Quote:
Originally Posted by edelweiss
Thank you renato for the reply.
I'm not wearing glass  btw.
So is it okay to use 4mm to view details on planetary? If I use barlow with my 10mm ep, that means I will get magnification around 5mm right? That is quite close to the 4mm.
Btw, I normally use my scope to view faint stuff too. So, does it means that wide angle ep will give me a better view?
Urmm..my focal ratio is f/5.9. Is it a good thing? I don't understand much about focal ratio stuff..sorry
PS: What size should I use for a perfect viewing of clusters?
Regards,
Eeda |
Hi Eeda,
The way it worked for me with my high power viewing in my C8 was I'd go out and use 200X on a planet. If the image was very sharp, then go to either 250X or 300X. If the image was still very good (pretty rare) then go to 400X.
The focal length of your telescope is 200mmX5.9= 1180mm
Divide the focal length of the telescope by that of the eyepiece to get magnification.
So,
3mm eyepiece (or 6mm eyepiece w/2X barlow)= 393X
4mm eyepiece (or 8mm eyepiece w/2X Barlow)= 295X
5mm eyepiece (or 10mm eyepiece w/2X barlow) = 236X
6mm ( ditto) = 197X
6.3mm = 187X
8mm = 147X
which gives you an idea of the eyepieces that could be useful at some time in your telescope, depending on how steady the atmosphere is.
I am very hesitant to recommend the Seben 4mm plossl, because you just may not like. Truth be told, I've since gotten a 4mm TMB eyepiece, which sort of has a little barlow built into it, and has a wider field of view than the plossl, and much longer eyerelief.
When you say perfect viewing of clusters, well, the perfect view is when the cluster fits into the field of view of your eyepiece. For big clusters and big nebula, you want low power with bright images.
Generally speaking for a bright view a 5mm or 4mm exit pupil is the way to go. Multiply the f ratio by the exit pupil to get the eyepiece that delivers that exit pupil.
Thus a
24mm eyepiece gives you a 4mm exit pupil.
30mm eyepiece gives you a 5mm exit pupil.
36mm gives you a 6mm exit pupil
42mm gives a 7mm exit pupil.
The bigger the exit pupil, the brighter the image and the lower the magnification.
The problem is, the image is brighter but so is the background light, and in suburbia, the image can look pretty yucky. Also, if you have any astigmatism in your eyes, you'll have to wear glasses to see sharp star dots. I do occasionally use 6, 7 and even 9mm exit pupils in some of my telescopes, but I always add a broadband filter to them to get rid of some background light. Your 25mm eyepiece gives close to the 4mm exit pupil, so if you want wider field and brighter image, you have to go for a 30 or 32mm eyepiece for a marginal increase in field or a 40mm 2" one for something more significant.
If you are only buying 1.25" eyepieces, then a 32mm plossl gives you the most field of view that you can see in your telescope. But if you have 2" focuser, you can get a wide angle 32mm or 40mm eyepiece which show more field. The cheap $49 GSO ones at Andrews Communication I'm quite fond of, but I don't know how they'd work in your telescope.
Your f/5.9 focal ratio is somewhat short. That means that while the fairly expensive wide angle eyepieces will work well in it, you may get a little or a lot of distortion at the edges with cheaper wideangle eyepieces. Some people can't stand that distortion, but it doesn't bother others who mainly focus on the central part of the image. If you'd had say an f/12 MAK, then even cheap eyepieces work fairly well in that one without much distortion at the edges. So, if you were thinking of getting a cheap Superwide Angle eyepiece from Seben or cheap GSO Superview, you'd only buy one in the first instance, to see how it worked in your telescope and how happy you were, before either deciding to buy others, or start saving for more expensive eyepieces.
I hope this has been of help. I've only given you guidelines to think about, because I don't own a telescope with your characteristics, so can't be definitive.
Cheers,
Renato
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oops ,,, apologies Eeda .
