Jump to content


Photo

Achromats & Apochromats - Cutting Past The Hype

  • Please log in to reply
179 replies to this topic

#76 keithlt

keithlt

    Vostok 1

  • *****
  • Posts: 190
  • Joined: 21 Dec 2013
  • Loc: Arizona

Posted 25 April 2014 - 09:51 PM

great info in this thread, so if a person is color blind an AR would better?

#77 Tony Flanders

Tony Flanders

    Voyager 1

  • *****
  • Posts: 11154
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 26 April 2014 - 04:55 AM

If we compare apples to apples, would 4" or 5" Newtonian cool just as fast AND be just as portable as a 4" APO refractor and provide similar CA free views?


As far as CA is concerned, Newtonians are always better than APOs. False color is inherent in refractors -- it may be too small for you to see, but it's always there. Conversely, Newtonians are inherently 100% color-free.

However, CA is not the end of the story. Newtonians have central obstructions, which degrade the image significantly, though in a completely different way from chromatic aberration. And unless they have first-class mirrors, they also scatter more light than refractors.

Putting it all together, I would say that a first-rate 5-inch f/5 Newtonian would deliver about the same planetary detail as a first-rate 4-inch f/6 APO. And with the aid of a fan, it would cool nearly as fast -- but probably still not as fast. In terms of portability, it would be roughly a draw. The Newtonian tube would be a smidge bigger, but it would require a shorter mount because you view from the top rather than the bottom of the tube.

However, a first-rate 5-inch f/5 Newtonian with a fan is not a standard commodity! You could get one custom-made for less than the cost of most 4-inch APOs, but it still wouldn't be cheap.

A more normal comparison is between a 4-inch f/6 APO and a standard commercial 6-inch f/8 Dob. Here, the Dob will cost much less and deliver significantly better planetary images. But it will take longer to cool and be significantly less portable.

#78 LDW47

LDW47

    Apollo

  • -----
  • Posts: 1036
  • Joined: 04 Mar 2012
  • Loc: North Bay,Ontario,Canada

Posted 26 April 2014 - 06:39 AM

If we compare apples to apples, would 4" or 5" Newtonian cool just as fast AND be just as portable as a 4" APO refractor and provide similar CA free views?


As far as CA is concerned, Newtonians are always better than APOs. False color is inherent in refractors -- it may be too small for you to see, but it's always there. Conversely, Newtonians are inherently 100% color-free.

However, CA is not the end of the story. Newtonians have central obstructions, which degrade the image significantly, though in a completely different way from chromatic aberration. And unless they have first-class mirrors, they also scatter more light than refractors.

Putting it all together, I would say that a first-rate 5-inch f/5 Newtonian would deliver about the same planetary detail as a first-rate 4-inch f/6 APO. And with the aid of a fan, it would cool nearly as fast -- but probably still not as fast. In terms of portability, it would be roughly a draw. The Newtonian tube would be a smidge bigger, but it would require a shorter mount because you view from the top rather than the bottom of the tube.

However, a first-rate 5-inch f/5 Newtonian with a fan is not a standard commodity! You could get one custom-made for less than the cost of most 4-inch APOs, but it still wouldn't be cheap.

A more normal comparison is between a 4-inch f/6 APO and a standard commercial 6-inch f/8 Dob. Here, the Dob will cost much less and deliver significantly better planetary images. But it will take longer to cool and be significantly less portable.

If you leave it in an unheated / vented shed / garage at the same or close to the same temp as the outside cooldown is negligible if at all. The key is venting / air circulation in your storage area especially in the more humid climates. Also a 6" Dob is still very portable for 90% of observers.

#79 Tony Flanders

Tony Flanders

    Voyager 1

  • *****
  • Posts: 11154
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 26 April 2014 - 07:08 AM

great info in this thread, so if a person is color blind an AR would better?


No, not at all! The fundamental problem with achromatic refractors isn't the appearance of color, but the fact that only (at most) two wavelengths can be in focus at one time. The malfocused red and violet light will smear the image regardless of whether you can perceive the colors themselves.

#80 schang

schang

    Viking 1

  • -----
  • Posts: 849
  • Joined: 24 Apr 2013
  • Loc: columbia, sc

Posted 26 April 2014 - 07:13 AM

As far as CA is concerned, Newtonians are always better than APOs. False color is inherent in refractors -- it may be too small for you to see, but it's always there. Conversely, Newtonians are inherently 100% color-free.

I think this should be put it in a bulletin or somewhere newbies can easily see. It is delusional for them to read those glorified reviews about APOs without thinking that it is still a refractor. Same for Newtonian, where it is CA free, but it has some quirks as well that can be posted in the same space for newbies to see all at once, rather than going thru the maze of big cannons and small "rifles".

#81 schang

schang

    Viking 1

  • -----
  • Posts: 849
  • Joined: 24 Apr 2013
  • Loc: columbia, sc

Posted 26 April 2014 - 07:22 AM

If you leave it in an unheated / vented shed / garage at the same or close to the same temp as the outside cooldown is negligible if at all. The key is venting / air circulation in your storage area especially in the more humid climates. Also a 6" Dob is still very portable for 90% of observers.

This is one point neglected by a lot of people. Due to the size and low cost of newtonians/dobs, most would put them in the shed or garage where the mirror temp is quite close to the ambient temp. I do not know if this is the case for expensive APOs. I'd think that people would store them in the closet or display them in the great room. :grin:

#82 howard929

howard929

    Member

  • *****
  • Posts: 3427
  • Joined: 02 Jan 2011
  • Loc: Low End of High Ground

Posted 26 April 2014 - 07:29 AM

Half of the time, 100 percent of first time buyers feel their requirements for said telescope are singular. Add to that a general feeling that as things change over time, there just may be something new and different to find out about. Long threads, short threads.. it's all good.

#83 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43444
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 26 April 2014 - 09:59 AM

If we compare apples to apples, would 4" or 5" Newtonian cool just as fast AND be just as portable as a 4" APO refractor and provide similar CA free views?

If it is pretty close then what are other technical reasons I would want to buy 4" APO and not a 4"/5" Newtonian at 1/10th the price.


This is my experience, Three scopes to compare:

4 inch F/5.4 apo TeleVue refractor, Orion 100mm F/6 achromat fitted with a 2 speed focuser, Skywatcher 130mm F/5 Newtonian. The Skywatcher is fitted with a JMI DX-1 2 speed focuser.

Thermally: The two refractors need very little cool down and provide very nice views the moment they are taken out the door. The Newtonian is a totally different beast. The effect of the cooling mirror is very evident and it takes 45 minutes or more for it to settle down so it performs reasonably well at higher magnifications.

There are a number of reasons that can explain this, most of them involve tube currents, convection currents rising from the cooling mirror that disturb the air in the scope.

- With a refractor, the glass which cools the slowest is at the open end of the tube so the convection currents are not trapped, they only have to escape the dew shield. Also, the light only passes down the tube once and for more of that distance it's well separated from edge of the tube where tube currents are most likely. With the Newtonian, that first pass most of the light is along the edge and there is a second pass to the diagonal.

- Optically, the central obstruction of a Newtonian is always a compromise between high power work and low power widefield observing. A small secondary mirror provides greater planetary contrast and is better for high powers but means the low power views will not be well illuminated at the edge of the field. A large secondary results in better low power views but loss of contrast for planetary viewing. The smaller the scope, the more important this is because the field stop size of the eyepieces available are limited.

With a 5 inch Newtonian the compromise is serious, a 30% CO is about right. With a 25 inch Newtonian, one has a lot of latitude. Mine has a 14% secondary (3.5 inches) and provides a well illuminated widefield view.

Back to the comparison.. Viewing Jupiter and Saturn, the 4 inch apo provides the most contrast and shows the most detail. For most double stars, particularly unequal double stars, the apo is the better tool. Once it is cooled down, the Skywatcher 130mm F/5 does a nice job viewing the planets and many doubles, the contrast is there and the view is clean and free from chromatic aberration. At the high magnifications, the 100mm F/6 achromat suffers, the chromatic aberration significantly affecting the planetary views and limiting it's capabilities splitting close doubles.

At low magnifications, those big, bright wide fields of view that smaller scopes can do that big scopes cannot.. The 4 inch TeleVue apo refractor is about as perfect a telescope as there is. Not only does it have that amazing color correction but it has a flat field of view so stars are in focus from the center to the edge. The 100mm achromat does a good job, it shows field curvature so the stars are not pinpoint across the field but it's a pretty fine view.

The 130mm Newtonian does a reasonable job as well but suffers from a drop in illumination at low powers. Still, I can see the entire Veil in one view...

Bottom line:

A scope like the NP-101 is very expensive, it's value is in the quality of the view. For a 4 inch telescope that is 26 inches long, it does just about everything a 4 inch telescope can do as well as it can be done.. If one is interested in high magnifications, it is limited by it's 4 inch aperture and there are many more affordable alternatives that are of a larger aperture that provide more detailed views, views with finer contrast of the planets and double stars.

At the lowest magnifications, it is arguably the best visual scope there is but others do a satisfactory job without the hefty price tag.

At telescope like this.. it's easy to use because it is compact, it provides wonderful views that approach perfection, when you look through the eyepiece, you know you are looking through something very special...

But all that said, when I want the best planetary views and I have time to setup and time for a Newtonian to cool down, my basic 10 inch GSO Dob provides the better planetary views.

As I said, I don't recommend expensive apo refractors to most beginning astronomers. But there comes a time when it is nice to have one.

Jon

Posted Image

Posted Image

#84 GOLGO13

GOLGO13

    Gemini

  • *****
  • Posts: 3036
  • Joined: 05 Nov 2005
  • Loc: St. Louis area

Posted 26 April 2014 - 10:21 AM

This is one point neglected by a lot of people. Due to the size and low cost of newtonians/dobs, most would put them in the shed or garage where the mirror temp is quite close to the ambient temp. I do not know if this is the case for expensive APOs. I'd think that people would store them in the closet or display them in the great room. :grin:


Personally I wouldn't store any of my scopes in the garage. To many contaminates from cars and such. Unless it were a garage that did not have cars or other pollutants in it. Sheds could work as long as they are secure and well maintained temperature wise. I don't have a means of storing mine other than in the house. Sometimes I will bring mine into the garage for a short time if the outside temp is really cold in order to phase the warm up.

#85 Abhat

Abhat

    Viking 1

  • *****
  • Posts: 506
  • Joined: 14 Dec 2013
  • Loc: Middletown, MD

Posted 26 April 2014 - 10:28 AM

Thanks Jon. As always, I truly appreciate the time you take to write this for us. And also your detailed and easy to understand explanations are very informative.

By the way those pictures look great. Few years down the line I hope I would have gathered enough courage and experience to buy that TV APO.

#86 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43444
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 26 April 2014 - 10:36 AM

Personally I wouldn't store any of my scopes in the garage. To many contaminates from cars and such. Unless it were a garage that did not have cars or other pollutants in it.



Cars in the garage? :)

In San Diego, cars mostly live outdoors all year long. No one has a basement, no one has an real attic, garages are for telescopes, bicycles and the like..

For those who live in the real world with real weather, I would also be reluctant to keep my scope in a garage with cars the like.

Jon

#87 obin robinson

obin robinson

    Mercury-Atlas

  • *****
  • Posts: 2624
  • Joined: 25 Oct 2012
  • Loc: League City, TX

Posted 26 April 2014 - 11:29 AM

Personally I wouldn't store any of my scopes in the garage. To many contaminates from cars and such. Unless it were a garage that did not have cars or other pollutants in it. Sheds could work as long as they are secure and well maintained temperature wise. I don't have a means of storing mine other than in the house. Sometimes I will bring mine into the garage for a short time if the outside temp is really cold in order to phase the warm up.


Personally i wouldn't store anything outside that i don't want spiders eventually taking up a residence in. If you don't mind the occasional insect crawling out of your focuser then go ahead and store your telescope in a garage. Garages also have temperature extremes that can ruin certain materials. Due to those temperature and humidity extremes corrosion of small parts will be accelerated in a garage. You also risk warpage of materials from heat. On top of that dirt and film from car exhaust gathers on everything which is stored in a garage.

obin ;)

#88 schang

schang

    Viking 1

  • -----
  • Posts: 849
  • Joined: 24 Apr 2013
  • Loc: columbia, sc

Posted 26 April 2014 - 12:47 PM

This is one point neglected by a lot of people. Due to the size and low cost of newtonians/dobs, most would put them in the shed or garage where the mirror temp is quite close to the ambient temp. I do not know if this is the case for expensive APOs. I'd think that people would store them in the closet or display them in the great room. :grin:


Personally I wouldn't store any of my scopes in the garage. To many contaminates from cars and such. Unless it were a garage that did not have cars or other pollutants in it. Sheds could work as long as they are secure and well maintained temperature wise. I don't have a means of storing mine other than in the house. Sometimes I will bring mine into the garage for a short time if the outside temp is really cold in order to phase the warm up.

There are contaminants all around the house, no better than the garage. If you cover your scope with a large plastic bag (I mean large one to cover from top to bottom), in addition to the lid, you will be fine. If you like the hassle of moving it in and out of the house, that is OK, you are not going to hurt my feeling... :grin:

#89 Widespread

Widespread

    Viking 1

  • -----
  • Posts: 635
  • Joined: 11 May 2011
  • Loc: Bowling Green, Kentucky

Posted 26 April 2014 - 01:21 PM

Personally I wouldn't store any of my scopes in the garage. To many contaminates from cars and such. Unless it were a garage that did not have cars or other pollutants in it.



Cars in the garage? :)

In San Diego, cars mostly live outdoors all year long. No one has a basement, no one has an real attic, garages are for telescopes, bicycles and the like..

For those who live in the real world with real weather, I would also be reluctant to keep my scope in a garage with cars the like.

Jon


In the Bluegrass State, many keep riding mowers in the garage. Smells like gas, grass or... Oops... grease? (No one rides free)

But I suppose you could just leave the mower outside to outgas, or whatever, for a while before putting it away?

#90 GOLGO13

GOLGO13

    Gemini

  • *****
  • Posts: 3036
  • Joined: 05 Nov 2005
  • Loc: St. Louis area

Posted 26 April 2014 - 01:42 PM

I certainly cannot store my scopes in the garage in the summer. It gets super hot in there. But in the winter it may be better. However, salt and dirt is in the air as I store two cars in the garage.

Bottom line is it depends on your garage situation. As Jon points out (and I am extremely jealous) he doesn't have issues. Then again, the cost of living is about 1/6th the cost where I live ;) ... though for this hobby it may be worth it.

#91 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43444
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 26 April 2014 - 02:18 PM

I think the bottom line is there are a lot of different garages, some are good for storing telescopes, some not so good. Some folks store their scopes outside in a shed or even just under a cover. I wouldn't do that, too much dust. But I do store them in an unheated garage. It never gets too hot because there is just always a "breeze" blowing and it gets cool at night, this is in the mountains so there can be snow, but never too cold to store a scope.

Jon

#92 LDW47

LDW47

    Apollo

  • -----
  • Posts: 1036
  • Joined: 04 Mar 2012
  • Loc: North Bay,Ontario,Canada

Posted 26 April 2014 - 04:55 PM

I think the bottom line is there are a lot of different garages, some are good for storing telescopes, some not so good. Some folks store their scopes outside in a shed or even just under a cover. I wouldn't do that, too much dust. But I do store them in an unheated garage. It never gets too hot because there is just always a "breeze" blowing and it gets cool at night, this is in the mountains so there can be snow, but never too cold to store a scope.

Jon

As I mentioned it all depends where you live and your average climate compensated by good venting / air circulation in your storage area. I am fortunate to live in a pretty temperate climate with no prolonged extremes and it all comes together perfectly.

#93 Widespread

Widespread

    Viking 1

  • -----
  • Posts: 635
  • Joined: 11 May 2011
  • Loc: Bowling Green, Kentucky

Posted 26 April 2014 - 04:59 PM

Yep! :waytogo:

#94 Tony Flanders

Tony Flanders

    Voyager 1

  • *****
  • Posts: 11154
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 27 April 2014 - 03:55 AM

I think the bottom line is there are a lot of different garages, some are good for storing telescopes, some not so good ...


As I mentioned it all depends where you live and your average climate ...


Actually, I think it depends a lot more on what else you keep in your garage. Obviously, I don't have a garage for my city apartment, but I do at my country home.

Keep a car in the garage? Why ever would I do that? That's not what garages are for, is it?

#95 LDW47

LDW47

    Apollo

  • -----
  • Posts: 1036
  • Joined: 04 Mar 2012
  • Loc: North Bay,Ontario,Canada

Posted 27 April 2014 - 06:52 AM

I think the bottom line is there are a lot of different garages, some are good for storing telescopes, some not so good ...


As I mentioned it all depends where you live and your average climate ...


Actually, I think it depends a lot more on what else you keep in your garage. Obviously, I don't have a garage for my city apartment, but I do at my country home.

Keep a car in the garage? Why ever would I do that? That's not what garages are for, is it?

If you are unlucky enough to not have a telescope then probably ! LOL !

#96 REC

REC

    Fly Me to the Moon

  • *****
  • Posts: 5045
  • Joined: 20 Oct 2010
  • Loc: NC

Posted 27 April 2014 - 07:36 AM

Just an FYI...I had my C102 out last night for a short time. High clouds in and out. I used my 8-24 zoom EP and had very little CA. When I zoomed all the way in to 8mm at 125x the CA was a little more produced. I had 2 bands, but backing it off a little to like 12mm @80x, I had 3 bands and very little CA. Overall, it was not bothersome and the 4 moon where very sharp. BTW...this is with a $69 zoom EP.

#97 schang

schang

    Viking 1

  • -----
  • Posts: 849
  • Joined: 24 Apr 2013
  • Loc: columbia, sc

Posted 27 April 2014 - 09:34 AM

Just an FYI...I had my C102 out last night for a short time. High clouds in and out. I used my 8-24 zoom EP and had very little CA. When I zoomed all the way in to 8mm at 125x the CA was a little more produced. I had 2 bands, but backing it off a little to like 12mm @80x, I had 3 bands and very little CA. Overall, it was not bothersome and the 4 moon where very sharp. BTW...this is with a $69 zoom EP.

I used my C102AZ as well last night to try the modified AZ tripod. CA on the C102 is there at 167X on Mars, Jupiter, and Saturn, but not much. I saw color fringing on my Dob as well on Jupiter when it is setting low around 11:30 PM. Mars was high and I pushed it to 212X and still not much CA. Overall, I am not too concerned about CA with a slower refractor. In observing, bad seeing, low hanging objects and poor alignment would have much severe impacts on images than CA from a slower achromatic refractor.

#98 finisher604

finisher604

    Sputnik

  • -----
  • Posts: 34
  • Joined: 17 Feb 2014
  • Loc: Vancouver , Canada

Posted 27 April 2014 - 11:33 AM

I made the jump to an expensive apo. I use it every clear night. I too am spoiled like Jon and keep my gear in the garage. The scope it's self is in a case which would solve everyone else issues with bugs and dust. My eye pieces are all in a plastic case to keep the dust out. In the winter is heat the garage with a space heater just to keep the dampness out of the air. No problems yet and my motorcycle is showing no signs of rust either :)

#99 obin robinson

obin robinson

    Mercury-Atlas

  • *****
  • Posts: 2624
  • Joined: 25 Oct 2012
  • Loc: League City, TX

Posted 27 April 2014 - 12:05 PM

In the winter is heat the garage with a space heater just to keep the dampness out of the air. No problems yet and my motorcycle is showing no signs of rust either :)


That's the key right there. As long as you keep the moisture levels to a minimum you will be fine. On the other hand when we lived in New York I can remember people being surprised at how much their cars were rusting even when in a garage. Moisture + salt + dirt + temperature changes = corrosion. Keep something dry and clean and the corrosion process will be slowed.

obin ;)

#100 Abhat

Abhat

    Viking 1

  • *****
  • Posts: 506
  • Joined: 14 Dec 2013
  • Loc: Middletown, MD

Posted 27 April 2014 - 12:13 PM

Overall, I am not too concerned about CA with a slower refractor. In observing, bad seeing, low hanging objects and poor alignment would have much severe impacts on images than CA from a slower achromatic refractor.


Thats a good point. Seeing, collimation, cooling, dew, poor mount, light pollution etc.. There are so many factors that kick in before one starts wishing about the premium optics. In my limited personal experience optics is never the weakest link in the chain. That's why I hate to throw money at $300 eyepieces when you can buy a decent slow refractor with the same money.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics