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Posts Made By: Thomas Bennett

July 18, 2002 10:55 AM Forum: Equipment Talk

Binoviewer Vignetting

Posted By Thomas Bennett

Hi All,

I've read that binoviewers with smaller prisms can vignette when using long focal length and/or wide field eyepieces. But doesn't this also depend on the scope being used? For example, a F15 Mak-Cass scope will have a narrower light cone 5-6 inches (typical binoviewer position) inside it focal plane whereas a F6.3 SCT will have a wider light cone at that same position. So wouldn't it be possible for the SCT to vignette while the Mak wouldn't using the same binoviewer and eyepieces?

Thanks, Tom

August 9, 2002 03:55 AM Forum: Telescope Making

Will this mirror focus light?

Posted By Thomas Bennett

Hi All,

I've just acquired a grossly under-corrected commercial mirror that gives the Foucault test results below. Will be taking it to a star party this weekend to compare with a mirror from the same source that I refigured last year - and I thought that mirror was bad! - time to re-pitch the polishing tool.

Has anyone seen a commercial mirror worse than this one?

- Tom

TEXEREAU MIRROR TEST SHEET

Comments: Optical diameter: 16.2
Readings per zone: 1
Radius of curvature: 144
f/D: 4.44
Diffraction disc: 0.00011712

1 ZONE 1 2 3 4 5 6
2 h(x) 3.5800 4.8348 5.8253 6.6703 7.4197 8.1000
3 h(m) 2.5400 4.2074 5.3300 6.2478 7.0450 7.7598
4 hm**2/R 0.0448 0.1229 0.1973 0.2711 0.3447 0.4182
5 hm/4f 0.0088 0.0146 0.0185 0.0217 0.0245 0.0269
6 D1 0.2180 0.2360 0.2830 0.3270 0.4280 0.5000
7 D2 0.2180 0.2360 0.2830 0.3270 0.4280 0.5000
8 D12 0.2180 0.2360 0.2830 0.3270 0.4280 0.5000
9 D12 - 0.0898 0.1282 0.1462 0.1932 0.2372 0.3382 0.4102
10 Lamda c 0.0834 0.0232 -0.0041 -0.0339 -0.0065 -0.0080
11 Lamda f * 1e5 73.53 33.96 -7.60 -73.53 -15.86 -21.50
12 Lamda f / rho 6.278 2.900 -0.649 -6.278 -1.354 -1.836
13 u * 1E6 -10.21 -4.72 1.06 10.21 2.20 2.99
14 Wavefront -18.94 -22.56 -19.21 -8.28 -4.33 0.00
Reference parabola: y = -0.217853 * x**2 + 0.49017
passing through ( 1.50, 0.00) and ( 8.10, 0.00)

Maximum wavefront error = 1 / 1.0 wave at zone 2

April 4, 2003 09:00 PM Forum: Telescope Making

First Rouge Polishing Session

Posted By Thomas Bennett

Hi All,

I've just polished out a 10" F6.5 Pyrex blank with CeO and planned on doing a final polish and figuring with some ball-milled rouge. This is my first rouge experience so a new lap was made and I tried proceding using the same techniques as used with CeO.

The problem I've encountered is that there is a LOT more drag between the blank and lap with rouge. This makes it very difficult to control the movement of the blank. Will this condition get better (less drag) as the lap get broken-in. Am not sure I can get an accurate figure unless I find a way to get better control of blank/tool movement by reducing the drag somehow. Any suggestions?

Thanks, Tom

May 5, 2003 03:30 PM Forum: Telescope Making

Digital Camera for Foucault

Posted By Thomas Bennett

I'm too lazy to do the research on this and too busy figuring a mirror right now :-). Does anyone have a recommendation for a good digital camera that can capture Foucault images behind my tester, take pictures through my scopes, and use as a general purpose camera for everyday snap shots. The Foucault image is small and faint so guess I'll need a large zoom and shudder control range. Is there an older/used model of digital camera that will do all this and can be purchased on an ATM'er budget? Hope someone can help - Tom

July 8, 2003 03:55 PM Forum: Equipment Talk

Refractor Rationale

Posted By Thomas Bennett

Hi All,

After getting up very early this morning and setting up my 10" F6.5 Newt to take a peek at Mars, I noticed that the seeing was typical for this area (central coast of California). In focus, Mars looked like a fuzzy edged red blob with barely visible white polar cap and black features on its surface. Out of focus, it looked like a bigger red blob at the bottom of a rushing stream of water. Ground level winds were fairly calm but upper air winds were really blowing and fuzzing out the image. My TV85 show a more stable view but no more detail due to small aperture. Would a larger refractor help me get better planetary views in this environment? My 10" does great further inland so I will keep it but this coastal wind really reek havoc on seeing most of the time. Would a 5-6" refractor (whatever I could mount on my GM8) improve this situation much?

Thanks, Tom

August 5, 2003 10:46 AM Forum: Equipment Talk

Zoom + Barlow Experiences

Posted By Thomas Bennett

Hi All,

In the attempt to reduce my eyepiece collection, a Leica 7.3-22mm Zoom (with 2" adapter) has replace 3 fixed focal length eyepieces in that range and now I'm looking at doing the same at shorter focal lengths. The TV 3-6mm is not too appealing since I like comfortable eye relief. Adding a 2" Barlow to the Leica would produce an eyepiece of enormous size (not good) and I'd also like a dedicated high power zoom eyepiece without having to insert and remove a Barlow constantly. Has anyone tried combining a smaller high quality zoom (Nikon for example) with a high quality Barlow (Ultima 2X for example) to produce a shorter focal length zoom with good eye relief and how did it perform?

Thanks, Tom

January 12, 2004 03:02 PM Forum: Telescope Making

Kutter Schief as Solar Scope?

Posted By Thomas Bennett

Hi All,

I’m currently using a 90mm F15 refractor + 2X Barlow with a H-alpha filter for solar viewing and would like to get a little more aperture – maybe 5”. Seems like building a tilted component scope such as a Kutter would allow me to get a F30+ focal ratio in a reasonably sized tube without a Barlow. Has anyone tried using a Kutter for this purpose and is there some reason that it wouldn’t work well in this application? Also, is there some coating that could be applied to the mirrors that would perform the same function as the ERF on the refractor? Thanks for any info, Tom

August 3, 2004 10:14 AM Forum: Equipment Talk

Clueless Astro Salespeople

Posted By Thomas Bennett

Here's 3 real encounters I've had with Astro Salespeople within the last year that are good for a few laughs.

(1) Phone order with well-known astro-retailer:

Customer - I'd like to purchase a 24mm Panoptic eyepiece.
Salesperson - Who makes it?


(2) Overheard conversation at well-know astro/camera retail store with customer looking at an 8" Dob:

Salesperson - Reflectors aren't good for looking at the moon or planets … for that you should buy a refractor.


(3) Conversation at well-known retail store about buying a Paracorr for my F4.5 scope:

Salesperson - Parks once made F3.5 mirrors that were so well figured that they didn't have coma … the Paracorr didn't improve them at all.

April 10, 2005 12:53 AM Forum: Takahashi

Mewlon on GP Mount?

Posted By Thomas Bennett

My first Tak scope (a Mewlon 210) purchased on A-Mart is on its way so guess I'm joining the Tak club smile.

Does anyone know a good way to attach this OTA to a Vixen GP mount? Is the included Tak dovetail compatible with the Vixen saddle or can the Vixen dovetail be attached to the Mewlon OTA? The Vixen dovetail has a pair of unused central holes about 35mm apart - are these holes designed to fit Tak scopes. I'd love to get a Tak mount but my humble GP will have to do for now.

Thanks for any help you can provide, Tom

August 14, 2010 11:30 AM Forum: Celestron

NS11 GPS with Denkmeier Power Switch

Posted By Thomas Bennett

Hi All,

I recently obtained a used S1 Power Switch and tried it out with my NS11 last night thinking this would be an upgrade from using the Celestron 0.63 reducer/corrector. The PS is definitely more convenient to switch between powers but I'm still seeing significant field curvature using the reducer with a 20mm ES, 17mm Ethos, and 20mm Nagler T5. Can someone verify that that Power Switch reducer does not correct for FC like the Celestron unit, or could my PS be defective somehow?

Thanks, Tom