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Magnification calculation ?

Guest

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Guest

Guest

Posted
Posted

Hi all, bit of a strange one ere.

I got a SW Star Discovery 150p 750mm focal length and i was just mucking around tonight with my barlows and came up with quite a beautiful image of Jupiter, let me explain ;)

The star discovery comes with a pretty good SW Deluxe 2x barlow to which the end unscrews and you can use it as a roughly 1.5x add on to the direct end of an ep.

Cant remember where i read it was 1.5x but you may clarify.

I then treat myself to the Revelation (GSO) plossl ep set for crimbo and that too has a variant of the same barlow, i read here its slightly better then the SW one not that i

can tell, i also got a Revelation 68deg SuperView 15mm plossl which is just beautiful, as is the whole set to be fair, really pleased with this (GSO) built plossl set....

I digress....

So, i got to thinking, i put my SW barlow 'end lens' into my 15mm ep and then put that combination into the other barlow....follow ? lol

Now i got some proper magnification out of that with a lovely eye relief and surprising brightness with lovely focus, it worked very well but i have no idea what mag it was producing ?

Is there a way this can be calculated ?

Cheers.

Lee.

Note :- i will endeavor to try a pic when the frosts come bringing some nice crisp air to show you how well this worked for this scope.

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cathalferris

cathalferris

Posted
Posted

Do a transit time of a star near the celestial equator, from ingress through the field center to disappearance, with the drive (if present) switched off. There exists a formula to calculate the true fov once you know that time, I'll try and dig it out later when not on mobile.

Sent from my SM-G920F using Tapatalk

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baggywrinkle

baggywrinkle

Posted
Posted

The Barlow does not magnify per se. It increases the focal lenght of the OTA.

So a 1.5x would mean you have 1.5x750mm so now you have an 1125mm focal lenght. So if you were using a 10mm ep previously it would be 75x mag, now it would be 112x mag.

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Guest

Guest

Posted
Posted

6 or two 3's baggy lol its still multiplied the outcome ;) besides, i know what the barlow does technically but on paper its increased the magnification but thanks, i am sure some may not of known that.

Thanks cathalferris, i assume this time taken will then equate to a formula to which size and mag and be worked out ?

Need the stars back out tonight now :) fingers crossed.

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Stu

Stu

Posted
Posted

I think Cathal's method is the best way of doing this. Barlows are strange things, the effect they have varies depending on the distance from the focal plane of the eyepiece. Stacking the two will change the distance of the 2nd one, so will alter it from being a x2.

This site has some useful information in it:

http://www.nexstarsite.com/_RAC/form.html#2

Specifically this section gives you the formula needed.

f6917957d04070cba93e8e8c4052552d.jpg

Note that you need to adjust the formula if you use a star which is not at 0 degrees declination.

This gives you the true field of view. If you know the apparent field of view you can find the magnification by dividing apparent field of view by the true field of view which you have calculated.

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Guest

Guest

Posted
Posted

Thanks Stu :) i will pick an equatorial star and do it right on the meridian just to be sure, i will then post back for some needed help with that formula lol

Appreciate the help guys.

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Guest

Guest

Posted
Posted

Hi peeps. Finally got a chance to take some timings last night so here are the ep's and approximate magnifications i would like some idea on.

GSO 9mm Plossl 52 degree apparent field of view

Ep + 2x barlow + removable lense from a duplicate 2x barlow (screwed directly to the ep before insertion to the normal complete barlow).

Star on equator took 0:41 seconds to transit FOV.

------------------------------------------

GSO 15mm SuperVIew Plossl 68 degree apparent field of view

Ep + 2x barlow + removable lense from a duplicate 2x barlow (screwed directly to the ep before insertion to the normal complete barlow).

Star on equator took 1:36 seconds to transit FOV.

This was on a 750mm F5 150mm Newtonian.

Any help with the calculations formula that Stu kindly supplied will be most appreciated.

Lee.

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Stu

Stu

Posted
Posted

Hi peeps. Finally got a chance to take some timings last night so here are the ep's and approximate magnifications i would like some idea on.

GSO 9mm Plossl 52 degree apparent field of view

Ep + 2x barlow + removable lense from a duplicate 2x barlow (screwed directly to the ep before insertion to the normal complete barlow).

Star on equator took 0:41 seconds to transit FOV.

------------------------------------------

GSO 15mm SuperVIew Plossl 68 degree apparent field of view

Ep + 2x barlow + removable lense from a duplicate 2x barlow (screwed directly to the ep before insertion to the normal complete barlow).

Star on equator took 1:36 seconds to transit FOV.

This was on a 750mm F5 150mm Newtonian.

Any help with the calculations formula that Stu kindly supplied will be most appreciated.

Lee.

Well Lee, I got you into this mess, so I guess I should have a go at it [emoji6]

First example:

(41/0.9973*15)/3600 = 0.171 degrees

Mag = 52/0.171 = x304

Second example

(96/0.9973*15)/3600 = 0.40 degrees

Mag = 68/0.40 = x170

How does that sound?

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Guest

Guest

Posted
Posted

Well Lee, I got you into this mess, so I guess I should have a go at it [emoji6]

First example:

(41/0.9973*15)/3600 = 0.171 degrees

Mag = 52/0.171 = x304

Second example

(96/0.9973*15)/3600 = 0.40 degrees

Mag = 68/0.40 = x170

How does that sound?

Hahahaha thanks Stu, how does this sound ???? i have no idea lol but if those magnifications are correct as this is a maths thing then thats great :)

I would say that it makes a lot of sense, Your a star pardon the pun ;) now i just need some still crisp frosty air to use the 9mm but the 15mm is a pleasure at that mag.

Really appreciate your help pal, THANK YOU :thumbsup:

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Stu

Stu

Posted
Posted

Hahahaha thanks Stu, how does this sound ???? i have no idea lol but if those magnifications are correct as this is a maths thing then thats great :)

I would say that it makes a lot of sense, Your a star pardon the pun ;) now i just need some still crisp frosty air to use the 9mm but the 15mm is a pleasure at that mag.

Really appreciate your help pal, THANK YOU :thumbsup:

Great stuff. Let's just hope that no one who really knows what they are doing points out that I've made a glaring mistake!! [emoji6]

x170 should be a very nice mag for Jupiter, x304 as you say, probably pushing it but maybe fun on the moon on a still night.

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Guest

Guest

Posted
Posted

Great stuff. Let's just hope that no one who really knows what they are doing points out that I've made a glaring mistake!! [emoji6]

x170 should be a very nice mag for Jupiter, x304 as you say, probably pushing it but maybe fun on the moon on a still night.

Mistake ??? well i definitely wouldn't see it lol but you could give us a clue ;)

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