Planetary Neb.: brighter in f/4 than f/4.5 & f/5, alot brighter

mental4astro (Alexander) · #1 07-03-2010, 03:30 PM

Hi all,

Last night, Hickny and I had a session at Randwick (inner Sydney) were we chased down a three PNs.:NGC 2438, 2818 and 2467.

We had three scopes at our disposal, all reflectors, one open truss: 13.1" f/4.5, 8" f/4 and 10" f/5 (open truss). We observed each neb in turn with the same EP with a Lumicon Neb. Filter in each scope and found that the smallest scope, the 8" f/4, gave the brightest image!

Why could this be so?

The image in the other two scopes was fairly much the same: faint but discernable with a litte giggle of the scope. The 8" required no 'giggleing' to see.

Mental.

Wavytone · #2 07-03-2010, 04:00 PM

Why ?

The lower the focal ratio, the brighter an extended object will be. As will the sky background. This is not true for stars (point objects), where its aperture that counts. So yes f/4 would probably be discernably better than f/5, all other things being equal.

But there are a lot of other factors - the condition of the mirror and things that scatter light or affect the background (open vs closed tube) would all come into play - even the effect of having a white tube vs a black one, on your night vision.

Sounds like the filter you were using was a good match for the light pollution there.

Starkler (Geoff) · #3 07-03-2010, 04:31 PM

Brightness per unit area of any extended object is defined by the exit pupil size.

Exit pupil = aperture /magnification. The effect is that for a constant surface brightness, a bigger aperture scope will show a larger image of the same brightness.

Its like in photography f-ratio is the determinant of brightness. A bigger scope will show a bigger image for the same brightness.

mental4astro (Alexander) · #4 07-03-2010, 05:38 PM

What intrigues me is that the 13.1" scope is about 3 times larger in light gathering capacity of the 8", yet the 8's f/4 ratio killed its bigger brother hands down! Amazing, Wavy, how the ratio works on extended objects.

Also, we struggled to see anything of the Sombrero in the 13.1" last night- the moon was hard on its heels. Yet again the 8" f/4 prevailed! Only half a stop difference between the two.

Oh, and the 28mm RKE I got from you Wavy is such a sweet match for the f/4 (actually, Peter and I call her 'F4'). NO coma noticeable in it. Yet the 15mm 68* EP I have, the coma is very, very noticeable, almost useless. I love it!

Used it with the filter on Eta Carina, WOW. And from Sydney. Can't wait to take her to a dark site.

orestis · #5 07-03-2010, 07:14 PM

hi guys i was looking for a 8 '' dob or a 10'' dob.i'd like to buy the 8'' if its better because its cheaper .which scope is better for deep sky objects i'm confused..

.

thanks in advance.
orestis

mental4astro (Alexander) · #6 07-03-2010, 08:22 PM

Hi orestis,

You have to carefully read what is being refered to in this thread. It is related to the technicalities of the focal ratio, not the actual size of the scopes.

If I had both the 8"f/4 and the 13.1" f/4.5 at the same magnification, the 13.1" 's image will be brighter, hands down. It is just a relationship between the focal ratio and the focal length of the eyepiece.

If the same eyepiece is used in two scopes of the same size, but one has a longer focal length than the other, the image in the shorter focal length scope will 'appear' brighter, but in turn the object will be smaller in size (less magnification). If both scopes are at the same magnification, the image in both scopes will be just as bright.

In your case, the rule in astro is 'get as big a scope as you can afford'. That means, the 10" dob.

Wavy & Starkler just helped me understand this little quirk of optical properties.

Ta gents,

.

Orestis, if you can, get to a star party of a local club or one of those listed in the 'star parties' forum, and see what different scopes offer performance wise. If Katoomba is not too far for you (or your mum or dad), and weather permitting, I'll be at the Katoomba Airfield this coming Saturday night. The same three scopes mentioned below will be there, plus what ever others that come are bringing. Bring your scope and binos too, and a parent. We'll get your 130 humming too.

Wavytone · #7 07-03-2010, 09:33 PM

Alex, The RKE's were designed originally for the Edmund Astroscan which was f/4 or 4.5. I have never seen a test indicating whether they include some compensation for field curvature and coma, but from the way they work in fast Newts my suspicion is that the 21 and 28 must have been designed with a curved field in mind and quite possibly a degree of coma correction.

It makes some sense to do that in telescope eyepieces because in the era they were designed, refractors and cats were very much slower (f10 - f/15) and the only scopes faster than f/7 were Newtonians.

mental4astro (Alexander) · #8 07-03-2010, 09:55 PM

28mm RKE, 25 to 30 year old design, no coma: $20

30mm TMB Paragon, 3 yr old design, coma: $300
+
Coma corrector: $ 250

mmm....

ngcles · #9 07-03-2010, 10:42 PM

Hi Orestis,

As Alexander said, if cost and portability are not huge issues, the 10" wins over the 8" every day of the week. A good big 'scope will always beat a good little 'scope in light-gathering power and with that the ability to "see" increasingly small and increasingly faint objects. The larger 'scope (which will almost always have a longer focal length) only misses out to an extent on extremely large deep sky objects like the Pleiades etc because it just hasn't got the native field-width to take it in all in one go.

Also, make sure you have a read here:

http://www.iceinspace.com.au/forum/s...ighlight=ratio

I normally try very hard not to do hard-sell advertising here for the magazine I write but, I will make an exception on this occasion.

The new edition (April 2010) of Australian Sky & Telescope (in the paper-shops next week) has a superb, detailed article written by Tony Flanders on choosing a telescope/binoculars for beginners. It is near the back of the magazine.

Honestly, it is the best I've seen for a long, long time if not ever on the topic -- mandatory reading for first-time telescope buyers.

Best,

Les D

Dave47tuc (David) · #10 08-03-2010, 11:20 AM

Have a read of this thread, people have asked the same ? many times.
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1

My advice buy the scope you will use. Bigger is better but not when you don't use it.

Quote:

Originally Posted by orestis

hi guys i was looking for a 8 '' dob or a 10'' dob.i'd like to buy the 8'' if its better because its cheaper .which scope is better for deep sky objects i'm confused..

.

thanks in advance.
orestis

Satchmo · #11 08-03-2010, 02:42 PM

Quote:

Originally Posted by mental4astro

28mm RKE, 25 to 30 year old design, no coma: $20

I own two of them and the amount of coma /distortion I see is proportional to how dark my enviroment is. With F4 Astroscan scope with ambient light and mediochre sky , the edge looks great as my 4 mm or so my eye pupil is stopping down the 7mm pupil of the F4 scope to an effective F7 scope . The edge doesn't look nearly as good to my fully adapted eye under a good dark sky with full pupil used.

tnott · #12 15-03-2010, 10:17 PM

Quote:

Originally Posted by mental4astro

28mm RKE, 25 to 30 year old design, no coma: $20

30mm TMB Paragon, 3 yr old design, coma: $300
+
Coma corrector: $ 250

mmm....

Many (most) eyepieces do not handle the steep cone of light form a fast scope and show astigmatism at the edges (seagulls). A coma corrector will not help here. With a cheap widefield you won't even see the coma for the astigmatism!

As far as I know, no eyepiece will help with coma. Coma is related to the parabola-ised mirror - not the eyepiece - and will show itself as comet-like stars with the tails pointed away from the center of the FOV.

So a well-corrected eyepiece (like Naglers) will avoid astigmatism and a coma corrector will help with the coma.

mental4astro (Alexander) · #13 17-03-2010, 12:53 AM

Quote:

Originally Posted by tnott

Many (most) eyepieces do not handle the steep cone of light form a fast scope and show astigmatism at the edges (seagulls). A coma corrector will not help here. With a cheap widefield you won't even see the coma for the astigmatism!

It is 'seagulls' that I do see, come to think of it. Yet the RKE shows so little astigmatism! I'll have to recheck at another time for what distortion it does show. It really does perform very well in this f/4 scope. I love it.