Fast vs. Slow

Accurate collimation is even more critical for the XT10 which has a so-called "fast" focal ratio (f/4.7, as compared to a "slower" ratio of f/5.9 for the XT8, or even slower f/8.0 of the XT6).

So what do we mean by "fast"?

A telescope with a low ("fast") focal ratio offers lower powers and a wider field of view at prime-focus for a given eyepiece. When using the telescope for prime-focus astrophotography, smaller focal ratios mean shorter exposure times, i.e. more light is captured resulting in a "faster" exposure of the photograph.

Just the opposite is true for larger focal ratios ("slow" telescopes): higher magnifications result for a given eyepiece, but the field of view is more limited, and this in turn results in less light. Therefore exposure times for pictures tend to be longer and hence slower.

In the "fast" XT10, tiny adjustments to alignment can have large effects on the precision of the image. It also follows that knocks and bangs resulting from transporting the scope to dark sites will often move the heavy primary mirror out of alignment, not by much, but enough to be significant.

Collimation ScrewsThe optical axis of the telescope should be closely parallel to the mechanical axis of the tube assembly too. Orion's SkyQuest™ telescopes have consistently good quality mechanical components and assembly at the factory that facilitate collimation precisely upon delivery, if needed.

The mirror cell has 3 large thumbwheel screws for precise adjustment of the primary mirror, accompanied by locknuts to secure and retain the setting once adjustment has been finished.

The thumbscrews are spring-loaded to keep tension while adjusting. It is surprising how even as little as one 1/16th of a turn can make a significant difference when using a laser collimator to assist with the collimation process.

 

A simple "collimation cap" is supplied with the scope to allow you to perform basic collimation (It sounds more special than it is really! It's just a small black plastic cap with a reflective material on the interior face, and this fits into the eyepiece holder on its own, and a tiny hole drilled in the centre makes sure your eye is positioned centrally when performing the collimation process. The colli-cap is sufficient for aligning the secondary, and can be used for primary alignment too, but there are much better and more precise collimation methods described elsewhere on this site.

The Orion XT telescopes employ a rolled steel enameled tube finished internally in a matt black anti reflection paint, with the exterior in a contemporary deep bronze metallic enamel. The tube holds optical collimation well even with temperature or humidity changes.

Secondary Mirror with 4 straight-vane spider

The Secondary Mirror is supported in an adjustable holder by a 4-vane spider. The vanes are thin but sturdy metal so as to reduce obstruction of the light and image entering the telescope tube.

Some diffraction spikes are noticeable at lower magnifications or with wide-angle eyepieces, although this is to be expected with Newtonian designs, simply because of the placement of the secondary mirror and supporting spider vanes in the path of light. All spiders will cause diffraction. In a normal, straight, four-vaned spider, the diffraction will be in the form of four distinct spikes eminating from any bright object.

All telescopes suffer from "diffraction" in different ways. Diffraction is going to happen to some extent or another simply because there is an object in the way! Generally the trick is to "ignore" it, i.e. focus your eyes through and beyond it. Generally the closer your eye is to the eyepiece the less noticeable it is, so perhaps glasses wearers may find diffraction more visible, and should try viewing and focusing without glasses. The following resources provide some useful info on diffraction:-

A modification that some owners of Newtonians often make is to replace the standard straight vane spiders with curved vane spiders. Mechanically, just about all curved spiders are inferior to straight ones. With most curved vane designs, the total amount of diffraction will actually be slightly more than with an equivalent straight spider for two reasons:-

  1. There is more total vane in the light path.
  2. For equal mechanical strength a curved vane will usually have to be thicker in order to hold the secondary mirror (surprisingly heavy) strongly enough to prevent it from "drooping" out of collimation when the scope is pointed at low altitudes. Straight vane spiders are normally under tension from the supporting bolts, but tension cannot be created on curved vanes. Miscollimation will cause far worse contrast issues than having thicker spider vanes (You can check if a spider has this issue using a laser collimater. Put the collimater in, and see if the spot moves when you point the tube vertical vs horizontal).

The advantage of curved vanes is that the diffracted light is evenly spread throughout the field, and not concentrated into noticable spikes. Stars will look like dots, but because diffraction effects are spread out there may be some loss of contrast in the view.

However in certain circumstances the noticable spikes produced by a straight vane may be prefereable. For instance, if you are looking for the faint companion of a bright double star, the circular diffraction pattern from a curved spider could bury it, but with bright spikes you can place the star where the spikes aren't.

The big difference is aesthetic - if the spikes annoy you, getting rid of them can be a good thing. If they don't bother you, getting rid of them isn't worth the effort.

Secondary Mirror Collimation

Secondary MirrorAdjustments to the secondary are by means of three long 2mm Allen (hex) key set screws in the centre hub of the spider, and one central spring-loaded tensioning pivot bolt.

Secondary HubThe 3 set screws adjust the tilt of the mirror. The tensioning bolt adjusts the distance of the secondary from the primary, and this is used to align the secondary with the focuser tube.

 

Click here for detailed notes on Secondary Mirror Collimation.

 

 

 

 

Next section: Orion XT10 Dobsonian Telescope Base Mount

Other Topics in this XT10 Review: