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#26 azure1961p

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Posted 11 January 2013 - 12:52 PM

I'd personally get that 8" f/9 truss dob by Optcorp. Nothing in your want list could touch it. Too the mount would be much easier on you than the PARKS. At anyrate Id go with the 6" f/8 or the 8" f/9 both will yield very high definition in good seeing. The only thing I've seen outdo the 8" f/9 was a 10" f/7 Cave and another one bring a Parks and a Celestron C11. Not a Meade10" SCT nor 8" SCT. Those have been my experiences anyway.

Oh and collimating is NOT the bother it is with short scopes. It still matters to be sure but the disaster factor is lessened.

Pete

#27 Gray

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Posted 11 January 2013 - 01:56 PM

Pete, the only problem I'd have with that 8" F9 Dob is the tracking platform I'd have to buy. I really don't have space for that type of thing right now. Don't get me wrong though, I'd love to look through that instrument, but chasing a planet back and forth gets old.

#28 dpwoos

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Posted 11 January 2013 - 02:51 PM

Not to say that you aren't entitled to your own likes/dislikes etc., but whenever someone posts that they have difficulties tracking with a dob I have to wonder if they have ever given this a chance with a properly constructed mount, with minimal stiction. Many, many folks find tracking planets at high powers not to be that big a deal, and so maybe this is worth a rethink?

#29 Starman1

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Posted 11 January 2013 - 02:51 PM

The three "C"s of observing:
--Collimation. If you're not real certain on this one, getting good tools (e.g. Catseye or Glatter) can make the difference. Adequate collimation is OK but excellent collimation is required for small planetary details.
--Cooling. No 8" will cool by itself to ambient temperature if the temperature is falling during the night. At best, it will take 3 hours or more before heat issues disappear from the images. Active cooling is mandatory.
One rear fan will cut the period of cooling to 1/3 of what it would be without the fan. This is critical for planetary observations.
--Conditions. Seeing, mostly, and transparency. Seeing is the Biggie. I have seen nights where my 12.5" was aperture-limited and the images unbelievable. In those conditions, I've seen Jupiter with as much detail as a really good photograph. No, better, because what I saw was sharply focused and photos never are.

Collimation and cooling are within your control. Conditions are not, but there are some lessons to be learned:
--never observe an object directly above a roof top. Heat from the roof will rise and distort the image.
--seeing conditions are often best at 2 times of night: right at twilight in the evening when the heat of the sun is removed from the atmosphere; and between midnight and dawn when the atmosphere has settled into strata of temperature gradients.
--try not to sit with your warm legs directly below a steel tube. You will see heat signatures in the images. It helps to flock the tube internally. This won't slow the cooling of the tube, but it will reduce the heat images from warm legs under the tube, and it will help reduce tube currents as the steel tube loses its heat into the interior.
--resist the temptation to look at an object below 30 degrees altitude. At 30 degrees, you are looking through literally twice as much air as at the zenith, and the extra air will always show more turbulence and scintillation. Of course, with Venus and Mercury, this rule doesn't work, but for exterior planets, this is a good rule to follow.
--the center of a valley usually has steadier air than the periphery (where air is either rising or falling). I went from the center of West LA, where seeing was spectacularly good, to a mountainside above LA. Now I see good seeing one or two nights a year. If it has to be a mountainside, make it one facing the wind, not the "alee" side. a steady laminar flow over your site is much better than the turbulence after the air rolls over some form of obstruction. In a city, try to set up on the upwind side of a building for the same reason.
--look at the weather maps. 2-3 days after a front passes, the air gets calmer and more stagnant. If the pressure isobars are close together, seeing will suffer.
--check out the jet stream position. If it's overhead (+/- 50 miles), you won't get good seeing. If it's a long way away from overhead, you might.
--check out the high altitude wind speed. If it's dead calm or if it's really fast, you won't get good seeing. If it's steady and not too fast, the air flow may be quite laminar and good seeing will result.
If you learn to anticipate good seeing conditions, you will be rewarded with a lot more episodes of fantastic planetary images.

Prevent stray light from entering the bottom of the eyepiece bya adding about an 8-10" extension to the side of the tube opposite the focuser. This will improve contrast (like flocking does to the interior) by keeping direct light from the sky out of the bottom of the eyepiece in the focuser. It can be quite simple--even black construction paper or cardboard.

Last, I don't see anything wrong with an 8" f/5 on an EQ mount for planetary viewing. Of the scopes you mentioned, the Parks isn't available, the 6" Mak is nice but it'll be a gamble that you get a really superb one optically (and cooldown is WORSE than your 8" newt.). And the Skywatcher refractor is too small to show you the small details the 8" can (the 8" will resolve details 1/2 the size).

#30 Gray

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Posted 11 January 2013 - 04:01 PM

dpwoos, I did have a Z12, and after a couple of observing sessions during decent seeing, I grew uncomfortable constantly bringing the planet back only too be able to observe the object for maybe a second in the sweet spot. I don't discount what others can do or have been able to do, I'm just saying it's not for me. I appreciate your thoughts.

Don, so glad you stopped in to enlighten me further! I gather from what you say that Parks is no longer in business, even though their site is up and current? Now then it looks as if I should just upgrade my newt with fans, flocking, and I'm wondering what you guys think of a curved spider? 3 or 4 vane. I was thinking 3, for my 8" even though they are expensive at $94 from Jim at Scopestuff. I'm also wondering if I should replace the secondary or keep it. What would you suggest I upgrade my scope with altogether, for optimizing it for planets is a better question of questions. Or would another scope be a better option? Thank you Starman

#31 Starman1

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Posted 11 January 2013 - 04:14 PM

I gather from what you say that Parks is no longer in business, even though their site is up and current? Now then it looks as if I should just upgrade my newt with fans, flocking, and I'm wondering what you guys think of a curved spider? 3 or 4 vane. I was thinking 3, for my 8" even though they are expensive at $94 from Jim at Scopestuff. I'm also wondering if I should replace the secondary or keep it. What would you suggest I upgrade my scope with altogether, for optimizing it for planets is a better question of questions. Or would another scope be a better option? Thank you Starman

Parks' website is pre-paid but there is no one currently working there.
If you do a curved spider, be aware that most of them are not collimationally stable. Try the ones that look like )-( as in this site (go down the page--the pic is on the right):http://www.obsessiontelescopes.com/telescopes/12.5/index.php
That design does not appear to sag as you point lower.
Might as well keep your secondary.

If you upgrade the scope but keep it possible to use your mounts, an 8" f/6-f/7 would still work and the secondary would be a little smaller.
An f/9 would be getting somewhat heavy and too big a wind sail, not to mention having an inconvenient eyepiece position for comfortable viewing.
Frankly, by today's standards, f/5 is long. Imaging scopes are typically f/3-f/4.

And, as long as your scope tracks, you'll not need a coma corrector at f/5.

#32 rflinn68

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Posted 11 January 2013 - 04:19 PM

The Destiny 3 vane curved spider worked very well for me. I went ahead and did the flockboard from Protostar and put on a new 1/20 ptv Antares Optics secondary at the same time. My collimation holds just as good or better than it did before with the factory spider. The Destiny that I bought was about the same price but it also came with collimation thumb screws. Not sure if Jims do or not. I also got lucky with Antares Optics! I ordered the 1/15 ptv 2.6" secondary for my 10" and they were out of that one and out of the 1/18 so they shipped me a 1/20 mirror for the same price of a 1/15. Very good people to deal with for sure! Like I said before, I almost sold my scope but now I am so glad I didnt. I'm still wanting to get a big dob (16"+) and will start saving for it soon but I am quite happy now with my 10" for the time being.

#33 rflinn68

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Posted 11 January 2013 - 04:35 PM

I just checked and the thumbscrews are an option at ScopeStuff. I bought the Large from Destiny for $99 and they were standard. The Large 3 vane is $109 at ScopeStuff and the thumbscrews are extra. The medium 3 vane is $94. A medium now from Destiny is $69.95 but you'll need to see what options are included with it.

#34 Gray

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Posted 11 January 2013 - 04:39 PM

Thanks rflinn68!

#35 Gray

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Posted 11 January 2013 - 07:59 PM

I really appreciate all you fine fellows who have offered to help me with this interesting decision. It is fascinating to think of all the possibilities and humbling when I see how much money the good things cost. I hope others will read this and learn a bit from what I've seen wrote so many times before. I've learned quite a bit from all of this and am very thankful to you all. I'll end this thread with that. :jump: :fingerscrossed:

#36 Jeff Morgan

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Posted 11 January 2013 - 09:55 PM

If you upgrade the scope but keep it possible to use your mounts, an 8" f/6-f/7 would still work and the secondary would be a little smaller.
An f/9 would be getting somewhat heavy and too big a wind sail, not to mention having an inconvenient eyepiece position for comfortable viewing.
Frankly, by today's standards, f/5 is long. Imaging scopes are typically f/3-f/4.

And, as long as your scope tracks, you'll not need a coma corrector at f/5.



Rebuilding a commercial dob allows one to spread the money over time. Doesn't help resale though. The economics aren't that compelling, at that point DIY starts making a lot of sense. 50/50 call on that, there is a lot of value in being able to get into the game now vs. months from now.

The 8" f/9 in the link was a truss design, it's performance in the wind likely is reasonable. It would have a zenith eyepiece height of 72" (give or take a few, depending upon the build). The OP was interested in planets, and listed his location as Tennessee. Not much chance of a planet finding it's way to the zenith from that latitude. Close (perhaps 75 degrees elevation), but not quite. Might even be quite comfortable for someone of average height. (Then again, the OP could be short.)

If planetary is your thing, look at what the more prolific ALPO observers such as Troiani, Parker, and Beisch are using - Large Newtonians in the f/6 to f/7 range. First emphasis on aperture, second emphasis on long. I doubt many (or any) of them are sub f/5, even for planetary imaging. Especially for planetary imaging where image scale is desired.

#37 Gray

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Posted 12 January 2013 - 06:24 AM

Jeff, that is an excellent observation and yes the planets do not reach zenith at my location, more like what you guessed, at most 75 degrees. I do have a badder mpcc and an at ff as coma correctors, but have only used the badder yet with good results. With the truss design I would need a tracking platform and I do not wish to go that route at this time. I was looking around and if I actually wanted a 6" F8 newt, I would have to buy an Orion dob and put rings on it for around $300. I'm not sure I want to do that, but it is an option. Looking at a set of rotating rings, they will run me $450 for an 8" set. I've never used any before and wonder if they are worth it. I'm thinking my 8" should be tested out first, then flocked, fanned, and maybe a new spider with a set of Parallax rings. This is off topic but I would also like a instrument on a alt/az mount for comet scanning. I'm thinking either a 10" dob or a set of big bino's on a good mount, also need a good chair. Plant scope is almost concluded here with me. Thanks again Jeff!

#38 Jon Isaacs

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Posted 12 January 2013 - 06:52 AM

Graham:

A few thoughts:

- For correcting coma at planetary magnifications, the Paracorr is the one. It is my understanding that the MPCC adds spherical aberration and is basically for astrophotography.

- All total summed effect of all the modifications you can do will be small in comparison to just making sure that your current scope is perfectly collimated and thermally stable. The first step IMHO, is doing the small things which make big differences in getting your current scope to perform it's best.

- When you listen to old timers, you are listening to people who have memories of how things used to be rather than how they are today. Back in the old days, there were no coma correctors, eyepieces had problems with fast mirrors, and fast mirrors were generally not well made. The world has changed...

Jon Isaacs

#39 Gray

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Posted 12 January 2013 - 07:15 AM

LOL, wow, never would've expected that comment! Thanks for that tid bit on the coma correctors too. Paracorr, check. I do have a laser and a Cheshire but maybe I should look into better tools. I will check them out. I'll do further reading about rotating rings before I decide on them too. Fans are a biggie and I will also read what I should do there. I appreciate your thoughts Jon.

#40 Jon Isaacs

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Posted 12 January 2013 - 07:43 AM

I do have a laser and a Cheshire but maybe I should look into better tools.



A good laser is a good tool.. What laser are you using and are you Barlowing it to adjust the primary.

Jon

#41 Jeff Morgan

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Posted 12 January 2013 - 10:14 AM

Jeff, that is an excellent observation and yes the planets do not reach zenith at my location, more like what you guessed, at most 75 degrees. I do have a badder mpcc and an at ff as coma correctors, but have only used the badder yet with good results. With the truss design I would need a tracking platform and I do not wish to go that route at this time. I was looking around and if I actually wanted a 6" F8 newt, I would have to buy an Orion dob and put rings on it for around $300. I'm not sure I want to do that, but it is an option. Looking at a set of rotating rings, they will run me $450 for an 8" set. I've never used any before and wonder if they are worth it. I'm thinking my 8" should be tested out first, then flocked, fanned, and maybe a new spider with a set of Parallax rings. This is off topic but I would also like a instrument on a alt/az mount for comet scanning. I'm thinking either a 10" dob or a set of big bino's on a good mount, also need a good chair. Plant scope is almost concluded here with me. Thanks again Jeff!


I didn't know your exact latitude, so it was kind of a WAG. It doesn't matter whether it is 70, 75, or 80 degrees though. The planet only hits that elevation for 1-2 hours per 24 hours and due to diurnal and seasonal motion that will likely occur when you are not able to look (like during the day!) And even then Worst Case it is quite close to standing eyeball height for the average person. To make that a limiting factor in your scope selection doesn't make a lot of sense.

OTOH, comet sweeping is whole different animal where the shorter Newt would serve you better by virtue of covering more square degrees. It's a productivity thing, not an optical thing. Not to dash your hopes, but in this day and age automated searches discover most things like comets. But then again, you could be the one ....

Tracking is worthwhile. With patience you might score some deals on the used market. But consider that you need rings, the eq mount, and a balance rail. I think you would be hard-pressed to accomplish all of this for less than $1000 - the cost of a new Tom Osypowski Compact platform. True it will add 6-8" in height to whatever you place on it, but it is a very easy piece of equipment to live with. Years ago I wrote a review on it which you will find here. And getting back to the whole zenith thing, an equatorial platform does help alleviate some of the clumsiness associated with Dobson's Hole.

#42 Gray

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Posted 12 January 2013 - 02:16 PM

Sorry Jon, I had to goto bed, it was ~6am here on the east coast. I use a GSO laser type, that came with my old Z12. It seems to be aligned, I haven't tried barlowing it though. Maybe I should get a better tool? I've read that pro's do use Howie's tools. Do you think that a set of rotating rings would be worth the expenditure for me? I'm sure you've probably used them before.

Jeff, I gather from your writings that an EQ platform mounted Dob is the way to go for planet viewing. So my question is, do you think the 8" F9 truss dob would be a better scope for viewing than my 8"F5? That setup would run me $2k new with shipping and above and beyond what I would want to spend. But, it never hurts to know as much as possible in that realm. I think I should keep my spider after reading some on curved spiders, but fan and flock for sure. What do you folks think of buying a 6"F8 dob from Orion? I have a set of rings already. I would like to have a long focus newt but I am unsure if the difference between F5 & F6 would be really noticeable.

#43 Jon Isaacs

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Posted 12 January 2013 - 04:32 PM

Sorry Jon, I had to goto bed, it was ~6am here on the east coast. I use a GSO laser type, that came with my old Z12. It seems to be aligned, I haven't tried barlowing it though. Maybe I should get a better tool? I've read that pro's do use Howie's tools. Do you think that a set of rotating rings would be worth the expenditure for me? I'm sure you've probably used them before.



Graham:

Have you actually tested your collimator by rotating it in the focuser and watching the dot on the primary mirror? Just about every collimtor that came with a Dob I have ever seen is badly out of collimation.

As far as rotating rings, my 12.5 inch came with them. I bought a set of Antares ball bearing rings for my 8 inch F/5, they worked but they were heavy and quite frankly, ugly...

So, with a little time, a few tools and an extra ring, I did some grinding, replaced the felt on the stock rings with bondable Teflon, added some Teflon to take the thrust. I sold the Antares rings, these are just as good, lighter and look better...

Rotating "Wilcox Rings" with Teflon

Jon

#44 Gray

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Posted 12 January 2013 - 05:03 PM

Jon, I have not tried that method of testing, and can not believe I didn't think of that! I checked it against the wall in my kitchen...Seems the better test would be your suggested method. Thanks for the aesthetic review of your rotating rings. I will save that cash and attempt the Wilcox build instead! Looks good there. My laser does have adjustment screws if it's out. I appreciate you. Thanks

#45 azure1961p

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Posted 12 January 2013 - 05:08 PM

There is a difference between f/6 and f/5 in that the secondary needs to be larger than the f/6 scope and this doesn't help planetary contrasts too its easier to collimate the f/6 than an f/5 .

I'd personally rather have a 6" f/8 than an 8" f/5 . Sure it's just one fstop but one too many for me. It's a personal taste thing too. I've never been a fan of reflectors faster than f6. The 6" f/8 will provide excellent diffraction patterns with no coma and planetary images with refractor-like quality. I've never had a view at f/5 that could be called refractor like.

Some folks are content at f/4. A lot matters about how tolerant you are of the faster reflector issues. I am no fan. Love my 6" sct though despite the large CO.

Pete

#46 Jon Isaacs

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Posted 12 January 2013 - 06:52 PM

I'd personally rather have a 6" f/8 than an 8" f/5 . Sure it's just one fstop but one too many for me. It's a personal taste thing too. I've never been a fan of reflectors faster than f6. The 6" f/8 will provide excellent diffraction patterns with no coma and planetary images with refractor-like quality. I've never had a view at f/5 that could be called refractor like.



A few thoughts:

My collection includes both an RV-6 (6 inch f/8) as well as an 8 inch F/5 an 10 inch F/5 and a few others. It does take more care and effort to get the good views with an F/5 Newtonian but with the right eyepieces and a coma corrector, nearly perfect views are possible. Some might call them "refractor-like" but I avoid that comparison because refractors have their own set of issues.

At 200x, the diffraction limited field of view of 8 inch F/5 is about 31 degrees AFoV, on a driven mount, this is more than sufficient to avoid the effects of coma.

I have directly never compared the 8 inch F/5 to the 6 inch F/8, my sense is that the 8 inch provides somewhat more detailed views of Jupiter. The 10 inch F/5 is definitely the best of that bunch.

The way I look at it, getting the good planetary views with a Newtonian begins with excellent seeing. You can't see it if it isn't there. Next comes preparing the scope. A great mirror that is not collimated or has thermal issues is not going to perform.

Graham has an 8 inch F/5, I suggest learning how to get the most out of what Graham already has and then decide where to go from there.

Jon

#47 azure1961p

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Posted 12 January 2013 - 09:40 PM

Agreed Jon.

With risk of beating a topic into the ground, ill just reiterate I just got through installing a boundary fan along with the rear blowing fan and that has really really made a nice difference. Till now the Galilean moons have been the test bed in observing the reduction in flare. The rear blowing fan reduces it by a whopping half while the boundary fan proves to be more subtle but just as real in reducing the halved flaring to half of that.

The full time fan was something I finally started doing a year ago while the added boundary fan was something not even a few weeks ago. I had no idea how much flaring was NOT the sky but in the scope itself.

Anyway something to consider.

Good luck Graham!

Pete

#48 dpwoos

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Posted 12 January 2013 - 11:26 PM

One way to think about it is that it is best to use and learn from the 8" f/5, so that one can answer the question for oneself or at least ask narrowly targeted questions. I hope I'm not being too critical, but I do wonder if sometimes the best response to broad questions like this isn't to suggest spending more time observing, including if possible observing with others, and then coming back and telling everyone here why such and such piece of gear isn't working for this or that, and asking for suggestions on how to overcome those deficiencies.

Some years ago I was assisting in a wood shop class at a K-6 school, and the teacher told me to make sure that the kids asked me a question before I assisted them, and (most importantly) that "help me" is not a question. A moment of enlightenment that I have never forgotten.

#49 Jeff Morgan

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Posted 13 January 2013 - 01:48 AM

Jeff, I gather from your writings that an EQ platform mounted Dob is the way to go for planet viewing. So my question is, do you think the 8" F9 truss dob would be a better scope for viewing than my 8"F5? That setup would run me $2k new with shipping and above and beyond what I would want to spend. But, it never hurts to know as much as possible in that realm. I think I should keep my spider after reading some on curved spiders, but fan and flock for sure. What do you folks think of buying a 6"F8 dob from Orion? I have a set of rings already. I would like to have a long focus newt but I am unsure if the difference between F5 & F6 would be really noticeable.


German Equatorials are just not friendly to Newtonians. It's a size and mass thing. And equatorials in general require tilted mounts, which means off-center loads that need to be balanced. Extra weight to balance. Extra weight to make the mount stiff enough to hold the scope and the counterweight. An alt-az mount keeps the loads centered over the mount, leading to greater stability with far less mass. Figure out a way to make it track (like a platform or ServoCAT) and you have the best of both worlds.

If planetary is truly your main interest, longer focus will serve you better:

- Simple eyepieces like Orthos, Plossls, Brandons, RKE's (and a few others) are favored by the hard-core planetary crowd for good reasons. And they perform great at longer focal ratios, not so great at f/5;

- You can buy these eyepiece types at high quality levels for not a lot of money. The faster scopes require more corrected (and costly) glass;

- You get greater image scale at longer focal ratios before you have to resort to barlows. This is important because it keeps the glass count down, and simple eyepiece designs have limited eye relief in the shorter focal lengths; and

- Then there is a the whole collimation thing. It is the number one performance killer, and it is more forgiving at longer focal ratios.

It occurred to me that instead of fixating on this Orion scope, you should find yourself a Criterion RV6. It's the 6" f/8 scope you've been wanting, and it has an equatorial mount. They go used for perhaps $300. While I am a skeptic of mass-market mirrors the RV6 has a great track record and I suspect your odds of getting a fine optic are much better than with the Orion.

Perhaps you could talk Jon out of his :wron:

#50 Jon Isaacs

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Posted 13 January 2013 - 04:54 AM

It occurred to me that instead of fixating on this Orion scope, you should find yourself a Criterion RV6. It's the 6" f/8 scope you've been wanting, and it has an equatorial mount. They go used for perhaps $300. While I am a skeptic of mass-market mirrors the RV6 has a great track record and I suspect your odds of getting a fine optic are much better than with the Orion.


And yet Jon find that his 8inch F/5 provides at least as good an image as his RV-6...

So often the focus is on the equipment, slow scope, fast scope, big scope, little scope, 1 element eyepieces or 7 element eyepieces. You gotta buy this, can't live without that. These are things were can argue all day and all night. Jeff has his preferences, the things that work for him, I have mine, the things that work for me. I am not going to argue what works best in terms of equipment.

My point is that the important things, the things that make the big differences are the seeing and the preparation of the equipment, there is no need to spend money to get better views, better views are possible with the existing equipment. The first step is work on getting the best possible views with what one already has... It is the least expensive, the most instructive and can yield major improvements.

Jon






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