In the good old days, refractors are used to be worry free instruments. Even for those without adjustable lens cells, the scopes would be correctly collimated out of factory. However, this is not the case any more, especially for the entry-level models. This is hardly anyone’s fault because we are now paying one-third or even a quarter of the price for these telescopes than before. For example, the original Celestron Firstscope 80 f11.4 long refractor could easily cost over $600CAD during my college years while its modern counterpart, the PowerSeeker 80EQ, can be purchased today at about $200CAD (with inferior eyepieces, diagonal, finderscope, and an under-sized mount). Worse, the lowest priced ones among these entry-level gears were never meant to be equipments for observations more serious than “looking at the moon and found a few craters on it”. It could be for these reasons their quality control is so loose that many of them are mis-collimated out of the box. Poorly made and assembled lens cells, tubes, and focuser all contribute to this while the objectives themselves are mostly not to be blamed.
While I shouldn’t be complaining because of what I have paid for these “modern” refractors (the Celestron Travelscope 50 discussed in this post was bought used at $20CAD), I would hate to see potentially good optics (many of the times, they are) got wasted. Hence, in the recent years of me doing stargazing, I started inserting a cheshire collimation eyepiece (as shown above, which is a tool to collimate a Newtonian Reflector) into the focuser of refractors to check if they have been properly collimated. However, unlike a Newtonian or a refractor with an adjustable lens cell, collimating a fixed cell refractor is a long and tedious task though the results can be surprisingly fruitful. In short, this is a process called “shimming”, using layers and pieces of shims made by various materials to create small “tilts” and “shifts” to offset the collimation errors. Below are my two cents on fixing collimation by shimming:
- If the scope is performing reasonably well at high power (i.e. good planetary viewings under good seeing, splitting tight doubles within the resolution of the objective), then there is no need to start such process, which could take a total of 2~3 hours and span several days.
- To make sure you have a “stable platform”, you would need to fixed the scope on the mount during the process. A desk lamp with a directional focused beam of light could greatly help illuminating the cheshire eyepiece.
- It is better to start the “corrections” from the focuser side as you might be able to fix the collimation without touching the objective lens cell.
- To “adjust” the focuser, remove all three retaining screws and insert the cheshire eyepiece, then slightly tilt the focuser at different directions until the “donuts” in the view are centred.
- You should then be able to find out where you should put the shim (I myself used layers of masking tape) on and approximately how thick it should be. You then put back the retaining screws and tighten them up. After that, you re-check if the donuts in the view of cheshire eyepiece are still concentric. In some extreme cases, you might need to leave out one of the retaining screw to maintain the collimation. If this is the case, you can use duct tape to temporarily fix the position. Do not glue it permanently before you perform a star test as there will be no going back if you do so.
- You might need to repeat the above two steps for several times to fully fix the collimation. If your scope is long, you might find the donuts in the view are too small. In that case, using a smartphone adapter and your phone camera to enlarge the images might help.
- If adjusting the focuser alone couldn’t fully fix the collimation, then you need to move onto the objective side. There are two things that might need correcting. First, the lens cell might not be aligned with the focuser. The shimming process to correct this is pretty much the same as adjusting the focuser. Second, there is also the possibility that the objective lens is not sitting flat on the lens cell. This could caused by the “mold stab” not being fully removed (just like taking out a part from the sprue in scale modelling). In this case, you could polish and flatten it with sandpaper. However, you would need to use sandpaper with extremely fine grit (#500 or above) which can be bought from hobby shops.
- Again, just like after you performed shimming on the focuser, you would need to re-check the collimation with cheshire eyepiece after you have corrected whatever imperfection on the objective side.
- The final step is to take the tuned scope out at night for a test. Stars should appear pin-point. High power views of planets and a star test can also be useful in determining if the fine tune has been successful.
After the tune up, in order to “secure” the shimming corrections so that they won’t be offset by any accidental “bump” on the OTA during transport, I put drops of E6000 crafting glues into the joints and gaps. Then for aesthetic reason, I covered them with adhesive felt pads. The end product of the tuning up the Travelscope 50 is shown in the image above.
Also, if you have read my earlier post on upgrading the Celestron Travelscope 50, you would notice that I have removed the red-dot finder and reinstalled the peep-hole sight. This is because when the OTA is mounted on the NexStar, the red-dot finder would be positioned at an angle that makes the red-dot difficult to see. As I would only be aiming at 1st magnitude (or brighter) stars when aligning the mount, the peep-hole sight makes more sense and is in fact easier to use.