Entendue/contrast

[jetstream]

I'm not sure where to put this thread so mods move it if need be, thanks.

Mel Bartels observes very faint, very large objects effectively using some rather unique reflectors, unique in the sense of their f ratio and size. Entendue or light through put in telescopes is an interesting theory which might ruffle the feathers of those who think detectable contrast is only linked to object size (in the eyepiece) and eye illumination (exit pupil).

I have a 200mm f3.8 and can say it does show some things better than other telescopes like the IFN near Vega.

I hope some good discussion will result from posting this link,Gerry

http://www.bbastrodesigns.com/HET.html

Edited August 25, 2019 by jetstream
200mm not 300mm

[John]

40 years in the hobby and thats the 1st time that I've heard of the term etendue

Back to school for me ........

I'm not keen to get an F/4 or faster dob but my 21mm Ethos sure does work well on faint DSO's even with my LET (lowish ettendue telescope) F/5.3 12 inch.

I guess if I was looking for an 18, 20 or larger dob though, it would have to be F/4 to avoid needing a ladder !

Thanks for the link Gerry - I'm sure it will generate much interest and discussion

 

 

 

[Knighty2112]

Yeah, me too. Never heard of of the term in all the time I’ve been doing astronomy either. As I don’t have or plan to own any low f ratio scopes then just looking at the Wikipedia page on it was enough to put me off with all the maths involved on the page! Hehe!  how "spread out" the light is in area and angle. It corresponds to the beam parameter product (BPP) in Gauss

[vlaiv]

I'm not sure if it's the feature of the scope or the way our brain/eye system works.

There is a limit to how low magnification we can go without getting too large exit pupil and turning our eye into aperture stop.

On the other hand, different parts of FOV that we normally have (without use of any optic) have different properties - there is central vision which gives best sharpness, then there is peripheral vision for threshold detection at low light levels (averted vision).

Having light "crammed" into smaller space does increase our ability to detect contrast. If one needs to move eye from one patch with certain brightness to different patch of the image with different brightness - we might not be able to detect contrast change as having both patches into same part of our eye FOV - to see them simultaneously - it's much easier for our brain to see the difference if we have two things side by side.

You can test this by having very close colors but separated so you have to turn your head to look at each - you might not be able to tell the difference - but put them next to each other and you will instantly see that there is in fact difference.

These "HET" scopes enable you similar effect and are hence better for detecting these low contrast features - not because they are somehow better at rendering the contrast - it is just the fact that they compose image in such way that it is easier for our eye/brain combination to see that contrast.

I quite love idea of larger aperture that you can use while still comfortably seated down would not mind having something like 16-18" F/3 scope and appropriate corrector.

[jetstream]

Interestingly enough my 200mm f3.8 will show pink under the wings of M42 and with a dazzling mottled green core all in a small package @ 36-44x. The dust lanes in M31 are also very contrasted against the bright galaxy glow.