Posts Made By: Jeff Quinn

Hi Andy,

I have heard of this. Any compound optical system can be made better or worse by changing the radial alignment of its radially symmetric components. For example I see a tad of astigmatism at 500x through my 12" SCT so I am going to experiment with changing the centering of the corrector plate in its cell (with the cork pads around its circumference) as well as try a turn of the plate by about twenty degrees at a time. But remember that the secondary will rotate with the plate unless you make an effort to realign the secondary relative to the primary (keep its top facing up -remember to mark it on its top edge so you know how it originally was aligned).

So go for it. Just be careful to make sure you know how to set the original alignment if you find that rotating the plate and / or secondary does not help your images.

Here is another thing to consider: The SCT design only has perfect spherical correction for one focus position. So if you pull your diaganol out a ways (or view straight through) so as to change the distance from the focal plane to the secondary mirror, you will change the spherical correction of the system. Here is an example: when I view straight through (no diaganol) in my 12" SCT I see nearly a perfect breakout pattern of the secondary shadow, nearly the same size on either side of focus but slightly different. If I pull my drawtube out a little ways and view again after refocusing, I see perfect star test patterns. If I put my diaganol in, I see the star test becomes asymmetrical opposite of how it was when just straight through.

What really matters with a compound system like an SCT or Mak is not perfect spherical correction, it is smoothness of the optics. Rough optics in any scope will mess up your images BADLY even if the star test shows no spherical aberration. But a smoothly figured 12" SCT is spectacular on planets, easily beating a 6" APO refractor despite having a 33% central obstruction.

Jeff Quinn

Hey that's pretty nice. Looks like Saturn's incliation is steeper since last season.

I've never collimated one before so this is just a wild idea. Measure the secondary mirror relative to the front tube opening to ensure that it is exactly centered. Then using the circular opening of the end of your Cheshire as a reference, line up your focuser to center on the outline of the secondary mirror. You can make this adjustment by shimming the focuser's base like with a Newtonian. Then line up the reflection of your Cheshire's crosshairs back onto themselves with the secondary mirror's adjustment screws. Get it PERFECT. Then focus the scope on a star (real or artificial) and adjust the primary mirror to center the airy disc into the comatic flare that you will see if it is miscollimated.

That is how my experimenting mind would explore the situation. But I have never collimated one before so I can't say that it will work, just that I *think* that it will. Eliminate what you can align through conventional means and then do the rest with star images (like when collimating an SCT at 500x on Vega).

Hi Maurice,

I am just curious, do you find it necessary to manually guide the short 3 minute exposures?

And also if you don't mind me asking, how much better is the ST-8 compared to the 416XT which you used to employ? I can't detect a difference in the image quality, so other than an increase in field size, what is the benefit of the ST-8?

Thanks in advance, I have come down with CCD fever now that I have a mount which can track well.

Jeff Q

Hi Phil,

Well it was not a $3000 scope that you bought, it was a $900 scope. Let us clarify that. $3000 will not buy a scope like yours if one were to present this amount to Starmaster in the current time.

You are a craftsman, so am I. Just admit it, not everyone is meticulous nor demands such attention to detail. You got what you paid for. Go out and spend $3000 on a new Starmaster structure and come back here and report what you find. It is unfair to complain about bad craftsmenship on a $900 OLD Starmaster and keep throwing around this arbitrary $3000 number.

I personally would not buy a truss dob from any manufacturer. In my opinion they are all excessively large, inefficiently laid out structures. There is no need for a separate altitude bearing when the bottom of the mirror box can provide the bearing curve. Enclosed mirror boxes prevent efficient passive cooling of the primary mirror. Then there is that monstrosity of an upper cage structure. People complain about balance problems with their truss scopes while overlooking how FAT that secondary cage really is. Look up Mel Bartels on a search engine and see what can really be done with large aperture newtonian optics. There is a much more efficient way to support those optics than just copying the brand x boxy form. Where is the innovation? Are all these truss makers copying each other's 1980's ATM scope?

When I get the time I am going to build my own 18" f/4.2 or 16" f/4.8 around my own ideas. Why not just start making your own line of truss scopes? You could really improve on the Obsession / Starsplitter / Starmaster / Discovery / Night Sky / Sky Designs / Telekit / etc etc etc. clones. Take what you did with your mirror cells and do an entire scope around your ideas. This is not unlike how Astro-Physics was launched. This company's founder could not buy a product which was up to his personal standards so he began making hiw own product. Where is Astro-Physics now in the industry?

Jeff Quinn

Hi Ron,

Here is a one shot picture I took of Jupiter in my 12" LX200. I held my Kodak DC3400 camera up to a 13mm Widescan and this is one of the better shots. I also did a light color curve tweak and unsharp mask, but it still looked better to the eye than the image below.

When you stack multiple (30+) good shots of a planet, it can look like the first picture if taken through a scope with around 1/6 wavefront optics in good seeing. It increases the signal to noise ratio of the image, helping bring out the fine details.

Not bad Walter, what scope and eyepiece are you using?

Jeff Q

Well I have experience doing my own fixes to my used scopes, because they are all Meades and Orion is not the only co. who won't exactly sell you a spare lens like Tele Vue just did for my broken 35mm Panoptic :-) Al Nagler is a nice guy to talk to on the phone. The value brands just don't do things like the companies who are in the telescope optics business for love of the hobby rather than profit.

How about 10x90 seconds or 15x60 seconds? It would be more work, that is for sure, but maybe you could avoid the bloomed stars that way?

It looks great other than that :-)

Jeff Quinn

Hi,

You can call Tele Vue and buy them for $2 each plus shipping.

Jeff