Posts Made By: Roland Christen

My suggestion would be to not use a minimizing filter. It creates all kinds of strange string-like artifacts.

Rolando

Here is the full frame image of M101. Unfortunately it is only 1/6 size because of size restrictions for posting on Astromart.

There is something peculiar going on in the upper left side of the frame.

Rolando

NICE! Love the shark exhaust tips.

Rolando

>>Pardon my ignorance but could you please elaborate a little. Forgive me if this has been answered before but what is wrong with the current design of SCT's? The SCT's I found in ATMOS are pretty good. The one with the aspheric secondary (like what Celestron does) looks near perfect with strehl ratios in the high 9's. I have owned high-end Maks (up to 8") as well as SCT's (up to 14").>>

Ok, you're pardoned. But seriously, Celestron does not aspherize their design - it is basically an all-spherical design, not like any shown in ATMOS examples list. Secondly, the one you show in the ATMOS design uses an F3 primary and conical secondary. Nobody I know produces anything like that (if they did, it would be quite nice).

The Celestron design is a very basic all-spherical design using an F2 primary and 5x secondary sphere. There is considerable sphero-chromatism, and with 34% central obstruction there is an automatic reduction of Strehl Ratio to a maximum of .84 if the optics are figured to infinite perfection. The Celestron design actually mimics a Dall-Kirham, with all its known field curvature and off-axis coma. The new Meade SCTs mimic RCs with no off-axis coma, but with field curvature and twice the sphero-chromatism of the Celesctron basic all-spherical design.

Either way, you are not going to get the purity of image of a first rate Mak-Cass. If you had either of these F10 systems up against my 10" aspherized Gregorian Mak, you would instantly see what is what. Even at 250x you can see the difference, and at 600x it becomes glaringly obvious. Bring your's over and set it up beside the 10" Mak. I have a 10" Meade and have had C11 and C14s here also, so i know what is what

Rolando

I think I see a chicken . Another super result.

Rolando

Looks out of focus. In the core of M101 you should record 4 stars with your 8" refractor. Do you monitor your air temperature and adjust focus accordingly? If not, then I sugggest that you check focus between every exposure. A Bahtinov mask will aid you greatly.

Rolando

Excellent!

Rolando

>>I heard someone claiming that one model of 6" f/8 achromatic refractor had substantially less CA than another model of the same size and focal ratio. Is this likely to be true?>>

No, not true. A manufacturer can shift the correction around, but the amount of aberration cannot be lowered this way. For instance, he can reduce the blue halo but this increases the red halo. For dep sky, this may work fine. Since eyes are less sensitive to red at night, it may look like the CA is better because you see less blue on the brighter stars. However, such a lens would show poor planetary images because their surfaces are bright enough to trigger your daytime vision. You would then see very strong red halo and red defocus on the planet's disc, which tends to wipe out subtle detail, especially on the red planet Mars.

In fact, one can make a very good Mars scope by shifting the correction toward the red and letting the blue go out. Since Mars has very little blue, then one will see it sharply defined in red, yellow and green. However, since Jupiter and Saturn are much more white and have lots of blue light, these planets would now have a huge blue halo and blue defocus, and would look like they are veiled in a light fog.

No free lunch, I'm afraid

Rolando

I would venture to guess that the last owner "cleaned" the elements by removing them, and has placed one or both of them in backwards. That would give you the ghost halo.

Rolando

Galaxies in better seeing with the 206EDF refractor. Tony added diffraction spikes by placing string in front of the objective to give it an "artistic" look Oh well. The better seeing allows more definitive detail in the tiny galaxies.

Interesting galaxy shapes and faint detail, especially the intergalactic dust toward the bottom group, and the jet coming out of the side in the middle galaxy.

Image reduced to 40% size.

Rolando