The Celestron C90 gives you nice optics for the price in a small and very portable package, but additional accessories are needed in order to be useable for astronomy.
The Celestron C90 is sold by Celestron as a spotting scope but has long been used by those interested in astronomy as a small and highly portable astronomical telescope as well. A long-running discussion on the telescope has taken place on Cloudy Nights since 2012: https://www.cloudynights.com/topic/358286-something-for-nothing-celestron-c90/
Ed Ting did a great comparison between the C90, Meade ETX and the 3.5" Questar here that is worth reading as well: http://scopereviews.com/90mmComparo.html
The C90 can be a great telescope for city dwellers since it can provide great views of the moon and planets in a highly portable package. The image below shows that you can take pretty nice images of the planets with a C90, and this was taken before all three of these planets were at opposition (when they are closer to Earth and larger in apparent size) so even better images are possible.
I picked up the telescope from Amazon back in 2019. (You can buy it here: https://amzn.to/2XiJwHI as an Amazon Affiliate I earn a small commission from all purchases made through that link) I've used the telescope several times since then, and have used it as a travel scope as well. The telescope is packaged in a roughly 16" x 9.5" x 6.5" box as you can see below.
The side of the box lists the telescope specifications. Note that the numbers related to eye relief and field of view relate to the included 32mm Plossl eyepiece. The 16" stated length is the length with the 45-degree diagonal and eyepiece attached. The total length if using an optional 90-degree diagonal is just 13.5" and without the diagonal, it is just 11.5" in length, very compact in size for travel. I measured all the dimensions as follows:
Length bare: 29cm, 11.5"
Length w/90 degree diagonal: 34cm, 13.5"
Length w/45 degree diagonal: varies with eyepiece length, but about 15-16"
Width: 10.5 cm, 4.2"
Weight bare: 1791 grams, 63.2 oz (3 lbs, 15.2oz)
Weight with included accessories: 2092 grams, 73.8oz (4 lbs, 9.8oz)
Here is the picture again of all the included accessories. The telescope does come with a nice, but basic, backpack that will hold the telescope and accessories, along with some space for additional items. Besides the backpack and telescope, you receive a 45-degree erect image diagonal for terrestrial use along with a nice 32mm Plossl eyepiece that gives you 39x magnification with a 50-degree apparent field of view. An 8x21mm erect image (the image is right side up as you would normally see) finder scope can be attached to the top of the C90 to help you find objects. However, in practice, that finder is not very useful because the eye relief is very short, and you have to have your eye pressed up against it to see the full field of view, which can be hard to do and it is not useable with eyeglasses. So, that should be replaced with a simple red dot finder for ease of use, or a different visual finder.
The visual back includes T-adapter threads for attaching cameras which is a nice added bonus. The threads on the telescope are not standard SCT threads though, so you have to buy an adapter to convert the threads on the C90 to standard SCT threads used on Celestron's C5 and larger SCT telescopes.
I use the telescope with a different set of accessories than those included. As I mentioned, the finder is not very useful, so I use a simple red-dot finder scope. Additionally, for astronomical use, I stick with a 90-degree diagonal. I use one of my available 90-degree diagonals, a Celestron diagonal is shown in the picture below. Additional eyepieces are also needed besides the 32mm Plossl. When I want a simple setup, I will just bring along a Celestron Zoom eyepiece, giving a nice range of magnifications. I also have several Televue eyepieces that I can use with the telescope as well if I want a larger field of view and higher overall quality of view, but with the added hassle of bringing along additional eyepieces.
Here is a shot of the telescope with the red dot finder attached along with the 90-degree diagonal and Celestron 8-24mm zoom eyepiece.
The C90 package I have didn't include a tripod or mount, though you can sometimes buy one with an included tripod as part of the package. However, that included tripod is usually too wobbly for use at higher power and something. I usually use the C90 with a Manfrotto 190X tripod and MHXPRO-2W mount head that I also use with my DSLR. This makes for a very compact travel setup and is useable at higher power. In fact, I used this same setup when taking the planetary images shown earlier on this page. I have an Explore Scientific Twilight I mount as well, and will use that mount at home when size and weight aren't as much of an issue. That mount also has slow-motion controls, which make the mount much more enjoyable to use at higher power. It can be hard to track a planet at higher power with the photo tripod and mount, though it can be done.
Here is a shot of the C90 on the tripod and mount head. This entire setup weighs just over 10lbs, making for an incredibly light-weight setup. However, the Manfrotto tripod and head cost over $300, more than the telescope itself. My cheaper and much more sturdy Twilight I mount cost over $200, about the same as the telescope package. The panhandle on the mount head is needed to be able to pan the telescope around smoothly.
Here is a look at the objective lens on the C90. The metallic spot in the middle is the mirror coating used to reflect the captured light back through the back of the telescope and to the diagonal and eyepiece.
Since the focal length of the telescope is 1250mm, the included 32mm Plossl gives a magnification of 39X (1250/32 = 39). A nice mid-range eyepiece would be in the 13-17mm range, and a nice high power eyepiece would be in the 7-9mm range. You could use an eyepiece as short as 5-6mm in focal length for high power views of planets, but the atmosphere may not support that high of magnification, and given the size of the objective and the amount of light that makes it to the eyepiece, your views will be pretty dim at that range.
So, how does the C90 perform? Well, on the moon and planets, the views are great given the size and cost of the telescope. 90mm of aperture does limit what you are able to see, and so larger telescopes will almost always outperform this one, especially on deep space objects. That is always the case though, so it can be easy to end up buying a telescope that is larger than what is easy for you to use. That is why most amateur astronomers end up with multiple scopes to serve different purposes. But, you can easily make out multiple bands on Jupiter, see the Great Red Spot, and watch shadow transits or view Saturn and see the rings with the Cassini Division. Mars will show nice detail during favorable oppositions, like this year in 2020. Since the C90 doesn't have the chromatic aberration that most cheaper refractors have, you can get better views of planets like Mars and Jupiter than with similar aperture achromatic refractors. However, higher prices APO refractors of similar aperture that can focus red, green, and blue light together, instead of just two of those, will outperform the C90, though at several times the price. Keep in mind that the C90 is sold as a spotting scope, and it does well in that regard as well.
While sharp, the views through the C90 are noticeably dimmer than a Stellarvue 90mm refractor I once owned (and have always regretted selling) and seem closer to what you would see in a 76-80mm refractor at the same magnification. I haven't actually tried to measure that, but that is about what it seems like visually. The light transmission of a C90 is going to be lower than for a 90mm refractor since the mirrors and meniscus lens each reflect or transmit less than 100% of the light passing off or through those surfaces. Further, the obstruction caused by the secondary mirror spot blocks some of the light from passing through the telescope. So, what you have is a telescope that gives you the resolving power of a 90mm telescope, but the light transmission of an 80mm range telescope. So, if you are trying to view nebula or star clusters, the views are going to be dimmer than you might expect. That isn't much of an issue for planets or the moon since they start out very bright anyway.
Another issue is that the C90 can only give you about a 1.2 to 1.3-degree maximum field of view using a 32mm Plossl eyepiece or a 24mm widefield eyepiece like a Televue Panoptic. Some people have used an adapter to put a Celestron reducer lens on the back of the telescope to produce a wider field of view, but one that will also vignette the view some. My 8" EdgeHD telescope also maxes out around 1.2 degrees with my 31mm Nagler eyepiece. But, being an 8" (200mm telescope) instead of a 90mm telescope, the view is noticeably brighter. That is, you will see many more stars in the same sized field of view. Of course, you can't toss an 8" SCT that weighs 12 pounds without accessories in a backpack or carry it around a tree-filled backyard with one hand either. Still, that narrow field of view the C90 provides versus the wider field of view a similar refractor would provide can make it harder to find objects in the night sky.
So, should you buy a C90? Well, the low cost of the kit, nice optics, and compact size make it a great option for some uses. At $230 regular priced (often under $200 when on sale), the entry price is low for those who already have other accessories and a suitable mount for the telescope. But, if you need to pay for a red dot finder, 90-degree star diagonal, and additional eyepieces, the $230 telescope is going to jump to about $370, not including the mount. That will set you back another $100 at a bare minimum and more like $200-350 for something robust that will work well for you. Suddenly, the $230 telescope turns into a $500-700 telescope. For that money, you are probably better off buying a nice 8" Dobsonian telescope that will show you far more than the C90 will, though at a much larger and harder to move around size. You could also buy an Orion XT4.5 Dobsonian for only $50 more (link below), and not need any additional accessories to get started. Another option would be the Sky Watcher Virtuoso which features a nearly identical telescope to the C90, but on a tabletop mount: https://amzn.to/2XfHB6T
So, if you already have other telescope accessories you could use with the C90, and already have a mount you can use with the telescope and are just looking for something that is rugged and highly portable for quick peaks at the moon, planets, and bright DSOs, then the C90 is a great option and worth adding to your arsenal. But it may not be the first telescope to start out with unless the astronomical aspects of the telescope are secondary to
C90 Kit (Amazon): https://amzn.to/2XiJwHI
C90 Kit (Highpoint Scientific): https://www.highpointscientific.com/celestron-c90-spotting-scope-52268
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