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Barlows vs. short focal length eyepieces

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#1 Tom Baer

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Posted 05 January 2013 - 09:48 PM

This one might belong in the Eyepiece forum, but I am a beginner so I'll put it here. I have always been an astronomy buff (had a Jason 60mm refractor as a kid), and for Christmas I bought a 4.5 inch Starblast altazimuth (450mm focal length) to introduce my 10 year-old daughter to the planets and Messier objects. I am very pleased and impressed with this telescope, especially for the low price.

It came with two plossls (6 mm and 17 mm), and I got a Shorty 2x Barlow. I also got a 6mm Orion Expanse eyepiece with is for me is an improvement over the 6mm the scope came with, even though the original 6mm isn't bad at all.

At 150x with the 6mm Expanse and the Shorty Barlow, bright objects like Jupiter and the moon look plenty sharp and bright, so for me that is definitely not pushing this scope too far. The Orion nebula and Andromeda galaxy look like dim greyish smudges at 150x, but I think those objects are better viewed a lower power away from light pollution anyway. (I am in Seattle, not easy to see many stars)

I am wondering how close I can get to the theoretical max magnification of 228x of the moon and planets before the images get too dark and smudgy, and also wondering the best way to do it cheaply.

So, my question is... what would give me the best chance of a decent high-power image with this scope in good seeing: A Burgess TMB 4mm + the 2x Shorty, or the 6mm Orion Expanse + a moderately priced 3x Barlow? Is it a tossup, or will one definitely be better than the other?

#2 Gary Riley

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Posted 05 January 2013 - 10:11 PM

Hey Tom and welcome to CN! It is not uncommon for many of us not going over approx. 225x-250x power even with larger scopes due to the atmospheric conditions. So I wouldn't worry too much about trying to max out your 4 1/2 inch reflector. The statement of 50X per inch of aperture is generally under ideal conditions which is very rare in most cases. Probably 25X-30X per inch is more realistic. Enjoy your scope!

Clear skies!
Gary

#3 dpwoos

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Posted 05 January 2013 - 10:20 PM

If I were you I would observe with your local astro club, where you can try out all kinds of eyepieces, delivering whatever magnifications you want. Deciding based on what you see through your scope is better than any advice you can get here.

#4 BigC

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Posted 06 January 2013 - 01:58 AM

Tom,

Buy or borrow a 3X barlow to use with the 6m;that combo will give you 225X in the Starblast 4.5.

And probably is all you can use.I find even an image of the bright Moon loses so much contrast it is unviewable bove around 410X in my 8" scopes I had previouslt thought I was using much igher magnification but realized my calculations were wrong.An 8" gather almost 4 times as much light as a 4.5" so it can support higher magnification.

The 3X barlow in combination with the 15mm will give you 90X with better eye relief than the 6mm alone.

I wonder if a 3X barlow isn't a better choice than a 2X to begin with.

#5 izar187

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Posted 06 January 2013 - 02:44 AM

In my experience 150x is about right for high power in a 4" newt. Rarely have I found a better image at more magnification.

Next consider a low power option. Something in the range of 25mm to 30mm of 4 element or more in design.

Welcome aboard!

#6 BillFerris

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Posted 06 January 2013 - 12:52 PM

...for Christmas I bought a 4.5 inch Starblast altazimuth (450mm focal length) to introduce my 10 year-old daughter to the planets and Messier objects. ...It came with two plossls (6 mm and 17 mm), and I got a Shorty 2x Barlow. I also got a 6mm Orion Expanse eyepiece with is for me is an improvement over the 6mm the scope came with, even though the original 6mm isn't bad at all. ...So, my question is... what would give me the best chance of a decent high-power image with this scope in good seeing...


It's helpful to think about and understand magnification in terms of exit pupil. Exit pupil can be thought of as the diameter of the image at the focal plane of the telescope/eyepiece system. When you place your eye near the eyepiece to view the focused image, a column of light enters your eye. The diameter of this light column at the focal plane is the exit pupil.

What makes exit pupil so useful, is that it can be applied to a wide range of apertures. Large exit pupils correspond to low magnifications. Small exit pupils correspond to high magnifications. A 7mm or larger exit pupil is going to be very low power in any aperture. A 1mm or smaller exit pupil translates to very high power.

Observing a bright object, a 2mm exit pupil is going to magnify the image enough that you will be able to detect the smallest resolvable detail which can be seen in that aperture. A magnification producing a 2mm exit pupil will present that detail with an apparent size of just over 1 degree. A 1mm exit pupil will present that same small detail as being 2.3 degrees in size. A magnification producing an exit pupil of 0.5mm, presents the smallest resolvable detail as being 4.6 degrees in size. In the vast majority of situations, using higher magnification does nothing to improve your ability to resolve fine detail.

One way to calculate exit pupil, is to the divide the focal length of the eyepiece by the focal ratio of the telescope. Another, is to divide the aperture of the telescope (in millimeters) by the magnification the eyepiece produces. Let's take a look at a selection of magnifications with your particular telescope.

The focal ratio of your telescope is equal to the focal length of the scope divided by its aperture. So, the 450mm focal length divided by the 114mm aperture equals 3.95. You can round up to describe your scope as an f/4. Here, are the magnifications and exit pupils you can generate with the current eyepiece/Barlow set:

17 mm: 26.5X = 4.3 mm exit pupil (moderate power)
17 mm w/ 2X Barlow: 52.9X = 2.2 mm exit pupil (high power)
6 mm: 75.0X = 1.5 mm exit pupil (high power)
6 mm w/ 2X Barlow: 150.0X = 0.8 mm exit pupil (very high power)

Your current set delivers a nice range of magnifications from very high to moderate power. Adding a shorter focal length eyepiece or a 3X Barlow will allow you to make objects appear larger but, in most cases, will not allow you to resolve any new details. I doubt you'll gain much in return for the cost to purchase that item. At the other end of the spectrum, adding an eyepiece in the 24-28 mm range would give you a much-needed low power (approx. 6 mm exit pupil) view and, when paired with the 2X Barlow, a 3 mm exit pupil that will compliment the views your current set delivers. This one eyepiece will deliver two magnifications not currently in your set and the wider true field of view will deliver pleasing views of large, bright deep-sky objects that your current set cannot deliver.

FWIW,

Bill in Flag

#7 dpwoos

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Posted 06 January 2013 - 02:15 PM

Nice post! I like to (more simply but less completely) point out that an eyepiece with a focal length equal to the f-ratio of the scope provides a 1mm exit pupil, and that one can easily scale from there. Some folks like to think in terms of exit pupil instead of magnification, but the realities of seeing make magnification a useful metric for larger apertures, e.g. almost never being able to observe at more than 300x or 400x or 500x or ???x regardless the exit pupil.

#8 SteveG

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Posted 06 January 2013 - 02:27 PM

Hi Tom
The advice above is spot on. Around here, we rarely get to use high power on planetary viewing. You might see a slight improvement with a higher quality, short focal length eyepiece, such as a 6 or 7mm ortho, but you would be dealing with a narrow fov. Even with the scopes you see listed below, I rarely use powers above 200x.

I think your best bet is to keep your eyes on Craigslist, and find an 8 or 10" dob, which pop up real cheap from time to time.

#9 Tom Baer

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Posted 07 January 2013 - 12:16 AM

Thanks for the advice everyone!






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