Posts Made By: Roland Christen

Here's one more picture showing the scope under the mount in a situation where you might want to track and image through the zenith, past the meridian all the way to the opposite horizon. This would be of interest only to astrophotographers, of course.

Rolando

This shows the PE of the mount. Data was taken over a period of 20 minutes. Note how each run has almost exactly the same shape within a very small range. The curves lie smack on top of each other and show little variation from tooth to tooth (except for the inevitable atmospheric disturbances).

I also want to point out what a powerful program PEMPro is for analysis and compensation of periodic error.

Rolando

This can all be avoided with a quartz or ULE mirror.

Rolando

Here is a closeup of the bubble detail.

Rolando

I would suggest that you eliminate the (median?) filtering that has produced the strange artifacts in the image.

Programs like MaximDL have really great processing tools that can bring out just about everything that you can capture. I know Maxim is expensive, but it is really worth it. The images that get downloaded from the camera are not altered during the download, and the latest version will store the image at a very high bit level. There are numerous stretching routines that can be applied to make the image look its best. Highly recomended over every other software I have ever used.

Rolando

Good amount of detail in the nebulous regions. There is some weird stuff going on between close stars. What kind of filtering are you using?

Rolando

They cost 10 to 20 times that of ordinary bearings. But they are used in some mounts.

Roland Christen

A similar question has arisen on one of the Yahoo groups about refractors, differences, cost etc. Seems to come up periodically as new people wonder where they fit into the scheme of things, and that maybe with newer technology breakthroughs and simpler designs the costs of refractors will come down like they did for silicon chips. Anyhoo, here is my looongish answer:

There is no such thing as simple curves which are easier to grind and
polish. All curves whether steep or shallow require the same amount of
polish and figuring time.

The simplest design is characterized by the all-spherical TEC 140 and
the Tec 160ED oiled lenses, which use two normal glasses and one ED
element in the center. The oil spaced design becomes all-spherical by
allowing a small amount of residual color error in the violet. The oil
spacing allows the internal surfaces to be less stringently figured,
and only the outer two surfaces need to be coated (real multi-coating
is quite expensive per surface). One can make a higher corrected oil
spaced lens by either aspherizing or by the use of fluorite, but both
of those will leave the realm of simple curves and low cost.

Using 3 inexpensive glasses does not materially impact the overall cost
simply because the amount of work required to make a high performance
lens is mostly in the polishing and figuring. Of course, if these
surfaces can be left un-touched by human hands (i.e. the overall figure
error can be allowed to be worse) then one can save real money, whether
ED is used as one of the three elements or not. However, since
refractors don't have any aperture advantages over other types of
scopes, should we not try to give it an advantage in higher wavefront
accuracy?

For visual use, any of the above will work just fine. If you want to
extend the correction of the spectrum to include that covered by a
modern astronomical CCD camera, then you will probably want to go with
an airspaced design as exemplified by TMB-Lzos, Tak TOA and AP EDF
triplets (or the oiled fluorite triplets from Zeiss and TEC). The gain
may not be apparent visually, but you may quickly find that pixel
resolution from S2 to H-alpha through blue-violet will be much higher
in these specialized lenses. The difference may be as much as 2:1 and
even 3:1 compared with less well developed designs.

Making a highly corrected airspaced triplet lens is not so simple as an
all-spherical oiled triplet because all 6 surfaces have to be polished
essentially perfect in order for the final wavefront to be highly
corrected. They also have to be multi-coated, which can add
significantly to the price. Of course, there is also the simple ED
doublet, which is even less expensive to make with today's modern lens
making machinery, and even when you use the lowest rated ED (FPL51) the
result will be many times better color correction than an achromat.
Doublets have another advantage - less glass = faster cooldown. One
thing that has not been offered to the amateur yet is an advanced
highly corrected doublet. I can assure you that such a thing is
possible.

Today you all have many choices. Times have never been better.

Rolando

I think you mean lens cell. In any case, tilting the lens cell will probably not work in collimating a doublet refractor. Most doublets are corrected for coma, so if coma is present, it is not due to a tilt in the lens assembly. Coma is usually caused by a tilt in the spacer between the cell. This is not something that can be compensated by tilting the entire assembly. The other possibility is that the spacer is not tilted, but the two elements have shifted laterally with respect to each other. Then usually a small amount of lateral color is also present.

Trying to fix a tilted spacer is not a job for a beginner. You will need an autocollimation setup to do the job right.

If it is a lateral displacement of the elements, one can try a simple thing first before disassembling the cell. Sometimes a temporary fix can be accomplished if the front lens retainer is loosened, and the lens cell is gently rapped with a piece of wood. This will loosen a stuck lens element and may cause the two elements to realign themselves against the inner walls of the lens cell. Do it with the scope aiming approximately 45 degree (point it at Polaris, and use that star to see whether it has realigned itself). When you are done, try not to tighten the lens retainer against the glass. Leave just a tiny smidgen of gap so that the elements are not being squeezed together and can move around inside the cell. The lens should rattle a tiny bit if shaken.

It could be that some dirt has fallen down the inside of the lens cell and has displaced one of the elements to one side. In that case you will need to disassemble the lens and clean the inside of the cell.

Rolando

Depending on how loosely a manufacturer defines the term "APO", it is quite possible that one can make a 5" for almost any price.

My own thinking on refractors is this: since they suffer from aperture restrictions and thus have no advantage when it comes to raw light grasp, why would you not want to have it produce the very finest image for its aperture? If that's the case, then at least they should be constructed with all the care of a carefully crafted instrument, rather than being a high volume machine-made injection molded toy sold at rock bottom prices?

As amateurs and consumers, it is you who drive the market. Be careful what you wish for.

Rolando