Jump to content

NLCbanner2024.jpg.2478be509670e60c2d6efd04834b8b47.jpg

Big Scope ?

pcardinal42

Recommended Posts

pcardinal42

pcardinal42

Posted
Posted

So I started out with a Nexstar 130 SLT it is a great little scope but I wanted more. I found a Meade 2120 on craigslist that look great so I bought it. I have noticed that when looking at the same objects my field of view on the 2120 is much smaller. Then I found out that when you increase focal length your field of view shrinks. My question is would I get back the greater field of view I have through my 130 SLT by using a wide field say 80+ eyepiece?

Thanks

Link to comment
Share on other sites
sgazer

sgazer

Posted
Posted

hi, had to look up what the Meade was, it appears to be a 10" SCT, nice scope. The SCT has a focal length of 2500mm and the 130SLT has a focal length of 650mm. That means a 10mm eyepeice in the SCT will give 250x magnification and just 65x in the 130SLT. Hence what's why you are seeing less FOV as it's much more magnification. To get the same magnification in the SCT, you would need a 38mm eyepiece!

You can calculate the FOV by dividing the eyepiece FOV by the magnification, so for a typical 60 degree 10mm eyepiece, you'd be getting 0.9 degrees with the 130SLT and 0.24 degress with the SCT. An 80 degree eypiece in the SCT would still only give you 0.32 degrees FOV.

To answer your question, you need a much longer focal length eyepiece with the SCT to give you the same FOV as the 130SLT. A wide field eyepiece would also help, but f/l makes the most difference.

Link to comment
Share on other sites
swamp thing

swamp thing

Posted
Posted

Losing FOV is one of the downsides to gaining aperture I'm afraid. The bigger the scope the more FOV we seem to lose.

But this is also where you gain as well, for as your aperture and focal length grow, so does your image scale, giving smaller fainter objects a huge boost.

So you'lll never see the Pleiades like you can in a small scope with all that black sky framing them. But they'll never show you M13 like your bigger scope can.

Horses for courses as they say. That's why so many amateurs have more than one scope.

A small RFT is a great thing to have in your telescope armoury. But then so is a big scope.

Link to comment
Share on other sites
dharma66

dharma66

Posted
Posted

Hi sgazer.

How is it that an eyepiece with a wider apparent field would give a wider true field? I thought I was starting to understand this eyepiece malarkey, and I thought that AFoV and TFoV were pretty much independent?

My understanding was that AFoV was a characteristic of the eyepiece design, and was fixed regardless of the scope the eyepiece is attached to, and that TFoV was calculated just from scope fl/ ep fl?

How is that an 80 deg eyepiece would give a slightly wider TFoV than a 60 deg eyepiece? What am I missing :confused:

Link to comment
Share on other sites
swamp thing

swamp thing

Posted
Posted

Maybe this will help:

Two 20mm eyepieces one 60deg and the other one 80deg AFOV. The 80deg eyepiece will give a bigger FOV because it is showing more sky.

When you look into either eyepiece the 60deg one shows exactly that and the 80 deg one shows more.

HTH

Link to comment
Share on other sites
dharma66

dharma66

Posted
Posted

HI Steve,

I'm being thick. I thought that the true field of view was a measure of the angle of sky that was visible, and the apparent field of view was the angle subtended at the eye by the extents of the image (the amount by which you have to move your eye to look from one end of the image to the other), and that the two were not directly related. So having a larger AFoV is kind of like having a bigger telly. The picture is the same, but bigger.

I still don't really get how a wider AFoV shows more sky. A 60deg eyepiece isn't showing 60 degrees of sky, after all. It's showing field stop diameter / scope fl * 57.3 degrees of the sky? So a wider AFoV eyepiece with the same field stop, in the same scope, will still show the same TFoV, but the angle through wich the eye has to move to go from one side of the image circle to another, is greater.

This was my understanding, and I don't understand how having a wider AFoV impacts the TFoV, which I understood to be determined by the field stop size. I could well be wrong, as I'm a bit of a newcomer to astronomy, but this is the first time I've heard of a direct link between TFoV and AFoV, and I'd like to understand how sgazer calculated that an 80deg AFoV eyepiece would give 0.32 TFoV, versus 0.24 for a 60deg eyepiece in the above example, as it implies that the maths I've been using to workout what will be going on in my eyepieces has been wrong, or at least incomplete.

Link to comment
Share on other sites
dharma66

dharma66

Posted
Posted

Wow. That's a completely different formula to the one I've seen previously.

So that's saying:

TFoV = AFoV / Mag ?

The formula I've been using (from the Televue website) is:

TF0V = Field Stop / Scope FL x 57.3

These not only give different answers, but completely different relationships.

According to the first, true field depends on the AFoV, and is not dependent on field stop, whereas the second states the exact opposite!

No wonder I'm finding eyepieces so confusing!

Link to comment
Share on other sites
dharma66

dharma66

Posted
Posted

Just found a page (I can't post links yet I don't think). Chuck Hawks Telescope Formulas. He does indeed list both of these methods for determining TFoV.

I guess I just need to chill about it, but it makes me uncomfortable when there are two different methods that differ in essence, not just detail. It set's the (very poor) mathematician in me on edge. I'm going to have to find a book on optics now...

Link to comment
Share on other sites
dharma66

dharma66

Posted
Posted

Ball park maybe, but for example, on a 17.3 mm Delos, the 'usual' formula - AFoV/Mag - gives 1.4 degrees, whereas the Televue formula gives 0.81 degrees, a difference of more than the width of the moon!

Interestingly, for the Nagler 31T5 and 41Panoptic, the answers to both formulae are very similar:

Nag Usual formula: 1.69deg; TV formula: 1.60deg: diff ~5.6%

Pan Usual formula: 1.69deg; TV formula: 1.76deg: diff ~-4% (TV formula gives bigger number)

For the Delos mentioned above, and another couple of eyepieces, the difference is quite large.

Differences expressed as the percentage by which the 'usual' formula is greater than the TV formula:

21E Usual formula: 1.86deg; TV formula: 1.38deg: diff ~35%

17.3D Usual formula: 1.40deg; TV formula: 0.81deg: diff ~73%

10E Usual formula: 1.09deg; TV formula: 0.68deg: diff ~60%

10D Usual formula: 0.67deg; TV formula: 0.49deg: diff ~37%

So, assuming that the TV formula is applicable to TV eyepieces (and one hope it is), this shows that for some TV eyepieces, using the traditional formula could lead to a large overestimate of the amount of sky you would see in the eyepiece.

I suspect what this is showing is the field stop does have a limiting factor on field of view, that is not accounted for in the traditional formula I suspect that the traditional formula is an 'ideal' calculation, but that in reality, the field stop puts a limit onto how much of this is realised. Seems to me like this is a good argument for manufacturers to publish field stop sizes. Going from the usual formula, the 17.3D looks like a great wide field eyepiece at 1.4deg TFoV, whereas actually, the field stop limits it to 0.81deg. For fuzzies, that's surely quite a difference?

Link to comment
Share on other sites
dharma66

dharma66

Posted
Posted

You're absolutely right. Column mixup on my spreadsheet :embarassed:

That makes a huge difference, and makes the variation between the two methods negligible.

That's good. Phew, thanks for putting me straight!

Link to comment
Share on other sites
Moonshane

Moonshane

Posted
Posted

Best way to get more aperture and wider field is to use a fast dob. E.g. the focal length of my 16" scope is 1840mm (with the Paracorr) and with my 26mm Nagler gives me 1.16 degrees of sky in the eyepiece. In your scope at 10" it would give 'only' 0.85 degrees. That said, 0.85 degrees is still more than enough for the vast majority of targets.

Link to comment
Share on other sites
swamp thing

swamp thing

Posted
Posted

Good call.

Small scopes show nice big wide fields and are great for rich star fields, clusters and cruising the milky way.

Your bigger scope will pick out the small faint fuzzies. Great combo.

Enjoy

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.