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Messages - Dale Khan

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General Astronomy & Observing / Re: Not a rant, more a plea...
« on: February 08, 2018, 08:44:44 PM »
Well I guess we completely disagree about this. Again, I won't pretend to be wise enough to say what anyone else will think or do, should do. I speak for me, not the millions, and just wanted to say I think the dark sky insistence is poison for our hobby! This is not anecdotal, it is my experience in urban outreach with the under 23 yrs. old crowd - that is feedback - not anecdote.

You keep convincing folks what can't be done and I'll keep trying to say what can.

You didn't listen to what I said. I agree that people should go outside and look.
But obviously the light pollution isn't as bad where you live as it is where I and my brother live.
A lot of nights here, the only stars visible in Orion are Betelgeuse, Rigel, and the 3 belt stars,
and Canis Major is Sirius only.
Tell me what kind of observing can be done in that environment?

I've worked in the astonomy biz since 2005 and sold lots and lots of scopes to beginners and young people, but they all had one thing in common--they
lived in places where you could see several hundred stars (at least) at night. Most were thrilled to see the Moon well, and the planets.
Occasionally, I heard some feedback about the viewing of the Pleaides, or the Perseus Double Cluster or something equivalently as bright.
But none--zero--of them lived in the city I call home.
Maybe some of them will become solar system astronomers (common in cities).
But, talking to them before they bought a scope, almost universally they were drawn to astronomy to see
galaxies and nebulae--especially the ones in the Hubble pictures. What usually followed was about expectations, and starting with
the Moon, planets, large star clusters and globulars.
However, I also heard a lot about disappointment and the difficulty of seeing anything on the planets and how impossible it was to see the Hubble targets after they'd owned scopes
for a while.

What that points out, I guess, is that a little help from a mentor or fellow astronomer could go a long way to keeping the person interested in visual astronomy.
Perhaps steering the observing to those things that are possible in light-polluted skies with small scopes (and I agree there are a number).
There's nothing like having a Dad interested in astronomy to fire the interest of a young person. That will go a long way toward overcoming the obvious difficulties
of urban astronomy.

Beginners Forum / Re: Why larger focal length implies more magnification?
« on: February 03, 2018, 01:01:47 AM »
Hey temax! In my few days of absence, I appear to have missed some new posts!

Thanks for your kind words. I get that same satisfaction and gratitude when the experts here explain things I've always wondered about, and it's great to be able to pass it on.

Quote: "I have one question. I can understand now how the image is formed. If that vertical line where all the image points are being formed is in the prime focus, now all we need is at least *one* of those rays to get to the eye-piece so that the human eye can see that image point. To form/create the image point we needed *at least* two rayes to cross, but since that is already created, now we just need one ray for us to see the image point. Is this a correct statement?"

As Christian already pointed out, that first image formed by the objective (primary image) is a "real image" that forms in mid-air. I like to think of a simple test to verify the existence of a real image as the wax paper test. You can always put a piece of wax paper where a real image is formed, and see it there in full color. But the image is really there whether the paper is there or not (not so with a virtual image). The paper acts as a light spreader - when the sheet coincides with the location of the image plane, each point on the surface of the paper redirects the focused light beam impinging on that point to re-radiate in all directions. When you look at the paper from some oblique angle, your eyeball is picking up from each part of the paper the particular ray that just happens to be aimed exactly at your pupil. So to you, the entire imaged area of paper will appear illuminated with the constituent parts of the image, making all of it visible from your vantage point.

Without that handy diffuser (the wax paper) though, it's just as you say - you will only see the rays of the image that are pointed directly into your pupil. Since the pupil is small, that means you only see a very small portion of the image. You can verify this by looking into a telescope with an empty focuser from about 10" away. You will see an image, but only a tiny portion. If you move your head around, you'll catch more of the image, but the visible area is also limited by the opening of the eyepiece holder.

That's where the eyepiece comes in handy. My new diagram below (admittedly not exactly to scale) hopefully shows how the eyepiece lens (or set of lenses) act to bend and redirect these prime focus rays from the objective into a parallel bundle that points directly into your eyeball, flooding the retina with an image you can see.


Note that between the eyepiece lens and eye lens, the brown bundle of rays are parallel, and so are the green. The brown rays of the trunk are headed toward the lower part of the retina, and the green rays of the treetop are headed toward the upper part and come to a focus there.

At the exit pupil, located right at the iris opening, all the rays from all parts of the image come together, completely stacked and scrambled on top of each other. This helps illustrate why the exit pupil is such an important aspect of a telescope's image - all of the light information collected by the objective lens is concentrated in that little circle of light. If your pupil is not large enough to accommodate this entire disk of light, then you are missing part of the telescope's potential output (equivalent to truncating the objective lens) and getting compromised performance.

Remembering that your eyeball is like a camera that uses a lens to focus a prime image onto a retina sensor, notice that rays from any object you wish to actually see in focus must enter your eyeball as nearly parallel light. Those rays must also be directed into your eyeball through its only port of entry, the pupil, or iris opening. As you can see in the diagram, the eyepiece lens(es) does both - 1) it bends the primary's rays so they are directed neatly into your pupil, and 2) it makes the rays coming from each point of the object nearly parallel before they enter your eye lens, so that your eye can easily focus those beams onto the retina. Note how the tree appears right-side-up on the retina. As you may know, your brain internally turns everything on the retina "upsidedown" to account for the physical nature of optics, so as expected, this basic telescope shows us the tree upsidedown.


I have edited the text (hopefully for the better)

As a complete newb to all this optics stuff, your diagrams and explanations are EXCELLENT! Thank you Steve for taking the time to provide this! VERY VERY helpful!

Best regards!


ATM, Optics and DIY Forum / Re: Lens Cleaning
« on: February 02, 2018, 08:24:54 PM »
Well, thanks for that, but this still doesn't answer my simple question: what does IPA dissolve that acetone/methanol can't? I am not exactly a stranger to chemistry, so you can give me a technical explanation. :o)
Newport, a professional company, states very clearly that IPA doesn't evaporate as quickly and tends to leave drying marks (which is actually true). That's why they recommend acetone/methanol. Given that they use what they recommend, maybe it would be a good idea to let them know that you think their method is incorrect and see what they have to say.
May I suggest Zeiss Lens Wipes for general cleaning? You can purchase them in any store with eyeglasses or optical department and are recommended for cameras, binoculars, eyeglass, and any other type of lenses you can think of, and are very affordable. No streaks either. :o)

Beginners Forum / Re: Yet another beginner looking for a telescope
« on: February 02, 2018, 06:28:04 PM »
Get an Orion 4.5xt or a Zhumell Z130. She can carry those, perhaps.

At outreach, a 12 year old girl discribed to me her telescope, which sounded like a 6" dob. She said she can't move it. She has to wait for her father to get home before she can ever use it.

I just got my Zhumell Z130 and it's pretty big and bulky. At least for a kid. It's a shame that the Edmund Scientific Astroscan is no longer made. Or something similar to it in shape and size.

General Astronomy & Observing / Re: What to look at during a full moon?
« on: February 02, 2018, 05:54:38 PM »
I'll go out with just a blanket or a chair and lie down or sit and relax. Sitting under the light of the full moon with crickets and nature around you is about as relaxing as it gets. Even on a still winter night, being out with the moon reflecting off the snow is really beautiful. Plus you can study the features of the moon with your naked eye.

Sometimes forgoing the equipment and just soaking in nature is great.

Mounts Questions & Expirience / Re: New Software Capabilities You Want
« on: February 02, 2018, 05:04:24 PM »
AstroArt 6 with SP2 has some amazing scripting capabilities. Observation Manager can now also control PHD.
Lots of great stuff out there if you know where to look.

ATM, Optics and DIY Forum / Re: Tube End Covers for Hasting Tube?
« on: January 31, 2018, 10:41:18 AM »

Do you use a reverse mold. They look beautiful.


Thanks Pat,

Yes I use a "female" mold. Sometimes I've made "one-use" molds from wood, but I have a number of fiberglass molds for various sizes also, that will last forever. I used to make these for club members and friends, so if I thought there would be a need for more than one set of a particular size, I made a permanent mold.

ATM, Optics and DIY Forum / Re: using a slightly undersized tool
« on: January 31, 2018, 10:21:00 AM »
Best way to refine an 80 grind---(here goes again for the N'th time !!!!)----TOT, with a 7/8ths diameter tool !!!!!!!!!!!!!!!!!
This is a concrete RULE. Grinding or polishing, the tool size mentioned works either way to result in good spherical surfaces that don't change much over time. It works extremely well with an optic that has a diamond generated surface. (Precisely the way optical shops work- they generate the curve, then fine-grind and polish. It's the most efficient way to manufacture an optic timewise.)

I doubt that's set in concrete...only tiles are  Obviously you'd have great difficulty and danger using 7/8 size TOT on large mirrors, say 30" and up, and it gets heavy and hard to handle even in the 20-30" range. I have used tools from full-size down to 60% with equally good results. It depends primarily on the type of strokes and overhang you use, more than any particular precise tool diameter.

Here in the MD they close down all the parks at night. Every state park has a sign near the entrance that it closes at dusk. If you are out alone anywhere at night doing something outside the norm you are a real suspect. I've been messed with by the police so many times that I don't bother seeking dark spots anymore.

There's a spot in PA about 2 hours from me that offers Bortle 5 skies, and I put up with the campers, hunters, and "other" activities. But they are enough of a distraction (and sometimes fear) to make one wonder why I should be bothering chasing DSOs, and not serenely using my 4" in my red/white zone back yard.

There are several premium dob makers, several astrophotography vendors, more eyepiece brands and series than you can count (with several premium options, not just one or two), many premium scope brands, all kinds of different optical designs to choose from etc.

I guess we should consider ourselves lucky that we have so many choices and that the industry is so competitive

the real problem is they have no reps in the states.

if you get a bad mirror, you have to ship it back to Hong Kong.

too much hassle if something goes wrong.

they need a rep in the states that we can ship to instead of main land China.

Another real problem IMO is the questionable (also IMO) performance data they publish with their mirrors (those sold on ebay at least). Seems they pretty much all now quote Strehl's of 0.98 or 0.99. One question immediately arises - are these numbers based on interferometry or simply a knife-edge test? If the latter, I would suggest that is disingenuous as a knife-edge test does not take into account astigmatism and surface smoothness, both of which can adversely affect the image. Even so, even if based on interferometry, the numbers seem too high for a "mass produced" mirror, esp when compared to that offered by premium makers.

Anyhow, as I understand it, "Strehl", by definition, is supposed to take into account the entire mirror surface so a "knife-edge" calculated Strhel is a pseudo value and should not be used unless it is qualified as such, or maybe not used at all.

Above happily withdrawn if incorrect but I think it would increase confidence in Hubble mirrors if they would elaborate a bit on how they test their mirrors and what the numbers actually represent.
they test the mirror with a Knife Edge, and it looks like they use this program to crunch the numbers

in the zambutomirrorgroup Cal Marlet has a 3 files documenting CZ's writings, in one of them CZ corresponds with Steve Koehler [a zermike polynomial guru] about IF. They both agree that IF will show the erros more by a magnitude of 2.5x so a 1/10~KE reading will be 1/4~ in IF. But most IF reports do not develop an entire mirror profile cause they only measure the lowest and highest point on the mirror.

Mounts Questions & Expirience / Re: Pesky CGE PRO clutch knobs.....
« on: January 31, 2018, 01:23:08 AM »
I actually thought about doing that. I do plan to take some 10mm X 1.25 cap head screws and put them in my drillpress vice to see if I can drill the ends accurately enough to insert the tips from the stock screws. I think the tips in the stock knobs are teflon because they are pretty hard.


Grainger has M10x1.25 cup-point set screws:

Sadly, none in that size with conformable tips. Still the cup point may make drilling out for the insert a bit easier.


ATM, Optics and DIY Forum / Re: Mirror Question
« on: January 26, 2018, 05:24:55 AM »
Perfectionist principals call for grinding down all the way through the chip. But what you say is supersufficient (not a new word) since the chip at an angle scatters nothing into the light path, just like the bevel doesn't.

Obviously "perfectionist principals" are subjective  I like that word "supersufficient" even though the CN spell checker doesn't agree LOL

So as you can see, a larger and faster mirror requires a pretty big separation and I don't think 60 meters is nearly enough. Best to use a real star. For bigger and faster mirrors, the distances required for SA test get very large, very quickly.
Thanks for the info regarding SA and star distance tests. My artificial star is home-made and too dim to put it further. I'll try on a real one next time.Regarding the astigmatism - yes they did silicone the primary. Maybe for overseas shipping it's not a bad idea. I'm really busy lately so it will take me some time before I get to work on it again. Then I'll check if the problem is within the primary or the secondary mirror and post new photos after it's (hopefully) sorted out.

I don't think they realize that at some point, when I'm done tweaking everything, a review on this telescope will be posted on my website (my last review of Skywatcher BKP 250 is around top 3 results in google). Maybe I should let them know that?

By the way regarding the focuser - they acknowledged that it has an issue, and initially I was asked for 80$ per technician hour to get it fixed (obviously I haven't agreed to this and waiting for further answer).

I paid via pay-pal and have till September to open a case. I'm not sure I should to send the whole setup back since the shipping (from Israel to UK) would cost me a lot. However I can try to make them to refund the focuser. There is no other 12" OTA that my EQ6 would hold (the only alternative is TS one which is a bit heaver and way more expensive).

1) Tony Donnangelo and his 24" f/3.3 Starmaster Sky-Tracker Dob at Cherry Springs

2) 22" f/3.6 SDM Dob at Cherry Springs

The Starmaster StarStep Observing Chairs in both photos are 42 inches in height.

Dave MitskyAttached Thumbnails

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