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M20 Trifid Nebula in LRGBHa

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Posts Made By: Randy Roy

April 3, 2005 04:09 AM Forum: Equipment Talk

C8 & C9.25 - what is maximum fov?

Posted By Randy Roy
Max field = Field stop X 57.3/focal length of scope

The C8 and C9.25 field stop diameters are limited by their 1.5" port so use 38.1mm for the field stop. Therefore for a C8 at F10:

F10 Max field = 38.1mm X 57.3°/2032mm = 1.07°

Unfortunately, the use of a focal reducer further stops down the rear port due to the cell that holds the FR. Instead of 38.1mm, use 36mm for field stop.

F6.3 Max field = 36mm X 57.3°/1280 = 1.6°

These figures aren't absolute, because with a moving primary system the distance from the rear port to the focal plane is somewhat variable. However, it is close and coincides with what I've experienced using the star drift method of determining FOV. BTW--This means field stops of more thatn 38mm at F10 or 36mm at F6.3 will result in vignetting. Practically, a 27mm Pan or 22 Nagler T4 is about as big as you can get in a 2" eyepiece at F6.3 without vignetting.

To compute the above for a C9.25, simply substitute the longer f.l. in each equation.

Best wishes,

Randy


April 5, 2005 01:27 PM Forum: Equipment Talk

14" Tscope big jump from C9.25?

Posted By Randy Roy
Dave,

IMO the best way to measure aperture jumps is not in perceived differences in brightness, but in how deep each scope can go visually. I've seen nights where the brighter DSOs looked quite similar (in brightness) in my 16" and others' 14" and sometimes smaller scopes. However, when we point at really faint objects, such as the Horsehead (with H-beta filter) or Stephan's Quintet, I've rarely seen a smaller aperture scope keep up. Another test is fine planetary detail, but this often takes good seeing and specific planetary observing experience to discern critical differences in detail. Once again, IMO, most observers cannot tell differences in brightness of less than 15-20% on bright objects, but if you are viewing nearer the limit of your scope, the differences are more profound and if the seeing is decent the improvement in contrast (of a newt vs an SCT) and resolution of a larger instrument is very apparent to the experienced eye.

I would consider the jump from a C9.25 to a 14" Tscope to be huge based on all this. There are simply many, many more objects that can be viewed with the 14" that will not be seen in the 9.25" due to the superior contrast, light gathering and resolving power of the 14. But, you may not discern this so easily by simply comparing views of the brighter DSOs.

Best wishes,

Randy

April 6, 2005 12:52 AM Forum: Eyepieces

Which 1.25 Diagonal?

Posted By Randy Roy
William Optics makes an excellent 2" diagonal for $129.00.
http://www.buytelescopes.com/product.asp?pid=3229

You will need a 2" visual back like the one made by Televue or Astro-Physics.

Randy

April 6, 2005 08:17 PM Forum: Equipment Talk

It's official

Posted By Randy Roy
It will be interesting to look back on today in a few years. I'll bet we will remember today as the day a lot in our hobby changed.

Randy

April 15, 2005 12:41 PM Forum: Equipment Talk

Avoiding the shadow of the secodary?

Posted By Randy Roy
As Floyd mentioned, this phenomenon is a common problem related to exit pupil. It has to do with the exit pupil of the light coming from the eyepiece being larger than the entrance pupil of your eye. It is much more noticeable during daytime viewing when your eye's pupil is much smaller due to all the light entering your eye. An 11" scope can gather quite a bit of light on brighter objects and as we all know, the moon and even Juipter can be uncomfortable to look at through a large newt. This causes the eyes pupil to consrict (losing dark adaptation, bummer) and making the exit pupil of the ep, that may normally be fine for dark adapted eyes to be too large for the constricted pupils.

I've noticed this in my F4.3 dob, but find that it is not a problem when I use a polarizing filter or even one of those cheap moon filters. Frankly, I have not noticed it on Saturn and have to wonder if your eyes are particularly light sensitive, meaning they would constrict more than some.

Best wishes,

Randy

April 20, 2005 07:38 PM Forum: Equipment Talk

Buying New Scope

Posted By Randy Roy
If I were you, and I realize I am not, I would not settle for less than ZOC optics. I know others are capable of making optics as good as Carl's, but I also know that buying ZOC ensures that you are getting superb optics.

IMO, the Starmaster 12.5 ELT is the best scope in this size and price range. Again, I am speaking for myself. But, if I did not have my 16" Starmaster and I were making a similar decision to yours, I would not agonize about the purchase for second. I would call Rick Singmaster and order the ELT.

Randy

April 23, 2005 03:36 AM Forum: Equipment Talk

Promises promises

Posted By Randy Roy
It sounds like you did not have a real delivery date to begin with? If you still want the scope, I would contact him and let him know you really need to have the scope by such and such date (say 4 months from now) and if he can't commit to that date you really need to have your deposit back. If he commits, hold him to the date. If not, tell him you can't have your money tied up any longer without knowing when you will receive your scope and instruct him to return your deposit. I would leave room to be able to say that you did not "Cancel" the order, but rather the supplier simply did not deliver within a more than reasonable time.

If you have any difficulty getting your deposit, simply post the facts of the situation and his name. Frankly, if I had any difficulty getting that large a deposit back, especially after being as patient as you have been, I would have my attorney send a strongly worded letter. It will cost you a couple of hundred but it usually gets results.

Best wishes,

Randy

May 1, 2005 04:23 AM Forum: Equipment Talk

R.I.P. ... The USPS Destroyed My Mirror!

Posted By Randy Roy
Though I am a ZOC user and sometimes ZOC bigot, if I were you, I would at least consider buying a 10" F4.5 Discovery mirror. The ones I've seen lately have been quite good, are reasonably priced and I am guessing have a short delivery time. If you aren't happy with the figure, send it to someone to refigure it, or sell it when your premium mirror arrives.

Randy

May 5, 2005 02:45 AM Forum: Equipment Talk

2" Autocoliimator

Posted By Randy Roy
I recently received mine along with the 2" sight tube and the 2" cheshire. I was skeptical of the talk until I met Jim at the Mid South Star Gaze last month. I put his 2" autocollimator on my 16" Starmaster and was amazed at how much easier it was to get my donuts (now triangles) to all line up. The 2" just makes them easier to see. I sold my Tectron tools and have no regrets. In fact, I can't imagine I'll ever use a laser again either.

Randy

May 6, 2005 01:58 AM Forum: Equipment Talk

Yikes! Burned out focuser Cap

Posted By Randy Roy
Mark,

We astronomers give that warning to folks so many times that I fear we get complacent ourselves occasionally. Your pic of the burned cap is a great reminder of how quickly a wonderful and otherwise safe hobby can turn to tragedy.

Randy

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