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Maximum focal length for 60mm diameter

Equipment Barlow Jupiter
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Lorin avatar
Lorin avatar
Hello,

So I have a small 60mm refractor and many barlows and other extenders that I am tempted to
stack on top of each other in order to obtain larger images of the planets.
With its native 355mm focal length, a x2 barlow and a x1.7 extender and yet
another x2 barlow I arrive at a whopping 2.4 meters. Is this reasonable for a
60mm scope or am I just making a fuzzy image even larger? Here's Jupiter (one
shot) taken with this configuration: https://astrob.in/full/417354/0/ I would appreciate if a kind soul would enlighten me on this.
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Jeff Coldrey avatar
Jeff Coldrey avatar
Hi Lorain. It will depend a bit on the quality of your scope, quality of the barlows, and maybe how you sequence the barlows. But I suspect 2.4m will be pretty crappy, but no harm in trying.

A good place to test might be the moon … along the terminator between night/day where there is plenty of shadows/contrast around the craters etc.

Let us know how you go!
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Adel Kildeev avatar
Adel Kildeev avatar
Theoretically maximum useful magnification is 60x2=120. But again it depends from the quality of the scope and quality of barlow kense. I use with ny Meade 80/480 APO triplet Barlowx5 with great success for planetary imaging. smile
Lorin avatar
Lorin avatar
Jeff:
It will depend a bit on the quality of your scope, quality of the barlows, and maybe how you sequence the barlows. But I suspect 2.4m will be pretty crappy, but no harm in trying.


The scope is the baby-taka with sexy fluorite glass, so quality-wise it's allright, as are the barlows: two of them (x1.7 and x2) are dedicated for that particular scope, and the third x2 is a Sigma extender that I stuck in the lens mount adapter. I may have a high tolerance for cosmetic errors but frankly I'm quite happy with the 2.4m focal. I do, however, prefer the 1.2m variant, but that's just because of turbulence (I shoot from the city most nights).

I am planning to take a control shot with all focals available - probably the Moon; shall post here when the sky allows it.
Lorin avatar
Lorin avatar
Adel Kildeev:
Theoretically maximum useful magnification is 60x2=120.


Now be kind and translate that x120 in focal length / diameter? Saw your pictures - very pretty with the x5 barlow!
Benny Colyn avatar
Benny Colyn avatar
Using some calculators here:https://astronomy.tools/calculators/telescope_capabilitieshttps://astronomy.tools/calculators/ccd

the Rayleigh Limit for a 60mm objective is 2.3 arcsec. Rayleigh Limit is a measure defining the limit at which two components can be clearly identified as separate components. It defines the distance between the centers of two Airy disks where the maximum of one is placed over the minimum of the other.

with your camera (pixel size = 3.7) 330mm focal lengths will already put you in the range of 2.3"/pixel. Let's be generous and say we take 3 pixels for 2.3" then 1000mm would be the max that seems reasonable. I think you need more aperture, not focal length, to get more planetary details.

My 2c, probably someone will point out my errors in reasoning but there you have it :-)
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Lorin avatar
Lorin avatar
Benny Colyn:
Using some calculators here:https://astronomy.tools/calculators/telescope_capabilitieshttps://astronomy.tools/calculators/ccd

the Rayleigh Limit for a 60mm objective is 2.3 arcsec. Rayleigh Limit is a measure defining the limit at which two components can be clearly identified as separate components. It defines the distance between the centers of two Airy disks where the maximum of one is placed over the minimum of the other.

with your camera (pixel size = 3.7) 330mm focal lengths will already put you in the range of 2.3"/pixel. Let's be generous and say we take 3 pixels for 2.3" then 1000mm would be the max that seems reasonable. I think you need more aperture, not focal length, to get more planetary details.

My 2c, probably someone will point out my errors in reasoning but there you have it :-)


Thank you for this. Confirms what I was suspecting.
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Adel Kildeev avatar
Adel Kildeev avatar
Lorin:
Adel Kildeev:
Theoretically maximum useful magnification is 60x2=120.
Now be kind and translate that x120 in focal length / diameter? Saw your pictures - very pretty with the x5 barlow!
You are right: aperture is the key allowing magnification without blur and distortions. With my 80/480 mm APO triplet useful magnification is x160, but I pushed it with x5 Barlow much higher, with focal length up to 2400 mm. The same trick not work with SC, unfortunately.
Rich Sky avatar
Rich Sky avatar
Hi Lorin,
if i may ask, what camera are you using with your scope, what is the pixel width and height?
undersampling will be the issue.
r
bobzeq25 avatar
bobzeq25 avatar
Great eclipse panel.

5X (1800mm) is about as far as you can go, farther and you are indeed magnifying a blur.

You'll need a good planetary camera, like an ASI290 (your DSLR may or may not work), and "lucky imaging" techniques, to be able to use 5X.  Anything less, and 2X would be more like it.

This book will be a great help.  It will tell you whether your DSLR could work, and, if not, will tell you what you need to know about how to use the 290, even though it says DSLR.

http://www.astropix.com/gdpi/index.html

Scroll down to the three images of Mars, and see what "lucky imaging" does.  Magic.  <smile>  It also works on the Moon.
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