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Posts Made By: Mike Hosea

April 15, 2002 03:31 PM Forum: Telescope Making

Secondary size ??

Posted By Mike Hosea

Gary,

I'm sure you don't want a FULLY illuminated field in a 40mm Konig unless you're only going to do photography exclusively. Even a 2.6" diagonal will have some dropoff in illumination at the edge of a wide true field.

Selecting a secondary size unfortunately requires you to make compromises. Do you want excellent planetary performance or are you exclusively a DSO observer? I'm interested primarily in planetary targets and globular clusters, so I tend to prefer a secondary under 20% of the primary size, and I'm more than willing to accept a drop of 50% or even more in illumination at the edge of a wide true field to get it.

It depends on the focuser, specifically the distance from the axis to the focal plane, but I think I personally would go with 2.14" in this case. That's a 17% obstruction, and going lower is not likely to help planetary views much (and might hurt them if the edges of the secondary aren't as good as its center). It also probably gives about 60% or more illumination at the edge of the focal plane accessible to a 40mm UO MK-70 Konig. IMO, that's fine.
--
Mike

April 16, 2002 12:43 AM Forum: TeleVue

9mm Nagler

Posted By Mike Hosea

Until recently, or perhaps even currently, these were available new for $200 from dealers who received additional stock after the 9mm Type 6 was released. It's hard to price used, superceded models, but for one in excellent condition I think $150 is too low and $180 too high. Just my gut reaction. The advice is worth maybe what you paid for it. :-)
--
Mike

April 16, 2002 09:29 PM Forum: Telescope Making

Newtonian Project

Posted By Mike Hosea

If it's premium quality you want, I suggest a spider and secondary from Protostar and a primary from Mark Harry

http://hometown.aol.com/maralees/myhomepage/business.html

if you can live with f/6 or otherwise get Mark to make you an f/5. Otherwise, I hear Robert Royce

http://www.rfroyce.com/ast/products.htm

makes a good mirror also, but the price is rather higher.

A well-made Newt should outperform a quality MCT of similar aperture due to the larger obstruction on the MCT, and if properly made it also should handily outperform Chinese import scopes of the same size. Having said all that, the best MCTs can be quite impressive, and don't forget the Mak-Newts, either.
--
Mike

April 16, 2002 09:38 PM Forum: Telescope Making

Secondary Holder

Posted By Mike Hosea

I've heard it works. I think the main main difficulty to overcome is vibration--taught strings and wires vibrate rather well, as evidenced by their use on musical instruments. Note that good spiders are pretty thin to begin with and aren't nearly as good for playing music. :-)
--
Mike

April 24, 2002 01:03 AM Forum: Equipment Talk

Glass Mounted Secondary on Newts

Posted By Mike Hosea

Yes, but there are also some reasons not to do it. One is that the diffraction spikes from a thin-vane spider don't really harm the image much. Another is that you can get rid of diffraction spikes with curved spider vanes if you really want to. Finally, if you don't want to degrade image quality by using an inferior glass, it can be expensive to use an optical window since the glass really needs to be multicoated for maximum transmission and reduced internal reflection (which would reduce image contrast and could drop the performance below that of a well-made Newt using a good spider).

It will work, however. Markus Ludes has a large Newt made for him by Aries that is designed this way, and I am sure it is great. For smaller scopes it seems a whole lot better to me to go with the Mak-Newt design since you get better image quality off-axis without a coma corrector.
--
Mike

April 24, 2002 08:29 PM Forum: Eyepieces

Radian or good Orthoscopic?

Posted By Mike Hosea

I think it depends somewhat on the scope, but I preferred an 8mm Radian to a 7mm Tak ortho in my 10" f/5 Newt. But then again, I prefered the 7mm Pentax XL to the 8mm Radian on that scope also (though not on contrast). With Abbe orthos in faster scopes, it's critical to keep the planet or feature (in lunar viewing) centered. Otherwise, off-axis astigmatism in the Abbe design will degrade the image. Even in a Newt with its coma, I believe this astigmatism will quickly dominate other abberations. If you are using a fast scope and for any reason regard centering a difficult or annoying requirement, then a Radian is clearly preferable, IMO. The best performance is to be had in the center of the view, but the Radian is more forgiving in fast scopes if you look off-axis a bit. In slow scopes, I think most people would prefer the Abbe ortho. Many scopes are somewhere in-between what would be regarded a fast or slow. Can't help you there. Might as well try both and decide for yourself.
--
Mike

April 27, 2002 04:59 PM Forum: Telescope Making

quartz or pyrex secondary

Posted By Mike Hosea

The price difference varies, but it is pretty painful, particularly in the 1.3" size.

Supposedly quartz takes a smoother polish, and each of the quartz secondaries is individually tested and comes with an interferogram. The regular Protostar secondaries are random-sampled for intereferometric testing and 100% tested on a reference flat. So, some of the price difference is the cost of individual interferometric testing.

I see the ULS Quartz option for planetary observers who will do just about anything within reason to improve contrast and are willing to pay extra in the attempt.
--
Mike

April 27, 2002 11:04 PM Forum: Telescope Making

Need Help Calculating my Focal Plane

Posted By Mike Hosea

I've never taken a prime-focus astrophoto, but if I have the right picture in mind, i.e. just a the camera body and adapter with no lenses, then the difference in focuser position should be the same as with the ST80.
--
Mike

April 28, 2002 10:19 PM Forum: Equipment Talk

Brand new C11 OTA : WARNING !

Posted By Mike Hosea

OK, so they don't have the screws anymore. But couldn't they tell you the specs so you could pick up replacements from a hardware store or perhaps order them?
--
Mike

May 4, 2002 04:31 PM Forum: Equipment Talk

Brandons

Posted By Mike Hosea


Jay Freeman uses them. See part of

http://observers.org/beginner/eyepieces.freeman.html

Of course, Jay is USED to them after many years of use. Personally, I equate longer eye relief and wider apparent fields to indoor plumbing.

Jeff Medkeff did this comparison:

http://www.weatherman.com/jeffm.htm

--
Mike