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Messages - Jim Parker

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1
Light Pollution Topics / Re: LP Map (new to us)
« on: February 09, 2018, 10:31:47 AM »
TY Deranged...we didn't know about

http://www.lightpoll...it/indexen.html

2
Eyepieces Questions & Recommendations / Re: Clave 30mm
« on: February 09, 2018, 07:38:17 AM »
Hoi hoi,
Has anyone seen a Clave 30 mm in 2" barrel size ? I saw it in old Clave catalogues, but no actual photos.

3
Quote
In EU the Astronomik 2" is around 200€, the Ultrablock is ca 50€ cheaper.
By the way, does anyone know which is the source of the current Celestron's "UHCs"?

It used to be Celestron filters came from Baader (i.e. both companies got their filters from the same Chinese source).
Whether the current owner of Celestron (Suzhou Synta Corp) would source the filters that way or just get them
from their own supplier would only be known by people at Synta who procure the filters.

The images and filter housings still look the same as Baader's.
At any rate, Celestron's UHC/LPR filter is more of a broadband ("CLS-type"), not a narrower "UHC-type" like the Astronomik UHC or the Orion Ultrablock.

The Celestron UHC/LPR filter and the Baader UHC-S filter have virtually identical bandwidths, and both would be considered "broadband" or "CLS-type" filters.

4
Beginners Forum / Re: Star alignment problems - dec balance?
« on: February 09, 2018, 04:23:13 AM »
Quote
Had the same issue with aligning my Orion 150mm go to reflector. The issue was when I was inputing my latitude and longitude. .. it has to be in degrees and arc minutes. I was just using the degrees in decimal form. Once I converted into arc minutes my alignments were dead on.

Oh man, I think that I put it in decimals, because that is what google maps gave it to me as. I will have to check! But I did as an alternative put in my closest city instead which is only a few miles away. That didn't make a difference really, but I will double check. Thanks!

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I use a G8 for piggy-back (up to 300mm lens) and with my 80mm refractor. I had the older stock tripod and felt it was not adequate for AP unless fully retracted. I switched to a G11 tripod and that made a huge difference. After selling the G11 GEM I switched to the tripod that came with my Titan, fitting a G8/G11 MA into the Titan receiving head. Both arrangements were rock steady.

I have no experience with the new G8 tripod, it might be fine.

Ed

6
Beginners Forum / Re: Help me survive an F4.7 dobsonian
« on: February 03, 2018, 11:21:28 AM »
Yes Jon but don't you use a $500 coma corrector in all your Dobs? That is very different from using an Expanse w/o coma corrector at F4.06.

7
Beginners Forum / Re: Orion Short Tube 80 - A few questions...
« on: February 02, 2018, 09:36:52 PM »
Quote
While you can add all kinds of refinements to the ST80, the beauty of this scope is it's combination oflight weight, durability, wide field-of-view and simple construction at a price that's cheaper than most binoculars.
Quite so. And that light weight and low cost are largely a by-product of said "cheesy" focuser. Which, honestly, isn't all that bad. Especially if you use the ST80 for what it does best, namely deep sky at low to medium power.
Although I do understand why Jon married a 2-inch focuser looking for a home with an ST80 looking for a focuser, it somehow seems a bit like sacrilege to me. Sort of like putting a sports-car engine into my trusty old Dodge Dart, whose beauty stemmed from its simplicity, reliability, and low cost.

8
Mounts Questions & Expirience / Re: Just hit the "buy" button on a mount
« on: February 02, 2018, 06:58:20 PM »
Both heads are for vixen style dovetails.

Orion sells a short vixen dovetail bar with a standard 1/4"-20 screw for mounting a camera.
http://www.telescope...eyword=dovetail

9
Beginners Forum / Re: Advice for all purpose grab and go OTA?
« on: February 02, 2018, 05:54:10 PM »
Quote
The focal reducer doesn't actually change the focal length of the scope nor does it change the maximum field of view possible, those are determined by the optics and the geometry of the telescope. What a focal reducer does is change the effective focal length of the scope much the same way a Barlow does. You can get a wider field of view with the same eyepiece with a focal reducer but eventually you run into the fact that the rear port of a 5 inch Mak, of a 5 or 6 inch SCT is only 1 inch and diameter, vignetting is inevitable.

Over the years, I have been blessed with a large number of scopes and the opportunity to spend enough time with them to get to know them. In this size range, these include refractors from 60mm to 120mm, achros and ED/apos, Newtonians, 3 inches, 4.5 inches 130mm and 6 inches of a variety of focal ratios as well as Maks and SCTs, 90mm, 4 inches and 5 inches. This is the way I see it based on my experiences:

An all "purpose grab and go scope" needs to be reasonably portable, one trip out the door on the mount is my criteria. It needs to be capable of reasonably wide fields of view as well as clean, crisp high magnification views and it needs to be thermally stable, take it outside or to a local park, I want the scope to be giving me decent quality high power views essentially the moment I am out the door, seeing of course permitting.

Refractors, Cats and Newtonians can provide good quality high magnification views. both Newtonians and good quality refractors can provide the low power, wide field views.... But only the refractor is stable enough thermally that it can provide nearly it's best views within moments.. Newtonians and Cats take real time to become truly thermally stable and if the temperature is dropping, they may never thermally stabilize. To be rock solid at 300x, my experience is that a hour might be enough but some nights, it just never happens.

Not long ago I setup a Orion 127mm Mak before sunset. I waited about an hour to begin observing and I observed for about an hour, either the seeing was not good or the scope had not cooled down. I went inside and grabbed my 120mm ED refractor, brought it out, looked at the same target I had been struggling with, a close double star, and the refractor provided a clean view and made the split. Even with scopes this size, thermal issues are very important.

For me, grab and go.. it's a refractor.. Whether it's an 80mm, 100mm or 120mm, they can all do the job.

Jon

10
Beginners Forum / Re: Minimums to see a couple items
« on: February 02, 2018, 05:03:30 PM »
I guess thinking about it, I asked a pretty stupid question. I do not regret it however because there is a lot of interesting information in this thread. Thank you.

11
I'm into vintage cameras and lenses so there that aspect I have of Japanese optics as well. Most of my most prized lenses are made in Japan and some were very very inexpensive with a few Zeiss and Zenit exceptions.

12
Quote
Jon,
Thank you very much for your explanation. Your math makes perfect sense, you made me think, and I have learned something. However I don't understand why M-42 for example, is barely bright enough to be seen without a scope, but is quite a bit brighter with one.

Think of it this way: Imagine a single brick in the Sun, seen from three hundred feet away. It is very small, almost invisible. Now take a lot of bricks and build a wall, 30 bricks long and 30 bricks tall. The area is now 900x larger and it is very easy to see from a distance, yet each brick is no brighter than the single one from before, though the wall *appears* much brighter and more visible to the eye. Telescopes work in the same way. The surface brightness of an image of an extended object is not brighter than before, it just appears to be, because it is much larger and easier to see.

Stars are an exception to the rule. They really do appear much brighter, because they are point sources.Clear skies!
Thomas, Denmark

13
Beginners Forum / Re: Image size based on aperture
« on: January 31, 2018, 08:59:05 AM »
As others have noted, the idea of a fast scope vs a slow scope originates from photography. A camera lens have a diaphragm that can reduce the effective aperture of the lens. (Why would you want to reduce the aperture? To increase the depth of field, to increase the requiredexposure time to intentionally blur things, or to increase the sharpness-- most camera lenses are not at their sharpest with the diaphragm wide open.) To properly expose a picture, two things need to be set: the shutter speed and the f-stop,(the effective f-ratio with the lens ). Shutter speeds aretraditionallyincremented by doubling the speed, while the f-stops are changed by a factor of the square root of two. A typical 55-200mm consumer zoom lens probablyoperates at f/5.6 with the diaphragm open, while a relatively inexpensive 200mm prime lens mightoperate atf4 , and a profession 300mm will typically be an f/2.8. At maximum aperture (and minimum depth of field), the 2.8 lens can take pictures at a fourth the shutter speed of the zoom lens at 200mm. That's why it is called a "fast" lens. It also is much, much larger, and much, much more expensive.

You are absolutely correct that a 400mm f/5 scope with a 3x barlow and a 1200mm f/6 Newtonian will produce the same magnification with a 10mm eyepiece. However, the 400mm scope is now an effective f/15, so, its image will be dimmer. If we put a camera on both scopes (with the barlow functioning as a 3x tele-extender), the image sizes will be more-or-less the same, but the Newton will require a substantially shorter exposure (abouta fifth of the time required for the refractor with the barlow).

And this is where I suspect the idea that aperture is somehow related to image size comes in to play.As long as the type of scope remains constant, image size is directly proportional to aperture, because focal length will be proportional to aperture: a 10-inch f/10 SCT at prime focus produces an image with the subject twice the size of that of a 5" f/10 SCT (assuming the same type of camera). If I'm looking at f/5.6-ish refractors, the 120mm scope will have a narrower field of view than the 80mm. They are both 5.6 scopes, so if I take a photo with each of the same object, the exposures should be the same. However, my subject will be 50% larger in the photo taken with the bigger scope. Depending on the subject, this may or may not be desirable.

14
ATM, Optics and DIY Forum / Re: Foucault tester LED-lightsource
« on: January 31, 2018, 06:30:53 AM »
Further on the topic of light source size and contrast, sensitivity of the k-e test, I went back to the interferometric results obtained earlier, and described in #4. Based on the mapped wavefront (by the way, each pixel on that wavefront has an assigned height value!), one can also simulate the Foucault appearance, the star test performance, and the Ronchi test.

So here's the IF-based simulation of what the Foucualt test should look like at 25 μm (can't get it down to less than that)
and here is with a 102 μm source. The sensitivity of the test is clearly diminished. This is in agreement with actual test results posted earlier.
Mladen


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Beginners Forum / Re: nebula/ color
« on: January 31, 2018, 05:59:28 AM »
Indeed, Jon. Such color schemes as employed like the 'Hubble palette' can and will have even fairly similar-color, adjacent mission line wavelengths assigned rather different color in the interest of highlighting their contribution in both intensity and location. This is certainly misleading visually, but most useful astrophysically.

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