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Hypothetical 4" budget APO choices.. WWJD?

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#51 lamplight

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Posted 30 January 2013 - 03:35 PM

Is the OP now looking for a budget 4.7" APO?


yes i think so. ive learned budget is not the right word. thanks for all the help everyone, im on the right track (to bankruptcy!) :p

#52 Jon Isaacs

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Posted 30 January 2013 - 03:47 PM

ive upgraded. in my mind, to the 120mm size. it seems manageable enough still and is somewhat still grab n go for me.



I have owned a couple of 120mm refractors. I prefer a 4 inch, preferably faster, shorter. A 120mm F/8 refractor is about as much effort to deal with as a 8 or 10 inch Dob. A 4 inch F/6-f/7 more like dealing with an 80mm on steroids.

Jon

#53 Tony Flanders

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Posted 30 January 2013 - 04:49 PM

I have owned a couple of 120mm refractors. I prefer a 4 inch, preferably faster, shorter.


And I will repeat my previous question. As you move up in the aperture leagues, what's wrong with a 6-inch reflector? More portable than a 120-mm refractor, much cheaper, and closely comparable light gathering and image quality.

#54 Jon Isaacs

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Posted 30 January 2013 - 08:55 PM

I have owned a couple of 120mm refractors. I prefer a 4 inch, preferably faster, shorter.


And I will repeat my previous question. As you move up in the aperture leagues, what's wrong with a 6-inch reflector? More portable than a 120-mm refractor, much cheaper, and closely comparable light gathering and image quality.


Tony:

Indeed... over the years, the traditional 6 inch F/8 Newtonian gained the reputation as an "apo-killer." The classic Criterion RV-6 (last built about 1978) has a light weight phenolic tube, a small secondary and a simple clean aluminum mirror cell. The optical tube assembly weighs 8 lbs and these are very capable performers as planetary and double star telescopes.

Jon

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#55 Paco_Grande

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Posted 30 January 2013 - 09:47 PM

My apo-killer :D I'm still a refractor fan.

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#56 Paco_Grande

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Posted 30 January 2013 - 09:57 PM


Sky-Watcher "equinox" 120ED (like the design/mechanicals better than american version), but will probably have to order from canada. is there a big tax increase? VAT or something or is that only EU?. my current "top pic". i just dont know yet what it would cost me.


Matt, I ordered my Sky-Watcher newt from Brandon Optics in BC, Canada. No special taxes. But shipping is more expensive and you'll have to pay someone to handle the customs process. If it's UPS, they'll give you a bill and won't deliver until you pay. :D $

40 for shipping to Northern Calif and $60 for custom's fees.

Bottom line, it cost me about $100 for shipping and customs fees to get that SW newt here. I have no regrets. It's one of those scopes you can't imaging selling, like a laid back Chihuahua. :)

#57 lamplight

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Posted 31 January 2013 - 07:52 AM

I responded yest but I guess I never pushed send.

Re: why refractor: I like the ease of setup, lack of maintenance (generally), fast cool down, sharp images and good contrast, the usual..my Achro has been my winter scope, But since I'm in LP I want more aperture..as much a s s reasonable to handle between this and a DOB I have coming.. Another ing pushing me to explore an APO is that I see the CA more lately... (Maybe just because I've been using this scope all winter every chance I can)

Jon the 4.7" range doesn't seem all that much bigger (??). The ones I'm looking at look really well built and everywhere I see good reviews of very low to no visible CA. I worry that if I went a triplet and stayed in the 100mm range (about same cost as these exceptionally good doublets) I would not get a noticeable upgrade in light gathering.

#58 lamplight

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Posted 31 January 2013 - 08:39 AM

The skywatcher 120ed is 11.3 lbs, my xlt 100 is 9lbs
The 120 ed (f/7.5) is 38", the 100 xlt is 39.5"

So in this comparison is really negligible increase in terms of what I and the mounts ill use can and would handle.

However, I'm looking at the beefier equinox and TS tubes which weigh in slightly under 16lbs with rings. Still very doable to me, and I don't see tube length info on these websites.. But it has to be similar to the skywatcher 120.

I THINK ;) this would be the upper limit of practicality in size and weight for me (for a portable refractor).

#59 Gert K A

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Posted 31 January 2013 - 09:00 AM

-- Matt:
I don't see tube length info on these websites..


Your link from APM say "The TS 120/900mm ED APO" so 900mm or 34½" betting on the dewshield is retractable

#60 REC

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Posted 31 January 2013 - 09:19 AM

Ah pretty:) It was my first real scope in 1964!
Bob

#61 Jon Isaacs

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Posted 31 January 2013 - 09:23 AM

Jon the 4.7" range doesn't seem all that much bigger (??). The ones I'm looking at look really well built and everywhere I see good reviews of very low to no visible CA. I worry that if I went a triplet and stayed in the 100mm range (about same cost as these exceptionally good doublets) I would not get a noticeable upgrade in light gathering.



It all depends on what you are looking for in a telescope. In terms of light gathering, the difference between a 4 inch and a 120mm is not much if there is a 10 inch Newtonian sitting nearby.

The fact that it takes as much effort to setup the 120mm refractor as it does the 10 inch Newtonian is clue that maybe the 120mm is on the big side of easy to use and portable. Myself, I look to refractors to do the things smaller scopes do best, easily portable, compact, wide fields of view, with the ability to go to higher magnifications, remembering that there is a limit to what scopes this size can do.

When small is good, bigger is not necessarily better.

For the things I use a refractor for, an 80mm is a better choice than a 120mm and a fast 4 inch is about ideal. In terms of fast 4 inch refractors for visual use, the NP-101 is about as good as it gets.

Jon

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#62 lamplight

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Posted 31 January 2013 - 10:53 AM

( is that a vixen head on your TV? nice !)

thanks.. well if it will be smaller than what i have but only a bit heavier with bit more light and less CA it seems a no brainer for my needs. except cost which is contrary to practical. "but..."

this has been SO educational thanks! this hobby is a continuation of not knowing what i didnt know.. how many times is this going to keep happening? sheesh.

#63 CJK

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Posted 31 January 2013 - 11:29 AM

Re: your photo -- Who knew Home Depot sold telescope mounts? ;)

-- Chris

#64 kenrenard

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Posted 31 January 2013 - 12:43 PM

I have owned a couple of 120mm refractors. I prefer a 4 inch, preferably faster, shorter.


And I will repeat my previous question. As you move up in the aperture leagues, what's wrong with a 6-inch reflector? More portable than a 120-mm refractor, much cheaper, and closely comparable light gathering and image quality.


Tony:

Indeed... over the years, the traditional 6 inch F/8 Newtonian gained the reputation as an "apo-killer." The classic Criterion RV-6 (last built about 1978) has a light weight phenolic tube, a small secondary and a simple clean aluminum mirror cell. The optical tube assembly weighs 8 lbs and these are very capable performers as planetary and double star telescopes.

Jon


Tony or Jon,
I have a question. Would a 6 inch reflector f/8 be better for planets than say a 8 inch f/6 or does the aperture cancel the longer focal length out? I have see quite a few ads for planet killer refractors f/11 or more. I have read how some people love these but the FOV is very limited. So would the a longer focal length help with planets or double stars on reflectors as well? I understand this would make them less portable. I am trying to understand the trade offs over wide field or better planet viewing. From what I am understanding it sound like only a select few refractors like the NP-101 does both things well.

Ken

#65 Mike4242

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Posted 31 January 2013 - 02:12 PM

The main benefit of a longer focal length is that you get a higher magnification with a given eyepiece; however, this also restricts your maximum field of view.

Focal length also comes into play with achromatic refractors because longer focal lengths reduce the amount of chromatic aberration.

In the case of reflectors collimation is more forgiving with a longer focal length.

An 8" f/6 should give better planetary views than a 6" f/8 if both scopes are cooled, properly collimated, and the seeing conditions permit. The 8" will be more affected by atmospheric seeing than the 6".

I'm probably missing some points here, but I'm sure Jon and/or Tony will chime in.

#66 Mike4242

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Posted 31 January 2013 - 02:14 PM

Here's a good article I just remembered about the pros and cons of short and long focal lengths:

http://www.chuckhawk...ocal_length.htm

#67 kenrenard

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Posted 31 January 2013 - 02:26 PM

Thanks Mike,
Good article. When I put the different focal lengths comparing say a XT8 f/5.9 and XT6 f/8 in an eyepiece calculator they have the same magnification. So in essence looks like a wash? At least thats the way I'm understanding things.

Ken

#68 Mike4242

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Posted 31 January 2013 - 02:29 PM

You have to look at the actual focal length value instead of the ratio for magnification. Since the F ratio is the focal length divided by the aperture, a larger aperture will have a lower/faster F ratio with the same focal length.

But, yes, it is essentially a wash. The collimation tolerance of the 8" will be tighter, but the 8" should give better views if atmospheric seeing permits.

#69 kenrenard

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Posted 31 January 2013 - 02:37 PM

OK I understand. Thanks for clearing it up. I found a few other old post explaining why we don't have long focal length reflectors. Size and weight seem to be the biggest answer. As well as better mirrors today.

Ken

#70 Jon Isaacs

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Posted 31 January 2013 - 02:51 PM

Thanks Mike,
Good article. When I put the different focal lengths comparing say a XT8 f/5.9 and XT6 f/8 in an eyepiece calculator they have the same magnification. So in essence looks like a wash? At least thats the way I'm understanding things.

Ken


Ken:

The article was comparing telescopes of the same aperture and is not really applicable in this situation because the question here is involves two scopes of the same focal length but different apertures. It is also rather dated so many of the comments about eye relief and eyepieces are no longer valid.

This is a favorite saying:

"The best 6 inch F/8 is an 8 inch F/6."

In terms of viewing the planets, the benefit of the larger aperture is greater resolution, increased planetary contrast and a brighter, potentially more colorful view. The resolution and contrast are proportional to the aperture, so assumming the seeing (atmospheric stability) is not the limiting factor, the expectation is that an 8 inch F/6 Newtonian will provide noticeably better planetary views than a 6 inch F/8 Newtonian.

This is very consistent with my experience. I do have the classic 6 inch F/8 but if want the good planetary views, it's not the 4 inch apo nor the 6 inch F/8 Newtonian but rather the 10 inch F/5 or something even larger.

Jon

#71 kenrenard

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Posted 31 January 2013 - 03:36 PM

Thanks Jon,
As always you are a wealth of information.


Ken






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