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Planetary observing help


T A WOW

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Hello peeps. I'm after a bit of advice to do with, as the title suggests, planetary observing (NOT imaging)

My question is to do with focal lengths.

After reading a post by Dawson, in which he comments on a Damien Peach DVD, stating that a focal length of about f20 is ideal for viewing Jupiter.

The question is, is there a difference in what I would see, if for example, viewing Jupiter in my 8mm Hyperion (150 x mag), or viewing it through a 4 x powermate and my 32mm Hyperion? (also 150 x mag I assume)

My scope is a skywatcher 250 pds F4.7. So with the 4 x powermate would it be F18.8 or does it just magnify the image and not alter th F length?

Thanks for any help

Toby.

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As far as I understand...............


 


A barlow or powermate technically alters the focal length of your scope.


 


So yes, you would end up with a focal ratio of F18.8.


 


But as you are just visual, I`m not 100% certain if the view would be massively better.


 


I stand to be corrected.

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I was also thinking about focal lengths earlier again, in the gym of all places! I learn best by using visual aids and i wish there was a website or a book which explained all these optical things more clearly but with drawings to show how the visual length changes with various EPs and Barlows. In fact i think here is scope for a book on the whole topic of modern astronomy written for the simpleton. Maybe it exists and i've just not seen it.

James

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I can post the DVD tomorrow to whoever wants it first.


 


James


 


Edit - it's not a highly complex DVD, it's all quite basic stuff but useful to new comers like me; I'm not sure how useful it will be for advanced astronomers like you lot.


Edited by dawson
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"The question is, is there a difference in what I would see, if for example, viewing Jupiter in my 8mm Hyperion (150 x mag), or viewing it through a 4 x powermate and my 32mm Hyperion? (also 150 x mag I assume)"


 


I may be about to waffle on about complete nonsense so sorry if this is the case, but this is how I thought it worked. No doubt someone with more knowledge with clarify.


 


I don't think there would be no difference in F Number between 8mm vs 32mm with 4x powermate, the clue is in the name, 4x magnification.


 


See it as Eyepiece focal length divided by the number, Also Barlows/Powermates work slightly differently but that's another matter.


 


E.g. 8mm Eyepiece with 2x Barlow/Powermate = 4mm Eyepiece


8mm Eyepiece with 4x Barlow/Powermate = 2mm Eyepiece


F Number = Focal Length / Diameter(In this case diameter of the scope)


 


So if you look at it like this,


Your scope has a focal length of 1200mm and diameter of 250mm


F Number = 1200 / 250 = ~f4.7


 


The only way to change the F Number of the scope would be to use the end cover with the hole bit taken off(I presume the 250pds has one as the 200p does).


 


Lets say the hole on the cap is 50mm(It might not be). Do the formula again:


 


F Number = 1200 / 50 = f24


 


Hope I am correct otherwise I've just made myself look like a huge idiot  :blush: 


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Eddy.

I'm sure I have seen on here somewhere that barlows/power mates while magnifying the image, they also effectively increase the focal length of the scope. When not being pestered by my 3 year old I will see if I can find the topic.

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Tobias,


 


It is Kim's DVD, but he said to pas it on to the next needy person. Send me your address and I'll post it in the morning, as long as you promise to pass it onto the next needy person (who I think is Pat).


 


James

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Pat I gave it to James for free - the only condition being that if he finishes with it he should give it to another astronomer/imager who can use it :)


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I've scanned three pages on the Barlow from Gerald Norths book called "Observing the Solar System", 2012. It's quite a hefty book , and at present a bit too detailed for me, but these pages seem to help answer the question a little:


 


 


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Could you cut a hole out of card with a diameter of 50mm, and plonk it on the end of your scope. You'd then have an aperture of 50mm, a focal length of 1200mm and a focal ratio of 1200/50 = f/24

Would that work?

James

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Aperture masks are quite common. On some scopes the tube cover has a cap covering a hole which can be removed for solar viewing with a filter or lunar viewing with reduced brightness. Mick made one for his old Lightbridge with three rotating holes of different diameter to do what you're suggesting :)


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I did indeed Kim, it was very simple to make I think there was a 3", 5" and a 6" hole. You have to make sure you miss the spider vanes.


 


Then do the maths involved and I think if I remember correctly the 3" hole made the scope became a F18 scope.

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Guest Ely Ellis

When I am viewing the moon (or any bright object)  I always fit the lens cap and remove the small 2" cap. It makes the contrast much better and also the focus much easier and clearer. Also as it uses the centre part of the lens it reduces any distortions, both colour and edge distortions.


Makes my F8 refractor into an F20 refractor.


Of course it does not increase any magnification as the scope is still 1000mm.


 


Cheers


 


Martin

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Around f-23 I reckon - but I never tried it to see the effect - certainly worth a try with different size holes :)


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Pat, 2350 / 80 = f/29


 


Kim, what would be the calculations for working out the FOV with an eye piece with this modified f/ value for Pat's scope?


 


James

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If I reduced the aperture of my 127mm Mak (focal length 1500mm) and used my solar filter, how would that impact on the size of the image of the sun on my DSLR sensor at prime focus?


 


When you talk about missing the spider vanes, do you make the new aperture off centre? What would I do as I've got a massive secondary mirror in the middle of my normal aperture:


 


http://www.eecs.wsu.edu/~ewang/PAS/erw-2120-corrector-plate.gif


 


(that's a Google image, not my scope!)


 


James

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Does the new aperture have to be just one hole? Could the mask consist of say two separate holes which collectively gave a new aperture of say 80mm, or would the light hitting the primary at two sites defeat the object of the aperture mask? (This is a theoretical question as I can't think of an advantage of having an aperture mask with two or more holes in).


 


James

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