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Messages - poithegepur

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Yes - put automotive wax on the aluminum alt bearings. When I do mine, the motions improve nicely. It wears off over a period of 6-12 months.

Completely disagree with a 1-deg (or any AFOV) rule for Messier Marathon. It's about exit pupil. Before I realized this, on one MM night from a very dark location, I completely failed to detect M74 using a 90mm f/7 scope and Nagler 9mmT6. AFOV just over 1-deg yes, but the 1.3mm exit pupil utterly inappropriate for detection of an extended-nebulosity object. I should have been using something like a 20mm eyepiece for 3mm or so exit pupil (minimum) on that one.

Here is a lesson learned. I bought the RAPAS about three years ago, aligned it and everything worked fine until a few months ago. During a guided imaging session I noticed declination drift that wasn't there before. After some mechanical analysis, the problem proved to be the reticle - it had rotated slightly. It is held in place by one set screw and the illuminator acts like a lever arm so if the set screw isn't reallytight, you could accidently rotate the reticle as you install and remove the RAPAS for an observing session. I plan on adding two more set screws this winter.

<p class="citation">Garyth64, on 11 Feb 2017 - 12:05 AM, said:<a href=";module=forums&amp;section=findpost&amp;pid=7698348" rel="citation">[/url]<blockquote class="ipsBlockquote built" data-time="1486764337" data-cid="7698348" data-author="Garyth64">Plugging in the data into Figure XP I get:8" f/3 primary:PV wavefront 1.42 waveTransverse error 13.23Encircled Energy ratio .087Strehl .016surface error 82.7
Just checking, you do have the fixed/moving light source correct?DaveEdit and what data did you input? Old or have you retested?[/quote]I have a fixed light source. In the hand written data, the light source moves (I believe from the readings since they are 1/2)For the 8" primary, that data is old, from '69. For the 10", the data is also old, from '75.

Beginners Forum / Re: Beginner's reading what?
« on: February 09, 2018, 01:10:40 AM »
I recommend several of the Stephen O'Meara books.

The Messier Objects
Hidden Treasures
Deep-Sky Companions: The Secret Deep

These are all more or less observing guides, but very well written.

General Astronomy & Observing / Re: Clear Sky Chart inaccurate
« on: February 08, 2018, 09:55:29 PM »
It certainly looks as if the cloudiness for the little box is based on the cloudiness on that exact pixel on the map. I've found that if I look at the map (by clicking on the blue box for that time), that if it shows spotted cloudiness in the area, that its basically right, even if the little pixel shows no cloud cover.

I wan't even aware you could do that. Thanks!


Hey Bob,

I can get that close with PHD2 drift but it takes a while. The NCP is only 18* above the horizon for me, well it should be anyways, atmospheric diffraction changes that by about 3* or so.

I think I will stick to my current method for now and drift align. I think I'll buy a new saddle instead.


PoleMaster could account for the atmospheric refraction. I have been able to achieve polar alignment as close to 13 arcseconds to the NCP in just 5'minuted. There is no occasion now I won't start my session without the PoleMaster.

Polemaster does account for it (if you enter your latitude in the provided window).

In the absence of any notable light source not coming from the sky, white materials have a surface brightness like that of the sky. If there is a strong light dome in some direction, the surface more or less facing it will have a surface brightness nearer to that of the light dome.

As seen against the generally dark ground, upward-facing white stuff looks to be surprisingly bright due to contrast.

I once placed a white sheet on the ground at a fairly dark site (Starfest), where the distant light dome of Toronto is fairly notable but not of searchlight brightness. Aiming my wide angle SQM from close enough to not sample the surrounding darker ground, and minimizing self shadowing, I got readings that were pretty near that of the sky brightness.

Not surprising, for a flat, high-reflectance white surface that scatters reasonably well will take on the mean surface brightness of the hemisphere it faces. Sky glow at even a pristine site significantly dominates resolved starlight, and so the sheet will have the averaged surface brightness fir sky glow over the whole sky.

If the sky itself is not considered deleterious to dark adaption, white objects under that light will not be as well.

However! When using smaller exit pupils especially, the dimmed image can be distractingly overcome by such extraneous light seen even well out in the peripheral regions. For that matter, even for large exit pupils and under dark skies it is not a waste of effort to block all such extraneous light.

Hi,I've had this mount for just a wee while and I am quite impressed with it. It has a few little issues but it works brilliantly (I was pleasantly surprised) with my 4" f/15 classic refractor as well as my 5" f/9 semiapo. I enjoyed some high magnification views of Jove through the 5" glass sitting comfortably and using the slow mo extension cables - an absolute must IMO for longer refractors. Tracking at high power is very good! I have also found that the head  is not raised off the ground nearly as much as I'd like. That's why I ordered up a 16" half pillar that mates to the tripod and allows the head to sit on top. An additional £89 UK cost plus shipping.Should be here tomorrow. I'll post some pics once it's all set up.Link here:http://www.widescree...n_for_EQ5_.htmlFirst pic shows my 5" refractor. Regards,Neil.

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General Astronomy & Observing / Re: Tree in the way... ethical dilemma
« on: February 03, 2018, 03:01:04 AM »
I can't say if I would cut it down or not but a good arborist should be able to prune the tree without weakening it. Discuss your concerns with them.

I think that Hyperion diagram might be for a different focal length, or maybe just a stylized rendition. Either way, the diagram doesn't seem to match the pieces I have.
The diagram in _not_ for a 24mm Hyperion, since that doesn't have the last field negative doublet in the 1.25" part. That's a generic 'Hyperion' image that is only more or less correct for the range up to 17mm (and if my memory serves me right, that's the 17mm).Apart from that it _is_ more or less similar: on top are the two plane-conves central lenses (that just touch with the correct spacer in between) and then you have three cemented doublets: the field doublet and two convex/concave groups.It's not exactly identical, though.

General Astronomy & Observing / Re: Clear Sky Chart inaccurate
« on: February 02, 2018, 10:23:14 PM »


Well, CSC worked ok last night and I hope it's right tonight. It's telling me I have a 5/5 for transparency and a 4/5 for seeing! We don't get many nights like that here in NC and no moon to deal with!

I sadly won't see conditions like that for a long time I'm afraid. Watch out though, the CMC is also predicting a rather large cirrus cloud moving over northern parts of NC throughout the evening. Maybe not an issue if it stays on course though...
Huh, didn't see that warning, but it was right! It was super clear out until about 9pm and high cirrus clouds did start to move in all of a sudden? I was surprised to see that? Well, I was freezing anyway and it gave me an excuse to go in! Glad I started out early, dark out by 7:30 now.

What is CMC?
CMC = Canadian Meteorological Center, which is the data source for Clear Sky Chart and my service Astrospheric. Taking a look at the map yesterday this is what I saw around 10pm.

Not exactly sure where you are in NC, but given the size of this cloud there was a good chance it was going to cover a big portion of the sky!

Eyepieces Questions & Recommendations / Re: Spacewalk eyepiece
« on: February 02, 2018, 09:07:48 PM »

There is no spec per say. Some EP manufacturers will state minimum reccomendation but that's uncommon unless they are touting performance on very fast scopes. All will "work" with a fast scope. What happens on some EPs is that the stars at the edge of field will get a bit soft (fuzzy) on fast scopes. Usually the best indicator is price unfortunately. The more you spend, in general, the better...

I don't have any experience with fast scopes so hopefully others will advise on what to avoid. But at f/5.92 your scope isn't extremely fast so you should be good with any of the 82 degree EPS out there. Especially since you actually need to try and find the edge of field at that AFOV so it's not something you should be overly concerned with.

As with most things, you pay a lot to get from very good to perfect...


How about this one?
The Celestron Luminos is something I can afford.Screen Shot 2017-03-21 at 4.37.39 PM.png

23mm will frame everything deep-space related? Or 31mm?

But I also might want to use my nebula filter on it which is 1.25 (so it wont work).
I do have a 2in light pollution filter though. So I could use that.
There are two different strategies you may want to consider (they are not mutually exclusive btw!)
1. Optimize for maximum exit pupil first, and then maximum AFOV (or TFOV) within that.

2. Optimize for maximum TFOV first, and maximum magnification within that

Strategy #1 will give you the *brightest* view possible with your telescope, without introducing light loss due to the secondary mirror shadow. This works best if you're in a very dark sky location or if you want to use LP filters as you had mentioned. The downside is that you can lose a lot of wow factor since a lot of objects may be smaller than they have to be.

Strategy #2 will give you a true field of view that will frame most of the major objects with 1 or 2 exceptions, and the maximum magnification within that TFOV means the eyepiece will also pull in medium and smaller objects better as well. This tends to give you a more versatile eyepiece that works well on a broader spectrum of objects.
There are a number of options that will result from crunching the numbers, so I won't list them here. You'd have to pick which ones make the most sense to you based on affordability.

I built a website that helps with the calculations: (disclaimer - in beta, still some errors with some EP values!) There's <a>also[/url] Both have databases of equipment so you can just do a search for what you need and see how it stacks up.

But your question gives me inspiration to build an eyepiece recommendation engine using the above two strategies!

Reflectors Telescopes Forum / Re: Largest dream scope?
« on: February 02, 2018, 05:23:44 PM »
For me it comes with the additional criteria that I do not want to be on a ladder or for that matter even a step. So the biggest for me would be a 24" &gt; f/3.5 with Lockwood or Zambuto mirror.

You know ZoC doesn't go below f4, what about lightholder, he can even make the cell

Would you like some cells with that primary?

ATM, Optics and DIY Forum / Re: a neglected design: Pfund telescopes
« on: February 02, 2018, 03:27:54 PM »
This is a nice overview chart of the 18" Wren-Marcario Accessible Telescope (WMAT) at the McDonald Observatory Visitor Center. I designed all the optical systems for it. By using two matching primary mirrors, we were able to cover the entire sky except for small keepout zones around each primary. Carl Zambuto made both 18" f/8 fused silica primaries (incredibly accurately, BTW), and the 24" perforated flat received final figuring at the Zygo Corporation to a flatness well below 1/10 wave P-V. Las Cumbres Observatory made the tracking mount, and it points and tracks like a dream. The staff at McDonald say this telescope consistently gives the sharpest images on the mountain. We dedicated it in July of 2010, and even had several very excited wheelchair-bound visitors look through it on the inaugural night. It was a wonderful project and team to work with.

Amazing set-up!

I'm curious if one did not want to install two identical primaries and could accept 1/2 sky coverage if the steering and primary mirrors could not be oriented east-west instead of north-south?

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