By: Vlad Fedosov
For most armature astronomers a large dob is about the only cure for a serious case of aperture fever. Refractors are astronomically priced in anything over 6”, and SCT’s really need to be permanently mounted with anything much larger then a 12”. Going larger for most of us, leaves us to the dob telescope design. With my aperture fever never quite quenched by my 12” SCT’s I too had to add a 16” dob to my stable. The dob design does however introduce an unwanted visual artifact called coma due to its fast optics. To put this simply all the stars in the outer portion of the FOV are not quite in focus. This is where a comma corrector comes in. I had the privilege of owning and testing both a TeleVue Paracorr II and an Explore Scientific coma corrector.
For this test I took advantage of a very rare clear December night in the Northwest to take my 16” Meade Lightbridge to one of my astronomy clubs dark sky sites; Stub Stewart State Park. This site has around mag 5-5.5 skies and almost a 360° open horizon. This particular night had average seeing and good transparency. For the test I used my two most used eyepieces an Explore Scientific 100° 25mm and 14mm.
The primary object that I used for the evaluation is the Double Cluster in Perseus as this nicely filled even the huge 1.37° FOV that the 25mm 100 Eyepiece provides. The view that I got of this DOS with no coma corrector seemed fine to me as I had yet to see what difference a coma corrector would make. Stars up to about 60% of the FOV looked in good focus. Past that stars started to degrade. I first decided to try the TeleVue Paracorr II. While setting up the corrector I tried a variety of settings and concluded that the view was the best around the ‘B’ setting. As of no surprise the view was cleaned up nicely. Stars looked in focus almost to the very edge of the FOV. I did find that I did not feel like I ever got a very sharp focus overall anywhere in the FOV.
Next up was the Explore Scientific corrector. After a fairly long process of trying to find the optimal position of correction with the very slow screw barrel design I was pleasantly surprised by a view that was simply breathtaking. Stars where perfect pinpoints out to about 95% of the FOV(at which point I believe the design of the eyepiece introduces some distortion). The view was really refractor like with stars that where finely focused little points. I was more than impressed. Switching back to just the eyepiece I was greeted to a view that was much much uglier. It was hard to believe the difference. Interestingly enough even in the center of the FOV the stars were not as sharp as with the Explore Scientific corrector. Putting the TeleVue unit back in, again confirmed that stars where not quite as finely focused. They looked like little circles as where with the Explore Scientific unit they looked more pinpoint like.
Switching back to the Explore Scientific unit I proceeded to examine the FOV more. The first thing that I noticed is that fine focus seems to be easier to achieve then without the corrector. I also noticed that the sky background got darker for about the last 10% of the FOV. Switching between the two correctors and just the eyepiece confirmed that with the Explore Scientific unit about 10% of the outer FOV is darker. The TeleVue does suffer from this as well but in only the last 5% of the FOV. This effect was not seen at all when using the 14mm 100° eyepiece. I’m assuming that this is vignette because of the fact that the 25mm 100° eyepiece really pushes the boundaries at extracting the widest possible FOV from the 2” eyepiece format.
Overall I find that the tuning mechanism of the TeleVue unit is much easier to use then the Explorer Scientific. Built quality of the units is comparable as they are an all metal design. I think my findings do not indicate that the Explore Scientific corrector is better than the TeleVue. I’m almost sure that the reason that I got better results with the Explore Scientific unit was that I was using Explore Scientific eyepieces. I guess the takeaway here is that if you have a dob I would strongly recommend a comma corrector if you are using wide angle eyepieces. Also buy the same brand corrector as most of your eyepieces.
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