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Celestron CPC800 8" SCT
Submitted: Friday, 25th March 2011 David Roser

This review is my 'first light' impressions of a recently purchased Celestron CPC 800 50th Anniversary Edition, ‘versus’ comparable Dobs and SCTs. The comments relate firstly to the telescope itself and secondly to the ‘purchase context’ which determines the value a user gets out of a telescope as much as its engineering specifications.

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Celestron CPC800 8" SCT

The Telescope

Basics

The Celestron CPC-800-50 is a 203.2mm (8") Schmidt Cassegrain with GOTO technology (f/10, 2032mm focal length). From my reading the basic CPC-800 has been available since 2002/2003. It employs a GOTO controlled Alt-Azimuth mount but there is an optional wedge which can turn it into an equatorial instrument. Why purchase the 50th anniversary? It was slightly spur of the moment. I intended provisionally when I went into the shop to get an LX90 but in the end opted for the 800-50.

At the time of writing the cost was ca $3200 (Bintel and Andrews) v. $2900 for the basic CPC model reflecting the wild (and currently beneficent) exchange rates between the US and Australia and China at the time of writing (Oct 2010).

The main difference to the basic model CPC 800 model appears to be its use of carbon fiber tube, a cast metal lens holder which is excellent for suspending a 12 V battery, the inclusion of 5 Celestron objectives between 32 and 4 mm and a set of 7 filters (6 dye/colour and a moon filter). I also considered the Meade LX90 or LX 200 and mulled briefly over the clearly less sturdy Celestron SE and Meade LS series. The LX90 (and LS series) appeared substantially less stable than the 800-50 on display and the CPC 800 was not much more in price than the LX90. In hindsight I didn’t fully consider the alternative of a basic CPC-800 however. The LX 200 appeared superior but was more expensive (prices have come down since then of course). Accessories I purchased with the telescope included a dew shield, heater and 40X wide angle 2 inch eyepiece. The unit was available ex-stock and came in two packages, the telescope+mount+motor drive and the tripod.

Trial assembly from the box took 5 minutes. The battery power pack is oddly an optional extra though that suited me. The telescope can be operated manually but you could get the same basic optical tube via the SE series and get GOTO or use a much cheaper Dobsonian. So there seems no point? Perhaps like me many people have a spare 12 V battery lying around and want to save a some money.

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Before assembly

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After assembly

The images above show the before and after assembly of CPC (on floor) with 12” Lightbridge and chair for size comparison.

  1. Ruler is 30 cm. It is located on the floor (left) and eyepiece holder (right)
  2. Black forward part of tube is a dew shield.
  3. Battery is suspended beneath eyepiece holder in plastic container.
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Close up of CPC from viewing end showing battery, eyepiece holder, mounting with computer, hand-controller and finderscope

Field Assembly and Transport 

An appealing 1st feature is the convenient size transformation which doesn’t compromise on stability. Fully expanded the 2m tall telescope dominates a small living room and doesn’t wobble even on a polished wood floor. Broken down it forms two cylindrical units of 85x20 cm ( tripod) and 60x20cm (scope assembly) (Figure 1).

These can easily be stored in a cupboard or sit on the back seat of a small car (Astra) without fuss leaving plenty of room for accessories, chairs and tables, 2 passengers and food/drink.

Manual transport of the 8 kg tripod in the field and at home was without fuss. The legs are secured with a clip strap at the extension point. Closer to the apex is a big triangular solid cast aluminium accessory tray which locks the legs tightly pre and post assembly which older reviews indicate was once plastic. Leveling of the tripod is done first using the inbuilt bubble level and leg clamps located on each leg. The tray is solid enough to suspend my 4.5 kg 12V 13AmpH deep cycle lead acid battery immediately underneath adding to stability.

The scope/mount is equally refined. To manually carry the scope there are two handles. The starboard handle is designed so the scope can be carried like a piece of luggage. To assemble, the telescope is lifted using the two handles and the base of the mount is lowered onto a central pin, rotated slightly until three further pins engage and finally locked in via 3 screw clamps.

Weight was crucial for me as every set up requires carrying the telescope and related accessories. The importance of this is confirmed in many reviews and is reflected in the construction of trolleys for moving even within a backyard. The 800-50’s 41 lbs (19 kgs) appeared to be the most I can currently manage without risk of back strain or dropping. It contrasts with the 58 lb and 65 lb mass of the CP925 and CPC1100 respectively which use the same tripod and compares with the 51 lb limit suggested in California as a maximum repeated lift limit under ideal conditions (http://www.dir.ca.gov/dosh/dosh_publications/mmh.pdf). What’s in another 20 lb? For non weight lifters and older people a lot! The bigger the scope+mount, the more stable and the more light gathering power it has and I can lift 30 or 40 kg alone or with help if I need to. But if you routinely want to grab the telescope from the cupboard, and go to the bush or backyard, an extra 50% is a lot. You need to be able to carry it gently tens of meters without tripping in the dark and lift it to chest height and position it over the tripod’s centering pin while bearing the full weight. Think of lifting airline carryon baggage (10 kg limit) and the weight of your stuffed suitcase (20 kg limit) and you get the picture. The 800-50 is at this comfort/security limit in my older 185 cm opinion.

Alignment On-Site

So now the basic unit is assembled. Moving the tube pre alignment is simple thanks to the vertical and horizontal clutches. The controller slots into a holder on the port side where it can be used hands free if you don’t mind a little movement. The package comes with a 50x8 finderscope which is easy to attach and align. It has good optics but no right angle making sighting near the zenith hard.

A big selling point of the current CPC 800s is the in built GPS based alignment system ‘Sky Align’ additional to other GOTO options which require some knowledge of the constellations. Basically you turn it on, it figures where and when it is (can take a while) and you point it sequentially at three widely separated bright objects (moon, planets, stars) whose identity you don’t need to know or search for in the database. I used the moon, a bright star at about 45 degrees which proved to be Achenar and a bright star in Scorpius’s tail (probably theta). As ‘Star Align’ worked first time I didn’t fiddle with the other options. Alignment proved a little out subsequently but the discrepancy was consistent indicating I, not the telescope, was the problem. The manual says it should take a long time to get the GPS fixes but in my case it seemed virtually instantaneous.

Viewing

The first thing to look at was the half full moon at about 60 degrees elevation. With my Televue 32 mm+moon filter (70X) it was excellent but under my 8mm Orion Stratus (200X) seeing was good only sporadically due to air turbulence. Scorpius and Sagittarius were the easiest targets for DSOs in an inner city backyard floodlight by a back lane light. After selection it moved using GOTO between objects in 10 to 60s. The selected Open clusters (M6 and M7), Globular Clusters (M22 and M55) and nebulae (M20, M8) were immediately recognizable. Color differentiation between stars in M6 was clear.

A little later Jupiter was above my horizon. The 800-50 gave good images with the Orion 8mm and the Meade 15mm wide field eyepieces. Despite the suboptimal seeing conditions banding was clear especially when accentuated by a light green filter. All four Jovian moons were close to the planet and their discs were discernable. Finally I keyed in Uranus and Neptune. Both appeared in short order where I expected they should be in the field of view. Uranus’s color had the characteristic light green yellow tinge by way of confirmation.

Subsequently I managed a somewhat better session at a spot in the Blue Mts marred by trees at the viewing site. The Jewel box fitted its name. The resolution of Acrux as a double was much better than with the Lightbridge in the past probably due to alignment The Nebulosities of M42 and C92 were distinctive despite there still being light pollution from Sydney.

Operating Aspects and Limitations

I didn’t use the dew heater as the dew shield was sufficient as well being an excellent block for stray light. The mount was stable. The battery and leads did not tangle when the mount rotated. I had a minor problem in the mountains with the unit wanting to travel the long way to a star leading to a wire tangle and the need for some manual slewing. Not sure why this happens. Viewing objects near the zenith was easy as with low elevation objects. Having a 13Ah battery allayed my concerns about having sufficient power to leave the system running for several hours (and in the future running a dew heater).

The Celestron eyepieces were disappointing alongside the flat field wide angle eyepieces I already owned. I detected aberration at the field edges which is reportedly less of a problem with the Meade LX optics, but it was not a major concern. The filters were a more useful addition.

An unanswered question was whether there was significant advantage in having a Carbon fiber tube? The main rationale seems to be lightness and carbon fiber not changing shape or size with temperature. The reviews are as unclear about the latter as am I. My guess is it might be more important for photography. Either way my hope that the system would provide a good balance between having large light gathering power and being, easily transportable and storable, appear to have been borne out.

A limitation of the current Celestron inbuilt GOTO is you cannot access stars based on their Bayer or Flamsteed designation but must use their name if available (too few) or their SAO designation (lots and uninformative). It’s a bit like Goldilocks. Computer control is one solution. If you prefer hard copy and have favorite guides there are a range of resources to help at Asterism.  My first solution which addresses the SAO issue was the compilation of Robin Gatter of 2 medium sized indices for stars (37,000) and DSOs (21,000) in the form of downloadable ASCI files. The two can be loaded into a single MS Excel table and searched/summarized selectively using the autofilter and pivotable utilities and the SAO designation of a star by constellation and Bayer designation can be cross matched. The trouble is this is clumsy as you need to have the damn computer handy and power it. I will shortly see if a Skyscout is the way through this one.

Future Options?

Having become a bit of an equipment collector I did consider when buying the scope “Where next?” and the big question of might I indulge in astrophotography? Maybe in the future but the current reality is much more modest. Firstly as all good serious-photographers an Alt-Azimuth GOTO can follow the view field but it cant follow the angular orientation of the stars. Also if you want serious Malin prize winning pictures comparable with Stromlo in its early days you will need a CCD camera in the region of $5K.

The native 800-50 simply does not have this capacity because it is not equatorially mounted. However it does appear sufficient to assess whether you might be interested, before outlaying vast sums, provided you are would be satisfied with happy snap photographs with something like a Meade DSI II (another collectable). The light levels for the moon and planets are blinding so short duration exposures are possible. For nebulae and star fields there are three further alternatives. Firstly it is feasible to piggy back my small refractor (an EON 72) on the main tube. Secondly the main SCT tube has ‘Fastar’ compatibility where a camera is mounted at the first focal point of the main mirror. Thirdly there is the option of an equatorial wedge. The first two have the effect of greatly increasing the field of view imaged, and the light intensity per CCD pixel while reducing the impact of shake and tracking imperfection. The third is notionally optimal but would clearly reduce the setup convenience. Note that  all three incur significant additional costs. The piggy back telescope mount looks most interesting as it would not compromise the existing unit and would only require a couple of mounting brackets.

In regard to computer control and photography netbooks draw only 15 W so they can probably be powered easily. Personally though, I find computers a nuisance and reminder of work so have so far avoided their use and the hand controller seems sufficient.

Viewing and Purchase Context

Observer needs and circumstances are as diverse as equipment designs, brands, size and price to name a few factors that inform any review and purchase of a telescope. The following summarizes mine which I’ve included because it definitely does impinge of this and all other reviews.

First Light

For experienced observers seeing my small bag of objects over a period of an 1 hour would not be remarkable, however….

  1. The telescope was located in a postage stamp backyard in Sydney’s inner west with just 10 m2 to open sky area currently,
  2. the moon was bright, half full and near its zenith,
  3. the horizon was elevated in all 4 directions to about 45 degrees by two carports(one now demolished), a 2 story terrace and 5 m orange tree,
  4. a fluorescent street lamp is located on the west boundary at an angle of 50 degrees (5 m horizontal, 6 m vertical),
  5. the sky generally is illuminated by sodium vapor lamps from the nearby airport and Port Botany,
  6. atmospheric disturbance around the moon was impressive in its own right and this was evident with the Raleigh rings at high magnification.

Despite these limitations these objects were rapidly viewable and identifiable under classically rotten viewing conditions, even for where I live - these were though the first convenient clear sky conditions in 2 weeks so. Admittedly M20 was ‘marginal. But from previous viewing its clear 1) backyard renovation, 2) moonless nights, 3) stiller air, 4) proper dark vision acclimation, will make things much better than such suboptimal, even by suburban standards, seeing.

On the night, despite the appalling location, several things made viewing as memorable. Firstly there was seeing Uranus and Neptune for the first time. I’ve seen the four easy ones, Venus, Jupiter, Mars and Saturn aplenty. Theoretically I could have found U and N in the past but would I have been kidding myself? GOTO provides confidence that what you see is not just what you want to see.

As importantly the effort required for set up and object to object traverse was trivial – so more time was available looking and later returning and less in frustrated searches. And of course it stayed put. Importantly this helped in linking reference map/descriptions/pictures/finer points to GOTO instructions to the images in the eyepiece to the constellation. And it took no time to show my long suffering wife how to use and find objects.

Comparison with other Telescopes

How does it compare with other telescopes? For mass produced 8” SCTs I haven’t detected anything in reviews that suggest much difference for someone like me in respect to the optics. The price differences are also marginal could be seen thus as the telescope equivalent of car accessories. However, there is a little more too it. The 800-50 seems about as optimal for my needs and more importantly location constraints. In hindsight for example the inbuilt GPS seems to me like cruise control. Unnecessary to see a DSO or photograph it but without it I wouldn’t be able to exploit the limited high quality viewing opportunities I seem to get.

Regarding larger SCTs I have to say one is tempted – but there are 3 other light limiters that I consider mean viewing opportunity is as important as light gathering power. Firstly there is optical thickness of the atmosphere. Between the zenith and 45, 30 and 15 degrees there is a reduction in radiation, especially in the green and blue of ca 20, 30 and 60%. (if you are interested in precise wavelength details see.  GATES, D. M. (1966) Spectral distribution of solar radiation at the earth's surface. Science, 151, 523-529.). Then there is light pollution which reduces seeing by a factor of ca 10 (magnitude 3.5 in city v. 6.5 in country). Finally there is eye sensitivity (logarithmic) and dark adaptation which in my case takes >1 hours. Ideally you want all of these and a big mirror but the point here is I’m dubious the sacrifice of size for ease is best for a city observer.

What about cheaper observing options? I’ve looked through the 12” Lightbridge ,and a friends No Frills 10” fixed tube Dobsonian in sky brightness zones ranging from 2 to 8/9 (see above). The 12” and 10” was of course marginally more sensitive but they were trumped by the more difficult set up and transport and time needed to find objects and the problem of tracking when you are in company. The images weren’t as crisp either. These experiences suggested that GOTO with a good instrument isn’t simply an optional extra if you are time limited it’s a must.

Don’t get me wrong. Dobsonians are excellent value for money in skilled hands. I don’t regret having got one and haven’t sold it, and if I was thinking of something to show older children the skies, a 6 to 8” Dob would be at the head of a consideration list. However if you want to learn the species of things in the skies but don’t have much time to get into the rainforest then an 8” SCTs is hard to go past.

Conclusion

From reading the blogs there are many people, enchanted by the romance of the stars, who discover the capabilities of modern amateur instruments and get the bug but are uncertain about what their best choices are and where the balance between spending a lot on GOTO v. big mirrors etc.

I am most interested in basic naked eye observing and getting good views of DSOs if possible. In hindsight if, I lived or spent a lot of time in a country or rural area country I would be I think satisfied with the big Dob I bought 3 years ago. However as I am trapped in the big city I have come to the conclusion that I need something different. The city has its benefits – like convenient telescope shops - but it often means one is time poor. When you combine this with the problem of viewing condition impacts – moon, clouds, light pollution, limited access to dark sky, and the need to be social with family and friends – you need something different. I suggest the CPC 800-50 is a telescope that possesses the following requirements I’ve learned the value of - good light gathering power combined with compactness for storage and transport, no or little need for realignment, can exploit sporadic viewing opportunities, is socially/domestically unobtrusive, is rapid to set up and dismantle, is easy for complete newcomers AND it still impresses.

Disclaimer

I have not links with any manufacturer or distributor of telescopes or related equipment and the opinions expressed here are entirely my own.

Review by David Roser (Newtownian). Discuss this review on the IceInSpace Forum.
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