Celestron C8 and useful accessories: my experience. Part one

Over the past few years, I've owned several examples of the popular Celestron C8 (203mm f/10) Schmidt-Cassegrain telescope. For purely personal reasons, my current setup consists of an older C8 Ultima series optical tube mounted on yet another Celestar fork mount, featuring a wedgpod—a very simplistic but lightweight system popular in the 1990s, which features an equatorial support integrated into the tripod.

This mount dampens vibrations worse than the mount supplied with the ULTIMA series and also compared to a classic equatorial mount, such as the Synta EQ-5 or the old Vixen GP.

I did, however, come up with some simple remedies that enabled me to quickly put this twenty-centimeter diameter catadioptric telescope to use.

I suggest you reread my review of the Celestar 8 at the end of this article, which I wrote back in 1998. If you do, you will be able to gain more insight into the true performance of this telescope.

I would, however, like to write down some tips that will allow you to make the most of one of the best-selling optical tubes in the world.

Compared to the years when I purchased my first model (1996), there are dozens of very interesting optical and mechanical accessories available on the market, moreover offered at more affordable prices, compared to what was available only fifteen years ago.

The C8 on the Celestar mount is a featherweight, just over sixteen kilograms: in fact, it is possible to lift it without particular problems, carry it from one part of your garden to another and even go up and down stairs.

I usually keep it at home and take it out into the garden a few hours before dinner.

To try to improve the vibration dampening capacity, I purchased specific rubber feet, although I can confirm that the ones obtained from the rubber soles of beach slippers can serve the purpose.

Damping times are significantly improved. There are two other options that I've abandoned, but which are still easily achievable: inserting sand into the tripod tubes, or attaching two sand-filled anklets (found in sports shops) to the fork legs to slightly dampen vibrations.

On the other hand, since my main interest was to lift the largest telescope I could afford, I preferred to proceed in two ways, unfortunately more expensive but which in the end proved to be effective as well as capable of increasing the value of the telescope.

I purchased a JMI "Moto Focus" electric focuser (which can also be found on the used market). Thanks to this accessory, specifically designed for SCs, I've solved most of the problems caused by the "Celestar" tripod + wedgepod, as the most significant vibrations were generated during focusing. It can be set up in a few minutes, the batteries last forever, and it completely solves the problem of vibrations caused by manual focusing.

In addition to this remedy, I decided to perfectly balance the optical tube, constrained by the fork arms. For this reason, I used of the excellent kit designed by Geoptik (Also available for C9 1.4 and C11). A system as simple as it is intelligent: a small bar on which a weight slides that moves quickly, loosening the small tightening knob that prevents it from sliding.

 

 The motors have finally started working on both sides without any effort, and I can also track objects at medium to low magnifications even without the motors by unlocking the axes and rotating the telescope by hand.

Writing about engines, I can only mention the one in declination (optional at the time) that I purchased on the used market to give a touch of completeness.

Telescope setting

I use two methods. Usually, in the summer, I take the telescope outside before dinner, place the optical tube with the corrector plate facing the lawn, and use a micro-perforated cloth over the eyepiece holder to let the warm air flow out. In the winter, however, when the temperature difference between inside and outside is high, I use the well-known system: Geoptik astrocooler which now appears to be out of production.

 

I think it's a good product, it has never caused humidity or dust infiltration and in a few minutes the C8 is ready to be used, I turn it on often even when there are sudden changes in temperature during the evening.

 

Lunar and planetary observation.

Schmidt-Cassegrains, due to their high secondary mirror obstruction, are certainly not the most suitable telescopes for observing low-contrast planets, such as Jupiter.

For this reason I profitably use a binocular viewer, I have noticed, in fact, that despite a greater perception of the chromatic residue and a slight increase in diffused light, I perceive planetary and even lunar details more easily. On Astrotest.it you can read a in-depth article regarding two commercial binocular viewers.

 

After several tests I found the best compromise between price-performance, lightness and quality in the following system: Baader Zeiss prism with connection to the SC breech, + Baader Max Bright viewfinder and eyepieces Baader Classic Ortho 18 mm.

I believe it's pretty pointless to use higher-quality eyepieces, because any optical accessory inserted before the eyepiece always—in my opinion—results in some degradation in the overall quality of the image displayed in the field lens. For this reason, I use higher-quality eyepieces only for direct viewing.

 

Deep sky observation

Another very interesting eyepiece that makes the C8 a useful instrument for deep-sky observations is the Tecnosky Ultra Flat Field 30 mm eyepiece. I've tried several in similar price ranges and I think it's one of the best value for money. It's an eyepiece composed of nine lenses in five groups, all multi-coated on every surface. The apparent field of view is 75°, it's not particularly heavy—less than 600 g—and it has a comfortable 22 mm eye relief, which is very relaxing. With this eyepiece, you can barely exceed 65X, and the exit pupil is still perfectly usable, as it is 3 mm.

I compared it to various products in this price range and chose it for its greater sharpness both in the center and at the edges and for the better control of geometric aberrations. M42 under a beautiful high mountain sky is always fascinating, even when using a commercial SC lens opened up to f/10.

With this accessory I use various types of mirror diagonal, an old William Optics, dielectric, a more recent Excelsis with a specific attachment for SC and the Geoptik connection for Schmidt Cassegrains with the new Auriga diagonal which is an excellent purchase, for optical quality based on the purchase price, the fine finishes and the useful system for attaching the eyepieces, through a collar fixing system.

In the next few weeks I will also tell you about the new Auriga eyepieces that I am testing these days, observing Jupiter and the Moon.

Ultimately, after twenty-one years, I still find myself singing the praises of a classic that was born years ago.

Whatever anyone says, a second-hand C8, perhaps a fork-mounted one, is a perfect instrument not only for starting to observe but also for carrying out astronomical observations of a certain level.

Rather than spending money on small apochromatic refractors, I think finding a good one on the used market is a great idea. It's not a specialized instrument for any specific purpose, but if you're a visual observer, I think it's one of the most portable and usable telescopes, even on evenings with average seeing conditions.

I now leave you with my old review in the hope that you can glean more information about the performance of this telescope.

 

Celestar 8

test of Piergiovanni Salimbeni 
February 3, 1998

 

Back in 1954, Alan Hale and Tom Johnson designed the first Schmidt-Cassegrain, which was launched on the market in 1966, after some refinements. It was a splendid 200mm model with a focal length of 2000mm. 
Its latest successor, the Celestar 8, traditionally maintains the same characteristics: the primary lens is f/2, and the secondary lens consequently magnifies five times. The focal length allows for a compromise between the considerable obstruction and the usable field. It is highly inconvenient to attempt to reduce the obstruction, which is also considerable due to the internal lens hood located in the center of the primary lens. If you were to go below the 25%, the internal lens hoods could cause severe vignetting. 
However, with a 0.34 shutter speed, our C8 performs very well when compared to other commercial SCs. 
When I decided to purchase a new telescope, I was particularly attracted by its exceptional portability. Consider that, with a featherweight of just 17 kg, it allows you to easily move it from one part of your observing site to another or fit comfortably between your knees (obviously, just the tube with the fork!) while a friend drives. You can even use one of the two polystyrene cases that wrap the C8, in the box delivered by Auriga, as an economical suitcase.

SETTING 
The all-aluminum fork has proven reliable in purely visual use, the only flaw being the inability to adjust the height of the tripod legs, a prerogative of the more expensive Ultima 2000. This forces the observer to remain seated in most cases, and those who prefer direct observation will have to kneel on the ground or assume a contortionist pose when shooting subjects very high above the horizon. 
The right ascension knob is very reliable, the same cannot be said for the declination knob, as after a few hours of use it requires a good adjustment with a small pair of pliers. 
Disassembly is very quick: there are three screws under the equatorial base. By unscrewing the two lowest ones, you can easily remove the telescope from the tripod, making sure to slightly loosen the first screw. The entire operation can be performed by a single person, since the tube with the fork weighs only 10 kg. 
The ascension motor and hand control are included in the price. The hand control, unlike what I read in another industry magazine, allows not only fine adjustments for astrophotography, but also for taking a quick tour of the Milky Way or from one crater to another. Just be sure to hold down the left button and then press the right button to allow the motor to move quickly to the right, or vice versa to move left. 
Not being a keen astrophotography enthusiast, I couldn't verify the accuracy of the tracking, but when observing the moon and planets at 300x or higher magnification, I've never encountered any problems, as long as the telescope is properly positioned toward the celestial pole. The declination motor is optional. If you don't want to use a nine-volt battery, you can plug it into an outlet using the optional power converter, which allows you to use a twelve-volt battery.

COLLIMATION 
I remember trying it only once. The telescope had become slightly misaligned after a two-month stint on a very bumpy road leading to one of my favorite spots. Commuting amateur astronomers and Newtonian enthusiasts, repent! 
As for the adjustment, the procedure is the same that Alessandro Bertoglio had to carry out on his 10″ Meade (see ”Il Cielo” n. 4). 
After properly acclimating the telescope to the outside temperature, observe the out-of-focus image of a star very high above the horizon at high magnification, at least twice the objective diameter in millimeters. If the diffraction rings are not concentric, adjust the screws on the secondary mirror. I recommend working carefully, as the three screws are very sensitive. 
The primary mirror tends to fog up, especially on humid nights. I highly recommend purchasing or building a dew shield.

ACCESSORIES 
They are certainly poor, with a medium-quality diagonal and a 6×30 finderscope that makes observing dim objects quite difficult. There is also a 25mm Kellner which, apart from insufficient performance at the edges of the field, does its job fairly well.

VIBRATIONS 
Observing Saturn and the Moon with a 10mm plossl and giving the telescope the classic tap, I calculated a vibration damping after five and a half seconds. 
 

MOON AND PLANETS 
A Schmidt Cassegrain certainly cannot offer the same images as the Maksutov Cassegrain of my friend Raffaello Braga; one could significantly increase the contrast by building it with a focal length of 3000 mm, but this would deprive the Celestar 8 of its traditional universality. 
However, I have learned a lot using this instrument: first of all, compared to a commercial Newtonian of the same diameter, it offers greater contrast thanks to the tubed optics, which allows for total protection of the mirror coating and sufficient insensitivity to turbulence. 
Compared to a 10cm refractor, especially a Vixen, on windier evenings the refractor offers more contrasty images, but on average and perfect evenings (very rare, unfortunately) it offers a superior quality of detail and contrast that I calculate to be about the 25%, although disturbed by a slight shaking. 
But let's see the performance on the moon: 
 Rima Alphonsus is clearly visible at 200x magnification, and the small pyroclastic craters inside the crater of the same name are also easily observable. 
The Rupes Recta, the Birt rime, the Galilei rime and the Marian rime are almost always easily visible, especially through a light yellow filter, which increases their contrast. 
I have never managed to observe the Sheepshanks rhyme and the Suess rhyme, while the rhyme within the Vallis Alpes has only ever granted me the honour of being observed 3 times in 8 months. 
Only on rare occasions have I been able to profitably use the 400x magnification; normally, our satellite can be observed between 250 and 300x. Most of the time, you can see details not even present in the excellent Rukl atlas, such as the micro-craters inside Theopilus.

PLANETS 
Seeing is also crucial in this case. 
On the evening of September 4, 1997, from 9:45 PM to 10:00 PM UT, observing Jupiter with a 10 mm plossl telescope, the following details were visible with a little attention: 
The red spot, which was characterized by a slight difference in color in the northernmost part, the SPR, the STZ, the SEB, the NEB, the NTB, the NNTB, and finally the NPR. At times, the EB could be observed.

Saturn, on the evening of October 22, was very interesting: the Cassini division and the subtle color differences in the B ring were clearly visible, the discontinuity near the A ring was also visible, but the Encke division was invisible.

 During last spring's opposition, Mars often proved a rather difficult subject, due to a tedious turbulence that made observations difficult. However, both the polar caps and key features, such as the Sea of Sidra, were consistently visible at 200x magnification. 
 

DOUBLE STARS 
On winter evenings, it's very pleasant to observe the four stars of the Trapezium in M42 and the fifth small star, called F, at magnitude 11.5. Rigel's companion is also very beautiful, aided, however, by a light green filter to dull its larger sister. In summer, with a 40 mm Plossl and a clear sky, you can observe the yellow and blue hues of the double star Albireo in the constellation Cygnus.

Speaking about the ”performance” of the C8, I too managed, like Mr. PG BARBERO (see ’Il Cielo“ n.4 page 63), to separate the Andromeda gamma, a 0.55″ double! This was thanks to a 2x Barlow, a 10 mm Plossl and a very favorable evening.

DEEP SKY 
I'll only present observations from a site with naked-eye visibility of the fifth magnitude. Obviously, in much darker skies, many more objects will be visible, but by saying this I might disappoint amateur astronomers who usually observe in their backyard or on a mountain a few kilometers from their hometown. 
At this time (January), M42 can be observed with great satisfaction using an oxygen III filter; the view is almost photographic. However, M1, the Crab Nebula, is visible without a filter as a faint, round cloud. I must say that it is particularly impossible to see it in the finderscope. M57 has poor contrast, while at 81x magnification, M31's companion, NGC 205, can be seen. M81 and M82 are also easily observed, while M51 and M101 are very faint. I remind you, however, that all of these can be observed from your backyard and without any LPR filter. 
As for star clusters, M13 appears very beautiful and fully resolved, while for a proper view of the Perseus double cluster, you'll need at least a 40 mm eyepiece, or even better, a focal reducer, which would also allow you to observe the Pleidians in full field. Under polluted skies, however, using magnifications in the order of fifty times brightens the sky background too much, making it difficult to observe. This, however, doesn't interfere with viewing galaxies under high mountain skies; in fact, it makes them even brighter.

CONCLUSIONS 
For its price (€2,065 in 1998), the Celestar 8 offers excellent portability and excellent anti-reflection coating. For lunar and planetary observation, performance depends heavily on turbulence. However, on windier evenings, it is possible to use the 150x magnification with reasonable results, also using filters, which in my experience slightly reduce turbulence. 
Even focusing does not create any particular problems of image deterioration, perfectionists will still be able to successfully apply a rack and pinion focusing, after having fixed the primary lens in the ideal position. 
Some improvements could be made: extendable legs would be very beneficial for amateur astronomers' backs, and a small attachment to hold the hand control would be ideal. 
Ultimately, this SC is a good all-purpose instrument; those looking for specific performance will have to look elsewhere. It's more comfortable to use than a refractor, all the adjustment knobs are easy and straightforward to use, and observing on a nice night won't make you miss more expensive telescopes. 
 

TECHNICAL DATA SHEET 
Diameter 200 mm 
Focal length 2000 mm 
obstruction c. 0.34 
Estimated limit on double stars 0.6 (in the evening exceptional Andromeda gamma doubled observing the diffraction image. 
theoretical limiting magnitude 13 
magnitude reached 13 
minimum focus 10 meters with 25mm Kellner and diagonal

CONTRAST AND RESOLUTION TEST 
MOON: Gassendi: Only the rims east of the crater are visible 
 Birt: rhyme Birt, Birt C, F,E,H 
 Alphonsus: the rhyme visible only on evenings with little turbulence, 
 pyroclastic craters visible. 
 Plato: the five internal craters are visible during the full moon phase. 
 Rima Archytas: visible 
 Rima Vallis Alpes: rarely visible 
 Calippus Rhyme: visible 
 Sheepshanks Rhyme: NO 
 Rima Hesiodus: visible 
 Rhymes within Pitatus only those at 23°W

 

ABERRATIONS PERCEIVABLE WITH THE STAR TEST
spherical absent 
absent astigmatic 
absent coma 
tensions absent 
collimation from the factory quite precise

AIRY'S DISC
the diffraction rings are very bright

RONCHI NETWORK
 the fringes show very slight spherical aberration.