Tribulations with the C14 EdgeHD optics!

Hello everyone,

it’s been a couple of month since my last post; mainly because of the weather which has been absolutely crap here, partly because, the scope was sent back to the importer, Médas, for a check and service. Here is why:

It has been about a year since I started using the C14 EdgeHD, and I have to say in all honesty that I still have not been able to get an half decent planetary image out of it. The main reason for this is that I live and observe in a “washing machine”, meaning on a mountain flank. What that does is that winds and mountains interaction causes alot of air turbulence, reducing seeing.Turbulence is easily identified looking through the eyepiece and watching a star “dance”. That said, it is hard to believe that, in the course of one year, there has not been a single night of good seeing. So, the second reason for not getting good images may be the scope itself; and that is the hardest part to evaluate. So, last March I really wanted to have the optics thoroughly checked and drove the telescope down to a company with an Haso optical testbench. What they did is measure up the wavefront accuracy at different wavelengths and metalback distance to quantify error of the C14 optical chain. They also tried to rotate the corrector plate to select the best position, which ended up being 120 Degrees from original’s. Here, you will find the test report. What it shows, when you do get used to looking at measurements in nanoMeters, is that the optics are quite decent for a scope that size roughly at Lambda/4. So, with newfound confidence in my telescope quality, after performing what I thought was  a good collimation, I set out to image planets, with the same result as before…

Since there is no substitute for my own eye, and I have never managed to see a star Airy disks on a real star, I set out to perform an extremelly precise collimation using an artificial star placed 50 meters away. Only that, with a focal length as long as a C14′s the star would need to be at least twice that distance from the scope in order to focus on it, which the terrain does not allow me to do… So, the solution is to place the eyepiece at a rediculus 30 cm (1 foot) distance from the scope metal back. To achieve this, I used a crayford, a barlow, a diagonal mirror and finally the eyepiece… And… For the first time, I was able to see diffraction rings! Here they are:

Then I was horrified! Indeed, those of you fluent in optical speak, won’t fail to notice that there is alot of trefoil and spherical aberration present, looking at the star. Apart from the fact collimation can be improved that is… How can that be, since the report shows that the optics are good? Well, this is because a C14 is not a microscope so, looking at a star so near it will magnify original optical defects tenfold. So, apart from allowing me to do a proper collimation, which is relative since the tube is horizontal, this sort of test is no use to evaluate optics on an SCT.

Still, the aspect of the star kept nagging at me, and I really wondered if the corrector plate might not had been better of left alone and not rotated. So, to set my mind to rest, I contacted the Celestron importer who is equipped (litteraly) to check SCTs and talked to them. They got the scope collected, and did a thorough check on it. What they do is that they own an high quality reference C14, stick a light at the back end, and place the tube to be checked in front of it. So, a fairly flat light wave front comes out of the reference instrument, which look like a star to the instrument beeing checked… Pretty crafty stuff which is a variant of auto-collimation. Then, you just stick an high quality eyepiece into the tube being tested, and you realign everything so that you get the best looking Airy pattern you can. It’s just about as good as an optical test bench without computer if you ask me. When I finally got the scope back after a couple of weeks, the corrector plate seemed to be back to it’s original position (hum… I wonder why).  Now, suffice to say that beside striving to achieve perfect collimation, I am done with messing around with my SCT, as the optics ARE GOOD. it’s the place where it’s being used as well as the user who are crap!

Now there is a very important point to be mentioned here about the EdgeHD variant of the C14 that impact performance tremendeously. this is what a unfocused image of the artificial star looks like after 4 hours ouside; notice the nice heat plume:

bilanun

Cooling and reaching thermal equilibrium is nearly impossible to achieve on an EdgeHD scope, because of the lens that now obstruct the baffle tube. There are no air currents inside the tube to help cooling, and the inbuilt air vents are there because Celestron probably knows that, but their effectiveness is next to nil. Also, the new fancy mirror cell casting won’t allow for rigging up cooling fans. The only way around I see is to make a fan assembly wich fits to the faststar cell, blowing air into the tube then out of the vents. That, and sticking a temp probe to the back of the primary through one of the vents and one stuck to the aluminium scope tube. When the 2 reads the same temperature, you can then be fairly confident your scope is at equilibrium. I’m planning to do this asap, so, to be followed…

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Tribulations with the C14 EdgeHD optics!, 9.1 out of 10 based on 11 ratings

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