by Brandon Wood
Some recent online comparisons between the Meade ETX90 and Orion 90mm Mak-Cass telescopes motivated me to capture my thoughts about another small telescope that warrants attention if you are looking for a inexpensive, but capable, grab-and-go telescope. Before diving into the details, I’ll point your attention to Guy Brandenburg’s review of the Meade ETX-60AT. Except for the obvious differences in aperture, I suspect similar comments could be made about the ETX90. I have never owned one and have only briefly looked through an ETX so I cannot comment on its merit. I can comment on Orion’s smallest dob, the SkyQuest XT4.5. It is one of the great values in amateur astronomy. Consider yourself warned; use your Back button when appropriate.
The Orion SkyQuest XT4.5 is the smallest in Orion’s series of Dobsonian-mounted Newtonian telescopes. It uses an f/8 4.5″ spherical mirror giving a focal length of 910mm. Orion includes a 10mm and 25mm Plossl eyepiece and a 6x26mm correct image finderscope. Their price is $199 plus shipping, and their marketing clearly aims this telescope at a young astronomer; but I have found it to be very entertaining as a second telescope.
Before I describe the XT4.5, you need to know why I decided to buy it 6 months ago. I had purchased an 18″ truss dob and included with it was an 80mm short tube refractor that I was going to use as a finderscope. I soon realized that my 18″ telescope was noticeably intimidating my 4-year-old son. He seemed to enjoy the time outside looking up, but had little desire to look through the eyepiece. One evening while camping, he warmed up to a camp-neighbor’s small telescope. I decided to “give” the 80mm refractor to him so we could play with it using a 45 degree correct image diagonal and a tripod. A number of problems quickly arose that taught Dad a few lessons.
The aperture did not catch enough light for much beyond the Moon and the bright planets and stars; but the short focal length meant it was difficult to get enough magnification to really be “wowed” by anything. Short focal length (high power) eyepieces are not easy for children. The small refractor tube combined with a cheap tripod could not handle the additional weight or magnification of a Barlow. Even on wide-field views, the tripod made it difficult for me to aim, impossible for him, and it was never steady.
Thus began my search for a cheap telescope that he could actually use. I figured it could double as a simple “quick look” scope for me. With my recent experience I had no interest in a small aperture refractor; and I knew that a tripod and equatorial mount would be complicated for a child, especially a young one, to use. I investigated many Mak-Cass telescopes, motivated by their easier high power views; however, they shared the same tripod/mount setup concerns plus, reportedly, took longer to cool down. To top it off, the small apertures available for my virtually nonexistent budget foreshadowed that we would be under whelmed with the views. I thought back to my childhood use of a 4″ f/10 dob and postulated that a modern version would be appealing. As you can read below, it was.
We ordered a SkyQuest XT4.5 online and received the well-packaged telescope not a week later. We had to assemble the rocker box but that was a fun, lazy hour with screwdrivers and packing material flung all over the living room. I read online about others replacing the Teflon altitude and azimuth bearings with small furniture feet sold as Magic Sliders. A trip to the hardware store and a 30-minute install improved the horizontal and vertical motions. I used 20cm squares cut in half for the altitude bearings. This easy modification improved the altitude slewing. I then installed 6 20cm squares slightly improving azimuth slewing. This was not necessary but I felt it was too easy to pass up. In addition, I removed the rubber and metal primary mirror supports and used black caulk to attach the mirror to the three supports. This cleared the mirror of the slight obscuration, with the resulting diffraction, of the three posts. I painted the interior of the tube with Ultra Flat paint. (Do not do this. The factory paint is sufficient. Go with flocking paper if you really feel you want a darker tube.) I also painted around the primary mirror and the cell. It is all an even flat black now, whereas it used to have some slightly shinier areas with silver screw heads visible. This probably helped a little with contrast. I painted the secondary holder, spiders, and secondary mirror edges. This probably helped a little too, but I suspect the most significant benefit was the black paint on the secondary mirror edges. None of this was necessary. (I’d been harboring a slight-ATM itch, and all this tinkering helped sooth it.)
I then collimated it. This does not take an MIT-educated rocket scientist but I did spend a little time getting all the mirrors carefully aligned. Looking through this $200 telescope, with a little extra materials and labor added, revealed a surprisingly good image. What I didn’t realize was how I would use it.
To make a long somewhat-disappointing story short, my 4-year-old soon lost interest in using the telescope. This was shortly after he realized nothing shot out of the “barrel”…maybe in another year or two. (Incidentally, it has provided one of my most memorable moments with my other, then 15-month-old, son. I pointed the XT4.5 at an 8-day moon and held him up to the eyepiece. He coo-ed, pulled back, and pointed at the eyepiece. I directed his attention to the Moon in the sky and he made that happy-baby-sound and pointed first at the Moon then back at the eyepiece. With that one motion, I recognized at once that he was not blind, not dumb, and definitely my son!)
But I digress; the reason why I found this telescope so entertaining is because of how I use it. I can take off the dust covers, mount an eyepiece in the tube, and carry a footstool (for sitting) outside all at one time. Thermal stability is never an issue. It can handle a 40-degree temperature drop in less than half an hour. Virtually any Spring/Summer/Fall viewing is immediately possible after setting it on the ground. There is actually no setup required; it is really only a “set-down” since the OTA is kept tight against the rocker box with springs. The XT4.5 is 17 lbs, making it easy to carry everything at once. The viewing position is ok, as long as you realize you will need to be sitting on something like a small kitchen stool. It also works on picnic tables, if any are available. My initial impression of the finderscope was that it was overly difficult to use. It requires some strange head angles for an adult to find the correct orientation. After some use however, I find it adequate. I installed a Rigel Quikfinder but do not find it as advantageous as hoped. A Telrad, being larger, would not fit on the small tube.
There are some interesting synergies between a 4.5″ f/8 spherical mirror and Plossl or orthoscopic eyepieces that enable a wide range of uses for this telescope. Let me first briefly explain the differences between a spherical mirror and a parabolic mirror. Virtually every Newtonian telescope uses a parabolic mirror because it focuses the light bundle to one point. (At least on axis, I will ignore off axis coma in this discussion.) A spherical mirror will not do this. Rather, the light hitting the outside area of the mirror will focus to a different point than the light hitting towards the center of the mirror. With slow focal ratio mirrors (f/10 and higher) and small apertures (~4-6 inches), the resolution will be limited first by the normal diffraction dictated by mirror diameter. In other words for slow, small mirrors a spherical mirror behaves to the viewer the same as a parabolic mirror. Why does this matter?
Well they are cheaper to make and Orion happened to use one in the XT4.5.There is no specific mirror focal ratio and aperture combination that still remains “diffraction limited” but a 4.5″ f/10 spherical mirror is generally considered “diffraction limited” vice “spherical aberration limited.” But wait, the XT4.5 uses a 4.5″ f/8 spherical mirror. That means that the far upper range of magnification is not limited by diffraction but rather spherical aberration becomes the dominant image degradation source. If it was “diffraction limited” (as it would if it had a 4.5″ parabolic mirror) the maximum magnification would be about 270x, if we use the 60x per inch rule. With the spherical mirror, my observations are that the images start to degrade around 200x. I suspect it would be impossible to tell the difference between a 4.5″ f/8 spherical and parabolic mirror at 150x. It would be slightly noticeable in the low 200’s. By 250x it is definitely noticeable, giving slightly fuzzy, lower contrast images, and beyond that the image obviously breaks down. So, in summary, my experience has shown that the XT4.5 has a maximum useful magnification around 200x.
This is where the synergies come into play with moderate to inexpensive 1.25″ eyepieces and this telescope. The mirror is capable up to ~200x. At that magnification, it starts to become difficult to steadily adjust the focus. It is also at this level that the alt/azimuth motions make it more challenging to maintain the object in the field of view. But, coincidentally, this also happens to be the magnification provided by a 10mm eyepiece through a 2x Barlow. In common moderate to inexpensive eyepiece designs (Plossl, orthos) a 10mm eyepiece is the smallest many people can comfortably use due to limited eye relief. So with two eyepieces (25mm and a 10mm) and a 2x Barlow, it’s possible to get 36x, 73x, 91x, and 182x. That is a range from 1.4 degrees field of view, which is wide enough for large open clusters like the Beehive or the Double Cluster, to 0.28 degrees, which is a little less than one quadrant of the Moon’s disk. Higher magnifications are limited by the above-mentioned telescope components but also by atmospheric turbulence on many nights. If 36x does not provide wide enough views for your interests, a 32mm (with a 50 degree apparent field of view) Plossl will provide 28x (1.8 degree field of view) and coupled with a 2x Barlow gives 57x.
In the time since I bought XT4.5, I have used it more nights than my 18″ dob. On more than one occasion, the observing sessions were very short. This past summer it often went something like this: 1) “I wonder how the jet stream is affecting seeing conditions tonight?”, 2) remove dust covers and insert eyepiece combo giving 200x, 3) place outside, 4) grab foot stool and maybe jacket, 5) sit down at scope to check atmospheric turbulence and any interesting terminator shadows on the moon, 6) swing over to Mars to see which side was facing Earth, 7) check the split on the Double Double, 8) carry scope and stool back inside, and 9) put away eyepiece and replace covers. I could accomplish all this in less than 10 minutes. Other times that quick glance resulted in more observing later with the XT4.5 or the setup of my 18″ with confidence that the atmosphere would permit higher magnifications.
Going back to the use of this telescope by children, I think it would work very well. However, and this is a big caveat, if you are considering purchasing the SkyQuest XT4.5 as your only “family” telescope, I would quickly recommend you consider the XT6 instead. Frankly, if you are going with a dob of that size, I would jump right up to the XT8. (It has the same storage requirements as the XT6, is slightly heavier and is more money but definitely worth the increased light and resolving power.) An XT8 would be manageable for a parent and child plus, with three times the light gathering power of the XT4.5, opens up much more of the night sky.
If you already have a large dob, or a telescope on a complicated mount, the SkyQuest XT4.5 will provide a cheap, quick, and easy way to get bursts of your astronomy hobby. It is capable of magnifications limited by decent seeing conditions (~200x) and yet provides wide-angle views (~30x) showing more stars than the similarly priced small refractors. It weights the same as an ETX and if you include the volume required for the requisit tripod, probably takes up similar space in a vehicle. It will never double as a spotting scope or as an airline transportable scope. It is almost completely useless for photography. It is sometimes awkward to use. It cannot provide Goto capability, or even tracking. But for only $200, its ease of use combined with its small size yet flexible magnification capabilities makes the Orion SkyQuest XT4.5 one of the great values in astronomy today.
B. Wood: Updated February 2004