barlow or eyepiece

[edelweiss]

Hi guys...which one should I buy first; barlow lens or eyepiece (4mm or 6.3mm)? I have a dobsonian 8" that came together with 10mm and 25mm (skywatcher). I want to use it for a greater view of the planets (10mm doesn't give much detail for the jupiter n venus).:thanx:

[brian nordstrom]

:) Are you talking about a Meade 6.3mm Plossl Eeda ? , if so yes these are a very good eyepiece , a little cramped in the eye relief but a very good eyepiece .

If you are talking about a 4mm Plossl , then its eye relief ( distance your eye has to be to see the image in the eyepiece ) is about 3 mm , very , very close , to close to be comfortable .

I would suggest a 2x Barlow so you get 25mm , 12.5mm 10mm and 5mm focal lengths as these would be fine in your scope .

One question , is your scope columated well ? because the 10mm is 120x if its an f6 and that's an easy magnification in your scope and the supplied 10mm eyepiece's are or good quality mostly and should give a good image .

Brian.

[edelweiss]

Actually, I found something like Obinar 6.3mm and 4mm ep (the budget one). But I haven't come across single ep for maede as mostly are sold in kit.

What do you mean by eye relief for 4mm plossl is about 3mm?

Do you know which one should I buy for barlow lens? I opt for seben since it's affordable but others can be considered too. As long as it's not too expensive. :D

Regarding your question,I just bought my dobsonian around 2 months ago. So i'm not sure whether it's columated well but so far, everything seems to be find when I use it.

Sorry, I'm quite noob regarding astronomical equipments.:sadeyes:

[astro744]

I'm not sure if it is columated but it may be collimated. Sorry, had to correct you both on this one. If no one says anything you'll never know.

I'm ready to be told off by those that do not care about spelling as happened once before but I see there is a thread on the forum at the moment about apochromat missing the 'h' so someone else must care.

The reason I think spelling on a forum is important is because it helps when searching for something. A typo though is another thing and can happen to anyone.

Now back to your question; yes a 2x Barlow is handy but buy a quality one as you will notice a difference.

3mm eye relief means that your eye has to be 3mm from the eyepiece to see the full field. This is quite close and uncomfortable for long periods.

[Renato1]

If you wear eyeglasses, get the Barlow, else I prefer eyepieces. But the problem with a standard 4mm Plossl is the very narrow eyerelief. I got one from Andrews and couldn't get my eye close enough to see the whole image. Eventually, I did - after I removed the rubber eyecup from the eyepiece. Then it became a good eyepiece, and I was happy with it.

Seben have lots of good inexpensive stuff. The 17mm Superwide Angle eyepiece I got for $34 was pretty good, not quite as good as my old Meade 18mm Superwide, but a fraction of what I originally paid for the Meade. Also, Seben's Flat Field eyepieces seem to be the same as everybody else's who sell them, but they are cheaper from Seben.

If you eventually want to hunt faint stuff like galaxies, it's best to get a nice wide angle or ultra wideangle eyepiece that gives a 2mm exit pupil. If your telescope is f/6, then that equates to a 12mm eyepiece.
Regards,
Renato

[edelweiss]

Thank you for the reminder.

A quality one like orion or bintel? I surveyed the price and both are around $39. Another one is Baader but that one is too pricy :eyepop:.

[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

[Renato1]

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

[brian nordstrom]

:D Edna , Renato's help here is spot on as usual , he is such a helpful guy , I applaud his words of wisdom .

Any way Edna , its very hard to describe colimation in words , its better to see it so try a 'Google' and type in ,

'youtube , collimating a newtonion telescope' and enjoy and learn from the 100's of clips posted there , its not hard if you take your time and remember ,, ' little movements ' .

Also there is no better thing than joining an astro society or club , is there one near you ? there are plenty of people that will help , but most of all , use and enjoy what you have and forget about getting more eyepieces , at least until you get more experience .

Brian.

[PCH]

Brian,

Edna is a girl's name - his name is Eeda :)

[brian nordstrom]

:shrug:oops ,,, apologies Eeda .

Brian.