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Posts Made By: Mike Hosea

May 5, 2002 05:20 PM Forum: Telescope Making

Secondary Size for 6" f8

Posted By Mike Hosea

With the elliptical secondary the measurement of interest is the minor axis. That corresponds to the obstruction diameter when it is mounted at a 45 degree angle.

I figure the actual size by looking at the illumination levels off-axis and considering the type of observing that I would like to optimize for. Mel Bartels has a calculator at

http://www.efn.org/~mbartels/tm/diagonal.htm

But SEC makes the comparison easier (see middle of page at)

http://skyandtelescope.com/resources/software/article_328_1.asp

Probably both 1" and 1.3" would be reasonable depending on your preferred type of viewing. 1" is a little aggressive, 1.3" a little conservative. Improvements to better than 1/10 wave would be very subtle or nonexistent, depending on your primary. The better the primary, the more I would be willing to spend extra on the secondary.
--
Mike

May 6, 2002 07:36 AM Forum: Equipment Talk

Focal Ratio Question

Posted By Mike Hosea

It is just a lot easier to design a fast scope to be capable of wider true fields of view. A slow scope can sometimes match, but the backend situation can get out of hand, e.g. you can get about a 2 degree field with a 6" f/5 apochromat using 30mm Ultima 1.25" eyepiece. A 6" f/9, on the other hand, would require a 2" 55mm Plossl or some such to get into that neighborhood. A 6" f/15 would require a 90mm/52degree eyepiece with a barrel no smaller than about 3.3" in diameter. Of course most reflecting designs are incapable of illuminating the required 80mm-wide image plane at f/15.

The speed thing is just derived from photography since the prime focus image plane of a faster scope is served up at a lower magnification than that of a slower scope and hence is brighter. Visually and for afocal photography, there is no brightness advantage one way or the other once magnification is equalized with appropriate eyepieces.
--
Mike

May 8, 2002 11:37 PM Forum: Telescope Making

Ray Trace..... where can I find info???

Posted By Mike Hosea

Bruce,

I presume what he's talking about is a program like Dale Keller's NEWT
http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm

However, unless you're thinking about changing or moving the existing focuser, I don't see the point. Once the focuser is placed, the secondary gets lined up with it, and the focus travel is not affected by the size of the secondary, only illumination levels.

Sounds to me like he was just speaking in general terms wrt placing a focuser and secondary. Now, if you are going to change the focuser or moving it, then we can talk about where to put it. I can't remember, did you ever report a measured lateral dimension, i.e. how far the from the center axis of the scope the tip of the focuser is when you have it in focus on an astronomical object? Should be about 6" or so, but if it is substantially greater, we probably should run the numbers again, and of course you can put all your measurements into Keller's NEWT program and see what is going to happen.
--
Mike


May 9, 2002 07:47 PM Forum: Eyepieces

Pentax 5.2

Posted By Mike Hosea

It happens to be my absolute FAVORITE, but I think I understand why. Being heavy and having a such a short focal length, it can not be popular for use in Pentax spotting scopes. This also makes me wonder about the future of the 40mm Pentax XL, but that rumor has circulated before.

But I am hoping that their desire to streamline production by eliminating the 5.2mm altogether has something to do with getting new products going.
--
Mike

May 12, 2002 04:54 PM Forum: TeleVue

Eyepiece Dew Management

Posted By Mike Hosea

I keep mine in a case. However, seems like you could just throw a hand towel over them, or if you want to get fancy choose the material of your choice to make a flap that attachs optionally to the caddy with Velcro. Covering them should delay the problem for a long while.
--
Mike

May 14, 2002 03:26 AM Forum: Equipment Talk

Freaking out on collimation

Posted By Mike Hosea

Yeah, don't panic. :-)

A cheshire is worth having. Get one. But for the moment if I were you I would

1) Satisfy myself that the secondary was placed approximately correctly underneath the focuser, centered (or correctly offset) in the tube, and NOT rotated away from the focuser. (Rotation of secondary is heap big problem for single beam laser.)

2) Use the laser to align the secondary.

3) Use the laser to align the primary.

4) Check collimation visually. Looks good? Use it. Tweak the primary in on a star if you want to get it better. Looks whacked? Check to see if the secondary is rotated. In any case, readjust the secondary somehow and repeat.

While this procedure can not be proven to terminate, it has always worked within a few iterations for me.
--
Mike

May 14, 2002 08:35 PM Forum: Eyepieces

Choosing eyepieces

Posted By Mike Hosea

Well, at f/5 I think you will do well to steer towards the better corrected eyepieces out there. I'm not that keen on any of the low power eyepieces you mentioned for an f/5 scope. I use a 40mm Pentax XL in my 10" f/5 and like it better than the 40mm UO MK-70 and 30mm Widescan II.

Siebert seems to use the singular word "element" to refer to a cemented groups of lenses as well as singlets, so it's hard to be sure how complex some of his designs are, but I am guessing that at f/5 the field correction of the 21mm would not satisfy me, even when considering only the central 65 degrees.

I do like the older 9mm Nagler a lot, as well as most Pentax XLs (especially the 5.2mm, 7mm, and 10.5mm), other Naglers with 12+ mm or eye relief, and the Meade Ultrawides. The Vixen LV Zoom is okay.

A small set of eyepieces needs to take into account your observing habits and preferences. For example, I'm mainly into planets and globulars with an occasional excursion to look at the showcase DSOs of other types. In my 10" f/5 I think I could get along fine with my 40mm Pentax XL, 10.5mm Pentax XL, and 5.2mm Pentax XL. I also occasionally use 21mm and 7mm Pentax XLs and a 3.5mm eyepiece made by combining a 7mm Nagler T6 with a Tele Vue 2x Barlow, but I'm sure I would not feel deprived if I had only the first three that I mentioned. Those who tend to regard the moon as light pollution would doubtless pick a different three, and of course being satisfied gets easier as you allow for more eyepieces.
--
Mike

May 18, 2002 04:24 PM Forum: Telescope Making

8" f/5 Ideas

Posted By Mike Hosea

Almost anything's possible, but you have to pay the piper. It implies a much taller rocker box, and it tends to make the OTA longer compared to its width, leading to more problems with vibration. I suspect after all is said and done you will need many more pounds of more beefy construction, not to mention the weights themselves, to achieve this with a standard Newt design.
--
Mike

May 22, 2002 06:26 AM Forum: Equipment Talk

Black Knight Telescope experience??

Posted By Mike Hosea

I'm sure Royce makes a great mirror. Those Black Knight scopes do look good, but those good looks do cost something more, and a truss or strut setup has some advantages when it comes to thermal issues. Furthermore, the Starmaster design with its low rocker box is probably a lot more stable. I'm not arguing against the BKTs, just pointing out that there are some strengths and weaknesses to both tubes and trusses.

The BKTs are pricey, but having just made a spare-no-expense 7" f/6.7 for myself, I have to say that the prices don't seem out of line at all to me. Even considering the great price on the primary I got from Mark Harry and the even better price on the Appleply scraps I got from a famous friend and neighbor, I probably ended up spending a total of $1250 or more, and that doesn't include labor. It does include some first-class components, like a TV Starbeam finder, a Wolanski focuser, a Protostar spider with ULS Quartz secondary--stuff like that. When I figure in primary prices, labor, and overhead, the BKT prices look about right to me.

My 10" Teleport is even pricier than the BKT options, but you're looking at a wait of no less than a year, probably more, for one of those. If I were in the market right now, I'd probably lean towards one of the 11" Starmaster ELTs. It doesn't look as good, but the bang-for-the-buck seems better to me while still providing premium performance.
--
Mike

June 12, 2002 05:15 PM Forum: Equipment Talk

Where can I find a shorty 1.5X Barlow?

Posted By Mike Hosea

Gary,

The Uranoport Barlows are the only ones I know about. Find Uranoport on the Astromart homepage. Haven't tried one.

I've heard that it is better for optical and mechanical reasons to Barlow a binoviewer up-front than use short focal length eyepieces (or equivalent using Barlows) on the back end. At first glance it might seem equivalent, but in fact one setup is more sensitive both to prism defects and collimation errors than the other. Of course it's hard to get 1.5x by Barlowing up front--would have to be a very weak Barlow lens.
--
Mike