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Messages - rennlispuring

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ATM, Optics and DIY Forum / Re: Coma correctors for fast Newtonians
« on: February 09, 2018, 08:27:16 AM »

If you want full coma correction (vs. partial suppression) for paraboloids you'll need at least a 3-element Wynne type corrector. Two-element correctors only suppress coma and usually have considerable chromatic residual -- unless expensive exotic glasses are used.


For this application I don't think extra elements would be a negative. Indeed, "whatever it takes".

Rather, I am surprised that with the growing popularity of electronic imaging there is not a solution on the market that offers good correction while maintaining the photographic speed of a paraboloid objective.

Maybe it is just not possible?

It's possible with multiple elements and exotic glasses but amateurs always want a cost-effective solution, which is much more difficult to achieve.

One reaosnable solution for small reflectors (up to 10-inches) is the Lurie anastigmat. It would require a full-aperture 2-element corrector which can be made with the cheap N-BK7 crown. It converts the Newtonian into essentially a visual Schmidt camera -- free of spherical, coma and astigmatism. The downside is of course, extra weight. However, the corrector is placed at or near the focus and not the RoC as in the Schmidt camera. This keeps the telescope compact. These systems with mirrors as fast as f/3 and cover wide fields. The corrector also has very lax tolerances which makes manufacturing a lot easier, especially for ATMs. Here's an example of a 10-inch f/4.2 covering a 2-degree filed (and could go much wider).
This configuration should not be confused with the more popular Houghton (aka Houghton-Lurie). Lurie anastigmats work only with aspheric mirrors, as opposed to the Houghton which works with spherical mirrors only, and is aplanatic (free of sphericla aberration and coma only), not anastigmatic (free of sphericla aberration, coma and astigmatism).


Mounts Questions & Expirience / Re: Different Sources of Tracking Error
« on: February 09, 2018, 12:53:58 AM »
Things that I’ve not seen mentioned here:

 Observatory mounted systems can pick up vibrations in the building and ground being transmitted to the telescope; things like astronomers with big feet clomping around, closing doors, and 18-wheelers driving past on a nearby road. Just like any other vibration there might be a harmonic effect where this error is at a max one time, and gone another.

 Systems set up on soft grassy surfaces can slowly ( or not so slowly ) sink into the soil. You might be tracking will - and a few hours no - because your rig has shifted.

 Large temperature changes over a night or season can impact everything from gear mesh to the attachment of camera to telescope. If it works in summer, it might not in winter without mechanical adjustments.

Yes George, you are correct, I have left out the diurnal temperature effect on mechanical equipment and the changes in focus position. The period for this effect is the same as that for polar alignment. Thanks!Seasonal changes always require some tune up from month to month, or even week to week. We have had a temperamental dome position encoder wheel causing us issues when the temperature gets really cold and the spring tension does not keep up with the contraction. total PITA!

General Astronomy & Observing / Re: Retiring under Scorpius
« on: February 03, 2018, 01:00:21 AM »
Just a personal ramble. I guess I "officially retired" today. I'm 67, and for the last 42 years, I've worked as a solo dentist in a small (pop. 1500) town... Something of a dinosaur. I didn't bring my job up much in here, or in the old sci.astro.amateur, because stargazing was an escape for me. Dealing very closely with people, sometimes under stressful conditions, often made the job stressful for ME, and I tended to find relaxation and perspective alone under the night sky, as I have all my life.
 Last night was clear and pretty, and a very pleasant 57 degrees F, and I stepped out with my 2x54 constellation binoculars. Enjoying my casual ramble, I found myself repeatedly being drawn back to Scorpius, rising in the southeast. The delights in his curving tail hadn't risen yet, but the distinctive little pentangle of stars above Antares, and the barely visible M4 were very pleasing. And the head of Scorpius is a wonderful sight in any binoculars. Just a nice grouping of bright stars, set off with such things as the little pair of omegas.
 Walking back to my house, the thought struck me that Scorpius had graced my previous "retirement..." the last college job I'd had was delivering pizzas. It was a good job for me, with hours from four pm to one am, and I got to be the proverbial "fly on the wall" in a number of situations. Late in May, close to the end of my college days, I was driving around Lincoln, Nebraska, delivering pizzas while enjoying a total Lunar eclipse in Scorpius. As much as I enjoyed the eclipse, the fun was complemented by the sight of the late night bar patrons wandering out and standing in the street marveling at the reddened Moon.
 After I got back in the house, I checked my 1975 Sky & Telescopes to see if my memories were accurate. Yup. The August Sky & Telescope (maybe things moved a bit slower in '75,) had a picture on the cover of the eclipse in Scorpius, and it had occurred on the night of May 24 - 25.
 I suspect from here on out, I'll always associate Scorpius with retirements. Funny how this stuff works out sometimes. 

Marty, congrats on the start of your retirement. Your next eclipse is just a couple of months away.

Light Pollution Topics / Re: Why light pollution will never end
« on: February 02, 2018, 10:04:23 PM »

And in my country, astronomy isn't the big impact to make people or government stopping planting lights everywhere, in fact government are waiting people to thank them for providing lights most places, and in fact in the radio everyday i hear people complaining about no lights in their areas or where they live, so it is desperately that astronomers in my country will be listened, and i am not against astronomers, but i can't be against those people who are seeking for lights in their environment, i just hope or wish that government can submit new good light that have less issues for sky or light pollution.

It would be OK for them to light their environment, just no one elses.

I was looking at the light pollution map for your country and noticed that most stretches of road are well-lit. You have some dark skies but no way to get to them and nowhere to set up if you could get to them.

Some of the artificial islands in the Gulf look to be in a green zone, are they accessible at night?
Mostly the streets or roads aren't owned by someone, and most houses are near/next to each other, if someone asking for the light in the street so he can reach house the government will never plant one or very few for him, they will plant the lights the whole street or most parts of it, so one request and the whole roads are done, and some houses maybe doesn't like, but i really never never hear in radio or newspaper anyone complained about having street lights next to his door, or no groups who are annoyed by lights, so it is look like one request for all, and those who really doesn't like or doesn't want will never talk, we don't have, but even if someone complain about the light, it is mostly they try to solve that light for him only, so even if they remove one light or 2 next to his house, how much that will solve the light pollution in the whole area anyway?

So you saw the map of my country and you saw the suffering then, and we hate to drive far anyway, even if we do it is not 100% dark or not even 90%, so we don't bother, i will try from my yard anyway, i can do the sun and moon and few planets very easy, so this one isn't a worry, i will only worry about DSO.

I am not sure about the islands, some can be accessable, but to be honest, visiting islands in my country is like asking to meet the queen, you need to go under long request or meeting so many people or submitting for a visit and waiting the answer, and it is way way long distance because mostly it is only 2 ports to transport from, one could be nearby, but the other i have to drive so far to reach and have an access or approval, not worthy for me, i rather go to desert with some light pollution better than seeking approval to visit an island.

There are areas from the map as i can see that is yellow or orange or even green, not sure how good are those zones compared to my ed zone, but sounds the country is small, so even in yellow or green zones it won't be that much huge improvement LP, but in winter with cooler weather and non full moon i can see better the sky, say about 20% better than now, and my urban isn't too much light pollution anyway.

Look at the example in next post

Reflectors Telescopes Forum / Re: GSO seconday holder rebuild ideas
« on: January 31, 2018, 12:10:26 PM »
The most simple fix is cut two milk jug washer the size of the stalk and place on top of the stalk .

You can slit the washers so you don't have to remove the stalk.

I just use nylon screws ... They hold very well and have that nice compression feel to them.

Beginners Forum / Re: Advice on planetary viewing with Nexstar 8SE
« on: January 31, 2018, 10:28:07 AM »
A 10mm eyepiece with a 2x barlow is equivalent magnification to a 5mm eyepiece.  And that 400x is about as much useful magnification as you will ever get out of an 8" SCT.  Yes, more magnification can make things bigger, but it will also make them dimmer, and will almost always make them fuzzier.

Now the 8mm WITHOUT a barlow may give you a better view than either the 10mm or the 10mm + barlow-- it is a better quality eyepiece and gives you 250x magnification compared to the 200x you get from the 10mm. If seeing conditions are good enough to support using the 8mm (and its not so much clouds-- although they are obviously a problem-- it is the amount of turbulence in the atmosphere), then it will be better.

I use (discontinued) Vixen Lanthanum LER (Long eye relief) eyepieces for most of my planetary viewing.  For me, they are crisp, and the eye relief is wonderful (20mm for all focal lengths), and they don't have ghosting or black out issues.  I also have a couple of "research grade" orthos that are even crisper. I only pull these out when the conditions are wonderful, because they require me to take off my glasses which is a major issue for me. I don't have anything shorter than than 7mm, and I almost never use my barlow (at least with my 8" SCT).

My recommendation is to find friends and/or a club so you try some different options to see what works for you.
If you live somewhere that makes that difficult, try buying used so you can resell at a minimal loss.  Also, observing skills definitely improve with practice, and its not like you are looking though junk eyepieces at present.

Mounts Questions & Expirience / Re: Tak Teegul Lapides
« on: January 31, 2018, 03:51:33 AM »
Yeah, I know. I think I'll just stick with my DM-6, UA Unistar, and Stellarvue M1 for now (written in order from most robust to lightest).

Eyepieces Questions & Recommendations / Re: Need advice on new zoom lens
« on: January 31, 2018, 03:10:49 AM »
I didn't note it when I went through that extensive typewritten listing but I'm tired and my eyes are tired so maybe I missed it. This appears to be the modern version of the one with the better glass from the early 1980's that I was talking about. It's more expensive but it is probably worth the extra bucks -- better glass by far and far better eye relief and a better eye cup to view through. It will be a whole lot more comfortable and a lot more enjoyable of an experience viewing through this one while seated. If I were going to buy a modern one -- this is probably the one to get -- a lot of bang for the dollar. The question is whether it will fit into your scopes; and, I don't know the answer to that but I think that it probably will.

Mounts Questions & Expirience / Re: Powering a Goto Mount
« on: January 30, 2018, 09:15:41 AM »
Harbour freight 17AH battery...$49! Works perfect and a good choice. Don't buy anything rated in mAHR, they are usually calculated at 5v not 12. The Harbor freight 17AH battery is a no brainer really imo. I used it for my ieq45pro and camera.

Eyepieces Questions & Recommendations / Re: Carrying your eyepiece's
« on: January 30, 2018, 02:43:16 AM »
I keep my oculars in their original boxes and if I intend to use several different ones I wear a vest designed for photographers that has lots of pockets. The lightweight vest works very well for toting eyepieces and filters. Mark

Bingo! All the tweaking paid off. I can put objects in the fov of my 31 Nagler consistently. I"d still like azimuth to be smoother. Not sure what else to try, but I'll keep tweaking.

Light Pollution Topics / Re: The ultimate light pollution?
« on: January 26, 2018, 01:40:26 AM »
Heralding the demise of night for amateur astronomers because of this purposed satellite? Give me a break! This has got to be one of the silliest gloom and doom threads I've ever run across on this forum. Did any poster even give a moments serious thought to the circumstances governing this satellite potential impact?

First off, at 170 square feet in surface area this satellite really isn't very large at all. It's reflective surfaces are flat like a common mirror's. The article, what ever its true degree of accuracy, suggests that at least one of its triangular reflective surfaces will be aligned so as to permanently reflect sunlight in the direction of the Earth's surface. It will be orbiting the Earth at a rough distance of 600 kilometers every 96 minutes, or so.

The consequences of these facts, considering that the satellite presents only a flat surface of reflection, means that it will likely only direct a very narrow beam of sunlight onto the Earth's surface. Given the relatively small reflective surface area, combined with the distance from the Earth's surface, will dramatically limit the size of the full illuminate spot projecting on the Earth's surface. With the satellite orbiting the Earth once every hour and a half this solar reflection spot will sweep rapidly across the Earth's surface. I suspect that even were the beam to intercept the observer's location exactly the flash of maximum brightness off the satellite would last no more than a few seconds anywhere along the path's center-line. Peak brightness would be preceded by a progressive brightening phase and followed by a similar decline, the entire event lasting a total of perhaps a minute or two.

My guess would be that even just at a few dozen miles off the path's center-line the observer would see an event no different from those we see now almost any night with the passage of various Iridium satellites which we have been viewing without harm to our skies for years now. Some Iridium passages supposedly peak in brightness at -8 right on their path's center-line. This would also be as bright as a quarter moon, yet I haven't heard of them ruining our skies. I would advise folks to think over any replies regarding the heralding of night's doom from any single satellite before posting and to worrying more about their own ever growing local terrestrial light pollution sources. These are what will bring about the end of night for us all.


Getting there...

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ATM, Optics and DIY Forum / Re: Help for coudermask
« on: January 25, 2018, 11:33:56 PM »
Model it in FigXP by setting up an ideal mask and then perturbing it while keeping the readings the same. You'll see it matters very little in practice.

I did not know that, Jon. You are absolutely right. A Coulter would be the worst choice for that project. I can hardly believe anybody would do that. Glen

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