New To Me Telescope! 2-3 July 2026

     I have a "new" 90mm achromatic refractor telescope. It's an Orion Explorer 90AZ. Anne  found it advertised on FB Marketplace in a town about two hours away. The seller had recently obtained it from a friend and didn't use it. I did some research and offered $100 for it, that they accepted. The telescope is a 90mm aperture, 910mm focal length (F10) achromat that has an alt-az mount with slow motion controls. The finder is a 6x26 straight through correct image finder with crosshairs. It came with the Orion padded carry bag for the OTA and tripod, an Orion accessory case, a 45deg correct image prism, a 90deg prism, 10mm and 25mm Orion Sirius Plossl eyepieces, a 2x Barlow, a moon filter, and the assembly tools that came with it originally. The manual and original purchase paperwork were in the padded bag with the OTA. The Orion catalog with the 'scope marked by the original purchaser was there also. The telescope and mount were purchased in December of 2006. The accessory case and some of the other items were purchased the following January. One of the adjustment screws for the finder was missing, and one of the thumbscrews to attach the spreader tray broke as soon as I tightened it, so I replaced those.

 

  Naturally as soon as I got it home the clouds came in each evening for a week. They cleared a enough tonight for me to do a quick set up and try it out.I set it up about11:30MDT. The Moon is bright at about 90% full. That's actually good because I wanted to try to get an idea of the level of chromatic aberration. I have read a lot of accounts of different achromatic refractors having serious chromatic aberration and others with very little. Longer focal length 'scopes apparently do better.

  The telescope and mount are easy to set up. I only approximately leveled the mount and started observing at about 00:10MDT. The mount has a lot of "bounce" to it when making coarse adjustments by moving the tube. When moving in altitude by loosening the lock enough to move the tube by hand there is a lot of slop and it settles quite a bit after I let go. I also noticed that moving the 'scope manually in altitude had enough torque to flex the joint where the legs of the tripod are bolted to the base for the mount head, That part can be at least partially addressed in the future by being sure those bolts are tightened and hanging a weight below it. The slow motion controls have some backlash. Basically, if I take care setting it up next time it should be more usable. I'll need to go over it and be sure everything is tight.

  I used the rising Moon to align the finder. This is the style with an o-ring in a groove on the forward part of the finder that fits in the leading edge  of it's bracket. There is a spring loaded pin at the back end of the bracket with two adjustment screws. I had to get replacements for those screws, one was missing. It proved difficult to get adjusted but I was able eventually to get it usable enough for tonight. In the future I need to take more time with it. I.m pretty sure I would eventually like to get a red dot finder for it.

  Started out with the 10mm eyepiece. The edge of the Moon was fairly well defined, but boiling some. It is only about a diameter and a half above the mountains. The air that low is probably fairly turbulent. Going to the terminator there is still some unsteady seeing, probably about Antoniadi III. I'm not seeing any noticeable chromatic aberration (CA). If I look hard at the limb of the Moon I can barely make out a very thin and faint blue along the outer edge. There is one smaller crater that is very bright that shows a hint of blue on the upper right and a hint of a reddish tint on the lower left of the rim. The color is very faint and none of the other craters show anything. A side note, the eye relief on this 10mm Plossl eyepiece is nice. 

   Using the 8mm Plossl from the Celestron kit there is still no noticeable CA. There is a very slight purple fringing at the edge of the limb and the same color effect on the one bright crater. Looking at the terminator the seeing still is about AIII.  Some high thin clouds make staying focused difficult. The same with the 6mm eyepiece. I looked at Tycho using the 8mm eyepiece. The crater rim was very bright and the rays easily visible. No CA that I noticed.

  I put in the 25mm Sirius Plossl. The Moon really popped, was very clear and in focus. The FOV (Field of View)  was at least two and a half Moon diameters. According to astronomy.tools The 25mm eyepiece has an FOV of just over 1.5 degrees.

  Moved to Arcturus to see how a bright star looked. The finder was difficult to use and could definitely benefit from illuminated crosshairs. That wasn't a problem with the Moon, but the built in crosshairs aren't really visible without a bright background.. No noticeable CA in the 'scope.

  Next was Antares. I was seeing some red and blue flashes but I was getting those with naked eye also. Antares is very low above the mountains.

  I shut down at 01:01 MDT. 

Overall I am very pleased with this telescope. Except for the effects of seeing, the views were very clear, with no problems with CA. The mount needs some tuning up and the focuser could use cleaning up the mechanism to be a little smoother.

 A quick note about Orion. They went out of business suddenly in 2024. They had also owned Meade Telescopes for a while. This is a long and convoluted story. You can read some about it here. When they went out of business, Highpoint Scientific bought out the remaining stock and eventually put it for sale through their website. There are still some Orion products available though Highpoint.