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Using a 2x Barlow ending up Beyond Suggested Maximum Magnification


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#1 Am33r

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Posted 12 September 2015 - 03:14 AM

Under good viewing conditions, and mainly to view certain features on a well-lit moon where an aperture of an LX200-ACF with UTHC should bring plenty of light; did anyone attempt to use a 2x or 3x barlow with maybe an 8mm or a 6.4mm or even a 5mm eye piece?

 

This configuration would put the magnification beyond the maximum practicle limits of the scope.  The maximum on the 16" is about 1000x and they go down 100x-125x from there on for every 2" reduction in aperture.

 

Example: Use a 12" Scope + 3x Barlow + 9mm eyepiece and the result is 1000x in magnification.  This is more than the suggested 750x maximum practical magnification of the LX200-ACF 12" UHTC Telescope!

 

What do you see beyond the sugessted max magnification? Is it just a pure mess? Especially with a well-lit object like the moon!

Is this rule truely a maximum practicle limit or is it just an educated suggestion that doesn't always hold true when ideal conditions are present?

 

Thank you



#2 Mark Sibole

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Posted 12 September 2015 - 05:06 AM

IM a believer that the limits set are really for seing conditions and not the limits of the scope.I have the LX200 10 inch and have viewed Mars and saturn with a 5 times barlow and a 9 mm EP.Some nights are great views and other nights its a wobbley mess.The contributing factior are seeing contitions.It can be clear but if the atmosphere is unstable you wont get a good view.I have had better views on a great seing night with less than optimal transparency.To get good seeing and good transparency is a rare thing any more.With people trying to play god with haarp i havnt seen conditions this bad for clouds seeing and transparancy in 8 years and it seems to be getting worse.* years ago I could get out for imaging 6 of 7 nights  now im lucky to get out 1 time a week due to weather.


Mark Sibole
MTSO Observatory
Fife Lake, Mi.

http://astronomy.qteaser.com

#3 MistrBadgr

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Posted 12 September 2015 - 08:20 AM

There are so many variables and interrelations between those variables, starting with the object one is trying to view all the way to the brain of the observer, that it is difficult to make any absolute statements. With really high magnifications, is seems like I am entering the realm of superstition when doing it.

For the moon with most scopes, I can generally count on going to 2.5 times the mm of the scope. Sometimes, I can go to 3.0 times the mm, but the image is normally not as good. I am going after seeing some partaicular detail that might show up at a higher magnification.

For double stars, with good weather, I can go very high with the right scope. The reason being is that I am viewing the behavior of light and how it implies a condition with the objects themselves. I have gone WAY up there for kicks, but the image of a bright double star tops out around 3.0X to 3.5X the mm of the scope under the best conditions.

Under light pollution, with larger scopes, the best contrast seems to be around 0.8X the mm, but that does not hold with my small scopes, which need a lower magnification on dim objects or the view gets to be surrealistic.

Well, I will stop. Hopefully you can pick the bit of information about my opinions that applies to your question out of all that.

My suggestion to you is to try those things at different times and conditions and see what happens. The results for you, with your particular scope and location will most likely vary a bit from what anyone else gets.

Bill Steen
Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

#4 Am33r

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Posted 12 September 2015 - 01:28 PM

I have the LX200 10 inch and have viewed Mars and saturn with a 5 times barlow and a 9 mm EP. Some nights are great views and other nights its a wobbley mess.The contributing factior are seeing contitions.

Thanks Mark.  So with good seeing conditions you went up to over 1,300x magnification (Twice the Suggested Maximum) and it was fine. That's awesome to get some good viewing beyond their suggested max.



#5 Am33r

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Posted 12 September 2015 - 01:40 PM

For the moon with most scopes, I can generally count on going to 2.5 times the mm of the scope. Sometimes, I can go to 3.0 times the mm, but the image is normally not as good. I am going after seeing some partaicular detail that might show up at a higher magnification.

For double stars, with good weather, I can go very high with the right scope. The reason being is that I am viewing the behavior of light and how it implies a condition with the objects themselves. I have gone WAY up there for kicks, but the image of a bright double star tops out around 3.0X to 3.5X the mm of the scope under the best conditions.

Under light pollution, with larger scopes, the best contrast seems to be around 0.8X the mm, but that does not hold with my small scopes, which need a lower magnification on dim objects or the view gets to be surrealistic.

Thanks Bill.  I do not understand the concept of "3x the mm of the scope, or 0.8x mm of the scope". I know the focal length of the OTA but that is not a measure of magnification / resolve / power. It requires an eye piece to determine.

If my scope has 3048mm focal length and the eye piece is 5mm, and I go and add a 3x barlow lense, what does that make my "focal length of the scope"?



#6 MistrBadgr

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Posted 12 September 2015 - 03:10 PM

I will use some simpler numbers for demonstration.

Some basics that you know or can easily figure out:

One my LX 70 8 inch reflector, it has a 1000 mm focal length and a diameter of 200 mm. The focal length of the eyepiece I want to use is 5 mm, then the magnification is 1000 divided by 5 or 200X.

The magnification is 200 and my diameter of the telescope is 200 mm, therefore the ratio of magnification to diameter is 1.0. If I stick a 2X Barlow in this system, then the magnification is doubled to 400X. 400x divided by the 200 mm is 2.0.

For 200X magnification on this scope, the size of the image as it goes through the pupil of my eye is decreased from the size of the primary mirror by whatever the magnification is. Therefore 200 divided by 200 is 1.0 mm.

Now for some other items:

Rod Mollise, in his book on Urban Astronomy, talks about a pupil diameter that seems to be the best for contrast with light pollution as being about 1.25. For larger scopes, I have found his number to be a reasonable guide for seeing things like nebula. For my scope, a pupil diameter of 1.25 translates into 200 mm mirror diameter divided by the 1.25 mm pupil diameter or 160X. To translate that into something I can apply to a different sized scope, I normally think of magnification divided by the mirror diameter. Therefore, the number I can carry to another scope is 160X divided by the 200 mm of my LX 70 8R or 0.8.

If I wanted to take that to my Polaris 130 to see what magnification would be best for contrast from my back yard, then I can multiply the 130 mm diameter of the Polaris by 0.8, which gives me a magnification of 0.8 times 130 or 104X. If I want to know what eyepiece to use to get that, I divide the focal length of the Polaris 130, which is 650, by the magnification, 104X, which give me 6.25 mm for the eyepiece focal length.

For observing most things from my back yard, there seems to be a practical atmospheric limit at my elevation for my LX 70 8R of about 400X. Divide that number by the 200 mm diameter and I get a ratio of 2.0. I know many people that use that same 400X limit as about as high as one can go with just about any scope, unless you go to high elevations. Smaller scopes may not be able to reach 400X, but larger ones seem to be limited more by atmospheric disturbance than small or medium sized ones, like my 8 inch reflector. The larger ones have more light gathering power, which allows them to see dimmer objects, but the magnification seems to be limited more for them.

For double star work, the 400X limit on my 8 inch scopes seems to apply to some extent, but I find I can go beyond that if the scope is perfectly collimated and either weather conditions are perfect or I am willing to be patient and look for those momentary glimpses when a good image shows up out of the chaotic wiggles. Even though I have gone higher than 600X with the 8 inch reflector to see what happened and I was able to see things as high as 888X, I really do not see anything more or better than I do at 600X, which is 600X divided by 200 mm diameter of the scope or a ratio of 3.0.

Another way to think about that is how big is the image as it passes through the lens of your eye. With my LX 70 8R, that 1.0 ratio we talked about is 1 mm, the 0.8 ratio I talked about is 1.25 mm. I have heard some people state that the optimum diameter that our eyes are designed for is a pupil diameter someplace between 1 and 2 mm, but I have no confirmation of that from an authoritative source. It probably depends on the individual a lot. The 2.0 ratio that I talked about as a reasonable limit for most things, like planets, has a pupil diameter going through my eye lens of 0.5 mm and the 3.0 ratio as a practical top limit for double star work has a pupil diamter of one-third of a mm. If you think about it, just about any imperfection in your eye that is located in the wrong place could affect an image that is only a third of a mm across.

Well, that is probably enough rambling. Please pick out the parts that mean something.

Bill
Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

#7 Am33r

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Posted 13 September 2015 - 03:08 AM

Bill, thanks again.

 

I understand the concept of using the 2.5 Factor. I just learned that it is also related to our eyes themselves and their limits.

 

It seems that's the same factor Meade uses to suggest their maximum practical magnification of 750x.

 

Here are the two examples this factor seems to be their choice too:

 

* On the 12" Telescope, the focal length is 3000, if I use the smallest super plossl they sell at 4mm then I have 750x

 

** 2.5 x (the diameter of the primary mirror which is 300) = 750x.

 

So clearely 750x is the general practical magnification, and it uses your 2.5 factor.

 

As Mark mentioned earlier, it seems that with a very lit object ln our solar system (like the moon or mars), coupled with having good seeing conditions, and a large aperture, one can resolve more with even higher magnification (seemingly at twice the suggested maximum).

 

Typically, there are objects out there that are best viewed at half of their suggested general Max Mgnf while others (like the lunar surface) can work fine with twice their suggested general Max Mgnf.

 

This is good news - I previously didn't know what will happen at all if I put the small mm eye piece into the barlow and attempt to view something beyond the general practical limit they suggested.  Now I know that there are occasions for a great window of opportunity to see certain things far beyond that number.  of course, practice, and starting with smaller magnification then going up are keys here too - depending on each particular object.

 

Thank you both for the tips, I now have a happy answer to my concern.



#8 Mark Sibole

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Posted 13 September 2015 - 02:45 PM

Transparency and seeing conditions(how steady the atmosphere is) are your biggest factors.On them rare good days ill use a 9mm eyepiece or camera and a 5 times barlow for good fiews of the sun with the PST but yoiu can tell the instant seeing goes to pot.Ive found for lunar and solar viewing early morning is the best before the atmosphere heats up.You can actually get some very nice views of the moon during early morning hours in the daylight.Actually I prefer lunar viewing during daylight hours/


Mark Sibole
MTSO Observatory
Fife Lake, Mi.

http://astronomy.qteaser.com




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