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Messages - Justin Prasad

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The Ramsden was an improvement on the poor color correction of the Huygens.
But the 3-element Kellner was a dramatic improvement on both.
I don't really understand why someone would subject himself to an eyepiece that was considered poor
in the 19th century since Kellners and Plössls and Abbe orthoscopics exist, even for the f/15 refractors out there.
Eyepieces have advanced since Huygens and Ramsden designed theirs.

I finally found a picture that shows my 6 inch on my homemade GEM pipe mount. That was many moons ago, kids are grown up and have their own kids now
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ATM, Optics and DIY Forum / Re: Folding a 7" f/12 R35 iStar refractor.
« on: February 08, 2018, 03:42:08 AM »
I used a coater once that did the large clip thing, which spoils the diffraction performance of the front surface. Clueless really. When I removed the coating using Ferric Chloride, it came off in a flash, and must have been quite thin indeed. Our club uses a ring that tightens around the edge of the mirror to hold it securely in the coating chamber, kind of like a hose clamp.

Majestic coatings does it the right way FWIW.


Silver also has a poorer response in the critical 400-500nm range, compared to aluminum.

The devil is always in the details and generalizations typically always false


Beginners Forum / Re: $ eye pieces
« on: February 03, 2018, 01:24:54 PM »
This is the way I see it:

"The best" is a very subjective thing. With very few exceptions, every eyepiece you can buy is quite good, particularly in an F/10 scope. The only way you can "buy the best" is by discovering just what it is you like and what is important to you in an eyepiece. Building a set of eyepieces is a learning process, how important is eye relief? how important is field of view? how important is off-axis correction, scatter, weight, physical size? How much eye relief is enough, how much is too much?

How does the eyepiece play with your scope? For example, a standard 8 inch SCT has significant field curvature. That means in the widest fields of view, stars at the edge of the field and stars at the center of the field are not simultaneously in focus. This affects different observers differently and it depends in part in the eyepiece chosen. The 35mm Panoptic will show less of the telescope's field curvature than the 31mm Nagler because the field of view is slightly narrower and the magnification is less.

Another example, you discussed AFOV and suggested that you thought 70 degrees was a reasonable upper limit. But this is not something you actually know, have actually observed in the field. It might be the first time you look through an 82 degree or 100 degree eyepiece, you are thrilled by the wide field of view. You cannot guess these things, you have to experience them.

Building an eyepiece set is a learning process, it is not only learning about eyepieces but about yourself, about developing your observing skills. Part of that process is figuring out what eyepieces work for you. I know what works for me, for observing the night sky in the telescopes I have, for observing the night sky under the conditions I face and the objects I enjoy.

Sure you can argue that if they have the best and don't know what it is, "Who cares?" I say this: I care and you, you don't want to be that person who merely has the equipment but doesn't really know how to use it properly. You want to be that person who knows how to get the most out of the equipment they have, the person who has the right equipment to match their interests and needs.

There is an active market in used eyepieces, basic eyepieces like Plossls are bought sold as easily, maybe more easily, than fancy eyepieces.

The 31mm Nagler versus the 32mm Celestron Plossl.

The 31mm Nagler plus a 2 inch diagonal adds about 3 pounds to the scope and can result in balance and clearance issues. The eye relief is shorter than a 32mm Plossl. in an F/10 SCT, both are equal except for the 82 degree versus 52 degree AFoV.


Rollo made a good point about people not willing to stay up past midnight
to take advantage of better conditions. In my case I find it difficult to enjoy
doing anything after about three hours straight. Often that limit comes
around midnight. Later in the summer of course. The three hour limit seems
to hold true for any activity, not just astronomy.

If I take a time-out break that still doesn't refresh me enough to want to
extend my observing sessions through the night. On those rare exceptional
nights where conditions are near perfect, I try to push myself longer.

In summery, after I get my initial rush satisfied I feel the night has been a
success and I feel happy. Extending the observing past three hours then
seems to grow steadily more tedious for me mentally.

I might connect this idea to what csrlice12 said: it is just a hobby (for most of us on CN) and is to be enjoyed!

In the city/at home, I usually go around 2 hours and, yes, it is darker in the early morning. When going to dark sites, I find it really hard to stay awake all night and stay alert/enjoy the views. I usually observe until around midnight or so, sleep for 3- 4 hours, and then go until sunrise.

A corollary points, as was suggested earlier on this thread: having a specific plan is often (but not always) important to getting more out of time under the stars. Knowing what objects you are looking for, what they might look like in your scope/in your conditions, and what they actually are/how long they have existed, etc.: all of these in my experience can really increase what you get out of the hobby.

And, now and then, it is cool to just go out and look around and see what you find...

Light Pollution Topics / Re: LP Map (new to us)
« on: February 02, 2018, 08:25:27 PM »
Ahh isn't that nice!  They decided to light up my spot on the map so I could see it better!

The problem is that no one can tell what the real question is. A larger scope should provide significantly better views (other than for wide field objects)assuming the aperture ratio is sufficient. "Significant" is a fuzzy term. Does that mean half a magnitude? A full magnitude? A quarter? Each has their own ideas of what significant is...and that is most often measured against cost and convenience.

Most people don't see budget as boundless. Most don't feel any scope they can afford on a large budget would also be manageable/convenient, or the best fit for the way they like to observe.

People tend to choose their scopes with budget and ergonomics being equally important as aperture and optical quality (and possibly aesthetics.)

If one ignores ergonomics and budget then there is no limit to size. But if one stipulates that the scope will have to be transportable, not require a ladder, or limit the ladder height to X,or be capable of being set up and taken down solo, or fit intoY auto,and/or within some reasonable budget, then things take on a whole different character.

I can't speak for anyone else, but when I bought my scope I selected one that at the time was the largest scope I was confident I could set up/take down/transport solo with the highest optical quality for a full range of targets from planets to DSO's. The budget was right although I could have gone higher.

With the experience I have gained I am considering going larger...again within some expanded ergonomic limits. But in this case to get as much step change as I want, it is unlikely that I will transport it...even though I anticipate it being trailer portable. My current scope will remain the mobile one.

ATM, Optics and DIY Forum / Re: Scopeless (warning! picture heavy...)
« on: February 02, 2018, 07:08:03 PM »
The Texas heat does not seem to be abating any time soon. Progress is definitely going to be slow until things cool off. But I was able to get a little done before it got too unbearable yesterday. Getting the miters cut is kind of a milestone for me. They have to be spot on or there will be gaps and I don't want me no gaps! I hope to be checking the fit in the morning before it gets too hot...
Edit to add: Forgot to mention that I need to get some black flocking for the inside surfaces. I will probably order that this coming week.

Beginners Forum / Re: New Eyepieces
« on: February 02, 2018, 06:16:16 PM »
Wow, thank you everyone for the replies. I'll admit, I got lost toward the end, but there was some really great information there.

I appreciate someone bringing up exit pupil, it is a term I had not heard before and definitely something to consider when selecting eyepieces.

As for budget, I don't really have one. I would like to buy something that will last as my experience grows. So, I would prefer to spend a little more upfront to get something that will be satisfying to use for years. I was considering around $150 for one eyepiece, maybe more if it makes a big difference in quality.

I picked the brands I did based on several hours of google searching. Those brands seem to be preferred for quality.

Thanks again for the help. Telescopes can be a little overwhelming, but it's nice to know there are passionate individuals out there willing to help a newbie.

So I have a spread of entry to mid-range eyepieces. Clearly I am no authority on the subject. Others here know a lot more than I do.

Of all the eyepieces I have I think the Explore Scientific 82 degree is about the best. If I had known then what I know now I would have purchased a couple more while they were on sale. And I have heard similar praise for the 68 degree eyepieces.

If you are looking for a set of eyepieces in this price range you could probably pick these and be very happy with them for a long long time. And they should serve you well in your future scopes.

The one I used had a 1/2 wave mirror, though tracked ok all things considered. I would ask $900.00 for it with local pickup. Shipping that beast will cost almost what it's worth. If it has an exceptional mirror I would ask 1200.00

That OTA needs a Cave 2.5" shaft mount really. Them 1.5" shaft mounts were not near big enough for anything over 12.5".

Thanks again guys for all your help and replies! Very thankful to have such a responsive community.

I think people who call basic plössl sets "trash" (no one in this thread did) should have all their high-end eyepieces taken from them and replaced with a set of huygenians and ramsdens for a couple of years, so that they may change their mind.

I observed with eyepieces from cannibalized binoculars, old huygenians probably handed down from Herschel's time and eyepieces of completely unknown designs but straw-like fields of view for several years, before I bought my first really good eyepieces: A 26mm and 17mm plössl and a 2x barlow (notice: No redundancies!) and compared to what I had, it was a godsend.Clear skies!
Thomas, Denmark

Poor man's AP900 really and a steal on the used market IMO. There's a potent Yahoo users group as well.

Mine's schlepping around a TEC 200ED with ease and the goto's are very accurate. The OTA as pictured with bino-viewers tips the scales at ~60 pounds and has that long moment arm too.  I may have to adjust out a little RA backlash that's crept in over the last few years but that's it really.

And I'd say the 90 pound payload rating is very accurate. I've seen many a photo of MI-250's pushing around 65-75 pound C14 imaging rigs.

I took these photos for a gent with another TEC 200ED who was trying to figure out the pier height he needed. It's a very compact mount really, like the APs.


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ATM, Optics and DIY Forum / Re: a neglected design: Pfund telescopes
« on: January 31, 2018, 01:46:55 AM »
Making a Pfund telescope track objects requires a simple 3D vector-sum algorithm in the control computer software.  You can't drive the Pfund flat with
conventional Dobson alt-az software.  Oh you can try, but it won't track anything.  With this algorithm, and accounting for atmospheric refraction, the Pfund will track an object all night.  The image does rotate over time, however, and this field rotation would have to be added to the algorithm, along with a precision computer-controlled rotation stage to keep from getting arcs around the edge of the FOV rather than points.

Regarding the tracking of the scope,  if the flat was mounted on an equatorial fork aligned with the polar axis, would it then track only requiring movment in RA?

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