider, the secondary support plate, the secondary housing and a number of bolts and nuts. None of my scopes ever suffered from such a problem (presumably because they do not employ any steel at all).
The focuser revealed itself as the worst part of the OTA. Apart from the very limited focusing range (1" or 25 mm) it stiffens in certain positions along its focusing range leading to a rather jerky focusing action. It also shows a considerable lateral focus shift, quite evident during the collimation sessions, possibly worsened by the non perfect centering of the optical train.
The finder, on the other hand, is a gem. Definitely one of the best finder I've ever found in commercial scopes. The chromatic aberration seems well contained and off-axis astigmatism and coma are bearable for about 2/3 of the field of view. The eye relief is generous and there is no problem in using it with glasses (this due also to the large eye lens of the EP). The helicoidal focuser (incorporating the EP) is very smooth in operation and the large section of it lends to easy fine focusing. The crosshair is very thin and basically of no use. There is no provision for adding a illuminating led anywhere. The dewcap is cheaply made (just a section of black plastic tube) but it nonetheless prevents dewing the lens.
Optical Quality and Performance:
The field correction tests were carried out on a second magnitude star. The star was left trailing through the
field of view of the 32 mm Ploessl and of the Pentax XL 21 EPs. Both EPs show a well corrected field without field
curvature or significant coma/astigmatism when used on aplanatic scopes with small field curvature (like Maksutovs).
The same EPs were used for comparison on the MK67DL and visual impression left was of identical performance off-axis.
The stars show in the TAL2K as round compact bright objects, without any hint of diffraction spikes or in a word,
pleasant. Overall the wide field performance (off-axis correction) within 1° of real field of the TAL2K is
in line with that can be expected, off-axis, from any well corrected telescope.
Observations of the Moon, of Jupiter and of Saturn (Mars being too low during this opposition at my latitudes to draw unbiased conclusions) at low and medium powers revealed a weird property of the TAL2K optics: that of showing every bright extended object with a yellowish hue, possibly due to the antireflection coatings of the corrector. This was mostly evident when the planet low in the sky (30°-40° ). Obviously this behavior does not depend on the ocular used and the same ocular (even those provided with the TAL2K) showed a bright whitish Saturn in the Maks or in the C8. One other notable fact was that the atmospheric dispersion effect seemed far easier to note in the TAL2K than in other scopes, as the Mars images shown in the following paragraphs show.
Out of view bright objects flooded the field with unwanted light. In focus objects had far more dispersed light that I think reasonable. I never did a critical observation where well baffling was of a importance but having used it through thick and thin I must say that I wouldn't bet on the TAL2K succeeding.
The Fresnel rings show something curious about the TAL2K. They show clearl color distinction within the each ring (see Figure 9 and 10), considerably more than for my C8 at about the same magnification. The same applies to the Airy disk. It shows clear colors at a magnification lower than what I was expected to see it. In focus low power views of Sirius do not bear out any chromatic aberration worth of notice, apart from that due to atmospheric refraction.
Using the relative size of the Mars images taken through the TAL2K and MK67DL (of which I know with some precision the focal length) I estimated the focal ratio of the TAL2K specimen in my possession to be about F/9 +/- 0.1, therefore about 1800 mm of focal length, quite close to the manufacturer specifications of F/8.7.
Star testing the TAL2K was a tough nut to crack. What strikes is the complete difference between extra and intra focal patterns. With average seen the intra-focal pattern is entirely dominated by a very bright inner ring (as shown in Figure 9), with the remaining rings faint and eventually dispersed by the seeing fluctuation. Only with the very best seeing you get the complete picture (as shown in Figure 9, as said). Outside the pattern resemble more closely a normally spherical aberrated pattern, with the outside ring just a bit brighter than the inner rings. In focus the residual astigmatism shows well (as one can gauge from the image in Figure 11, showing Sirius, as well from the 90° opposite ellipticity seen in Figure 9 and 10), as well a strong first ring and a much fainter second ring (on Sirius) and traces barely visible of a third one. This astigmatism seems huge but is in the order of magnitude of the seeing variations in a decent to good night. In Figure 11 seems huge because of the atmospheric diffraction effects.
My evaluation of the total spherical aberration of the system, carried out with the aid of Aberrator (thanks Cor) would be 1/4 of third order spherical aberration over-correction and as much as 1/3 fifth order spherical aberration over-correction. These numbers aside I don't think that what the star test show can qualify as a "diffraction limited" optics. In addition to this, the huge effective central obstruction (shown in Figure 12) of about 40-41% does not help in bringing out subtle planetary details.
Figure 9 - Star Test - Inside Focus Figure 10 - Star Test - Outside Focus
Figure 11 - Sirius - In Focus Figure 12 - Inside Focus Exit Pupil Image
Other proof of the barely acceptable (or not acceptable at all) optical quality of the TAL2K was apparent when focusing at high power. It never "snapped" into focus but was rather like finding an acceptable compromise between various kinds of opposite aberrations.
Double splitting was a mixed bag but not very promising. The double-double (eta Cygni) was any easy split with the 10 mm Ploessl. 36 Andromedae (0.9") was tougher but cleanly split as well (barlow 2x + 10 mm). Not so lambda Cygni (0.7") which was split with some difficulty, due mainly to the residual astigmatism. Completely out of question was splitting gamma Andromedae (0.5") far too tight for this scope.
Under the stars. A note on the method used.
The majority of the comparative observations reported below were carried out not at the ocular but through the use of webcam Philips Vesta Pro coupled to the optics under scrutiny. This to report in a less subjective way the image quality as seen through this or that scope. All the images displayed, unless noticed otherwise, were taken few minutes (usually about five minutes) apart form one comparison scope to the TAL2K to ensure to have about the same average seeing condition through a row of shots. Using a webcam is no much different, in terms of underlaying principles, from using an ocular and the eye. The typical frame rate used goes from 30 fps to 15 fps, 5 fps in some specific situations and is comparable to the threshold the eye-brain system has to distinguish continuous motion from a series of single "frames". As for a human observer is important to grasp those few moment of relative calmness and as for a human observer is important that those few moments last a modicum of time to be fully savored. A number of limitations apply to the digital observing which do not apply (or at least do not apply as much). Focusing is much more critical as is tuning the various settings (gain, intensity) of the CCD employed. The amount of time which can be employed for recording a single shot is limited by the rotation of the planets. At any rate I think these limitations are a rather fair trade-off with the gain in using digital processing tools to enhance the final image. While a 1-to-1 comparison between the eye and an electronic sensor is not possible nor useful I do believe that the conditions impacting on the final image quality apply equally to one kind of observations as they do on the other, so that a fair comparison can be made between different optical systems, as far as high resolution planetary performance is concerned.
All images were acquired with the most suitable settings (frame rate, gain, exposure) for the task. I employed the same barlow lens for all pictures taken for comparison between the C8 and the TAL as well as for the MK67DL vs TAL2K comparison. The MN66 needed considerable more focal extension than can be provided by a 2x/3x barlow, so in that case I used a PowerMate 5x. The other barlow used were the TAL, an Ultima 2x and a Televue 3x. Post-processing was carried out using Astrostack for Mars and Saturn while Iris was used for all Jupiter shots. The steps taken in processing the shots was essentially the same (excluding color saturation) and was only limited by the amount of noise that could be left in the image. All images were scaled to have the same size (exception made for the C8 vs TAL images) regardless of the original size.
Under the Stars: TAL2K vs C8, Deep Sky Performance.
All observations were carried out from the periphery of Berlin (Germany) and there do not bear out the effective performance that can be wrought out from each instrument. They are however indicative of the effective differences that can be seen in each instrument. For the test comparison I've used a Vixen 8-24 zoom EP. While I understand this not to be the very best EP for deep sky, it is however good enough and provides the needed focal length adjustment to compare across system with different focal length while having optically the same EP. All observations were carried out in late summer and very early autumn. Zenithal limiting magnitude was estimated at about 5 - 5.5, good enough to see the Milky Way bands and M31 as a naked eye object. Object used for the comparison were M15 (globular), M57 (planetary nebula), NGC891 (galaxy) e NGC6939 (open cluster). I used through the observations magnifications ranging from 100x to 250x. The perceived limiting magnitude tests were carried out in the M57 star field . Results are as follows:
The C8 resolved marginally better M15 than the TAL2K showing more contrasty views between the faint star halo and the sky backdrop. M57 was, again, better seen in the C8 than it was in TAL2K, with the same faint stars (up to a visual magnitude of 13.9) showing up in the ocular but less straining the eye than for the TAL2K. The open cluster NGC6939 was faintly detectable at low power in both instruments but, again, in the C8 was perceptibly easier than it was in the TAL2K, despite the better field correction of the latter one. NGC891 showed slightly better the round halo in the C8 and no hint of the central dust band was seen in either instruments. After a night long observing session I came out convinced that the C8, visually, was a better bet than the TAL2K for deep sky observing.
Under the Stars: TAL200K vs C8, Planets
Both visual and instrumental test were mainly carried out on Mars during the past (2001) opposition with some comparison carried out on Jupiter and Saturn late in October 2001. All Mars observations were carried out with magnification around 300x-350x. Due to the low altitude at my observing latitude (excuse the pun) the planet was heavily afflicted by atmospheric dispersion with lower than average seeing thrown in for good. Both the TAL2K and the C8 are afflicted by diffuse light to a point were it is difficult to tell them apart at first glance. However, a more careful examination (Jupiter limbs at 220x, Saturn rings at 300x) shows that the TAL2K is somewhat more afflicted than the C8. On Mars both instruments showed the same amount of details at medium powers while at high powers the TAL2K showed a better color saturation of Syrtis Major with a faint trace of a violet color but a the expense of less detailed overall view, as shown in the pictures below. I haven't used the C8 as extensively as the TAL2K or the Maks so my experience in planetary observations is quite limited. What I have seen, however, is sufficient to draw a clear line between the two. When seeing is above average the C8 delivers a superior performance with respect to the TAL2K, although the difference varies depending on the subject. Mars and Saturn seem where the C8 shows a clear edge.
Mars - 12.04.2001 - TAL2K (left) and C8 (right)
Mars - 30.05.2001 - C8 (left) - TAL2K (right)
Under the Stars: TAL200K vs MK67DL, Planets and Deep Sky.
As far as deep sky stuff is concerned the superiority of the TAL2K, due to its 40% more aperture, was in no doubt. It amazed me, however, how the distance between the two varied from subject to subject with some like globulars showing nearly as good an image in the MK67DL as was in the TAL2K. M4 was obviously easier to detect in the TAL2K than it was in the MK67DL but, due to the far superior contrast shown in the latter, more pleasant to see (although less detailed). In same instances I had to go back and compare again with the TAL2K to be sure that I wasn't actually seen more in the Mak. As said before, field correction is about the same (although the MK67DL shows more "real" sky due to the shorter focal length), nevertheless the overall appearance of star field was superior in the MK67DL than it was in the TAL2K, this due mainly to the darker sky background (at the same magnification) pulled out by the Mak.
Visual performance on all planets I've tried both at shows how clearly superior is the Maksutov. The MK67DL show a clearly more contrasty view of everything I've tried it at. At higher power it shows more details although not considerably more than the TAL2K. On Saturn the Encke minimum was wider and more easily seen than in the TAL2K as well as better defined was the Cassini division all around the rings. The C ring (despite the bigger aperture of the TAL2K) was easier to pick up in the Mak than it was on the TAL2K. The main banding around the planet's globe showed to be easier by a tad in the MK67Dl than they were in the TAL2K. In the end on Saturn (magnification kept at 300x to 360x) it seemed as if a veil was draped around the planet in the TAL2K showing about the same details but fuzzier than in the MK67DL.
On Jupiter the superiority of the MK67DL was still evident but not as much as it was for Saturn. Details in and around the GRS were easier to discern in the Mak. The satellite shadows seemed more black and better defined at the edges in the MK67DL than in the TAL2K as well as WOS in the temperate zones and in the SEB. On the other hand, it was easier to pick up blue festoons in the TAL2K than it was in the MK67DL. Subtle details in the SEB/NEB showed easier in the MK67DL than in the TAL2K.
Mars was the subject were the perceived performance of the two was found to be closest. Yet even here the superiority of the Mak stands out, although by a smaller margin. The examples shown below demonstrate this quite clearly, I believe.
Mars - 19.05.2001 - MK67DL (left) - TAL2K (right)
Mars - 27.05.2001 - MK67DL (left) - TAL2K (right)
Jupiter - MK67DL - 3.10.2001 Jupiter - TAL2K - 3.10.2001
Saturn - 26.8.2001 - MK67DL (left) e TAL2K (right)
Under the Stars: TAL200K vs MN66, Planets.
They simply don't and can't compare to each other so far apart are the visual performance of the MN66 from that of the TAL2K. I'll leave to the images below to better make this out. I just want to remark that the subjective visual appearance in the two system was far, far superior in the MN66 than it was in the TAL2K. On any subject. Period.
Jupiter - MN66 - 13.10.2001 Jupiter - TAL2K - 13.10.2001
Jupiter - 19.11.2001 - MN66 (above) e TAL2K (below)
Looking back at all my observing experience with the TAL2K my conclusions are, stepping aside the technical
appraisal of the effective optical quality, that a C8 (or a similar SCT) is a better all around choice. It is far
lighter, much more versatile for whatever use, easier to collimate and optically above or at the same level of
the TAL2K. U.S. customers will also find it a more price effective solution than the TAL2K. As far as planetary
performance goes a MCT or a MN in the 6" range offer a far better solution in all aspects (mechanical and
optical) at about the same pricing, considering that the TAL mount is not worth its money. The only advantages
I've found with respect to the comparison scopes was the quicker thermal equalization and the insensitivity to
fogging. Too much for the asked price. The TAL 200 K was sold.