Field view, mac vs newt

[tom33pr]

Folks,

A quick question...

What's the difference in the field view for 5" mac and newtonian. I don't expect anything dramatic, don't get me wrong.

Put it this way, can you fit M45 (for example) in one view using a large field ep like Hyperion 17? (I remember I was getting pretty impressive DSOs with my first SW130p + 17 BH), ...before I switched to a 10" DOB ...and then - long story short - my kids arrived

Kind of thinking of getting a small scope for quick back garden sessions, and while I had a go at 8" mac (great scope) number of times, I never really used the 5"...

[Freddie]

Try this

http://www.12dstring.me.uk/fov.htm

[tom33pr]

Great. Thanks.

[Moonshane]

all other things being equal, field of view (i.e. true field of view) is simply governed by the focal length of the scope, not the design. any scope with a 1500mm focal length will give the same field of view with the same eyepiece. personally I accept that if I want a 16" aperture then ignoring anything below f4 focal ratio, I have to put up with a maximum field of view with a 32mm plossl of 1600 / 32 = 50x and 50d / 50x = 1 degree of field

1600mm being focal length

32 being the mm of the eyepiece

50x being the magnification

50d being the apparent field of the eyepiece

if you want more field you have to reduce aperture and retain focal ratio

so a 6" newtonian with a focal length of 1600mm will give half the field of a 6" scope with an 800mm focal length

a 16" scope with a focal length of 1600mm will give the same field as a 6" scope with a 1600mm focal length

[tom33pr]

all other things being equal, field of view (i.e. true field of view) is simply governed by the focal length of the scope, not the design....

Totally agree, but then considering macs have generally way higher FL than newts (with the same aperture) we surely can say the field view depends on design

...anyway, I'm just wondering what can you get out of a 5" mac with a low mag EP and a large FL as far as DSOs are concerned (comparing to a 5" newt).

I'm thinking of getting a small mac and while I realise they are good job for planetary observing, I wonder whether or not I am to be disappointed with DSOs.

[Moonshane]

no it doesn't. it's possible to make a scope of any focal length within reason, you just have to create a different depth of curve on the mirror or a different shape in the refractor lens. there are plenty of f15 refractors about, I have a f11 6" newt and have seen f12 newts too. field of view is determined by focal length no matter what design.

anyway, assuming the Mak has a f15 ratio (is that correct?) and therefore a focal length of : aperture 127 (?) x 15 = 1905mm and assuming a 1.25" focuser, you could get 0.84 degrees with a 32mm plossl. anything this size or less would be in view (if the aperture and sky conditions allow it to be seen).

for a 130mm f5 newt with a focal length of 650mm, you'd have the same brightness of image and a field about three times as wide.

[RichieP]

With a five inch Mak and the right eyepiece you can still get a degree of fov. Enough for most DSOs.

[John]

There are mak-cassegrains which tend to have slow focal ratios (eg: F/11 or slower) and mak-newtonians which are much faster (eg: F/5.9 or faster). So it depends which "mak" we are talking about

A mak-cass 127mm is going to have a max true field of around 1.1 degrees when used with 1.25" eyepieces. Many DSO's will fit in a 1 degree field of view of course.

[Moonshane]

ps

if the DSO fits in the field there would be no reason you'd be disappointed, the view would be the same (roughly) as in the newt at the same magnification.

[tom33pr]

...all makes sense, thanks.

ps (I meant the design of macs vs newts generally).

[Moonshane]

I suppose if talking standard products then on average it does appear that design affects field of view but it's just an illusion

I should add that I know quite a few people buy the small maks as travel scopes and are very happy with them. all I would say is that planets, double stars and moon are easy at home. if travelling then a wider field and more aperture might be better but I see the attraction of 'small'. make sure you have a mount that can handle the focal length though.

[tom33pr]

Thanks.

I know the mount may be challenging; however, I have a few ideas on how to address this, sure.

I simply need 'small' and quick.

I and my two boys (1 and 1.5 yo) got together today where i was told the following:

- Dad, we all know you don't have time and energy for any fancy setups and BIG DOBS, so go small... and remember... if you stay observing too long we will be catching up with you first thing in the morning ...

[Moonshane]

ha ha cute.

beware that Maks can take a long time to cool to the point where they are usable if taken from a warm house to a cold garden.

[ollypenrice]

With a five inch Mak and the right eyepiece you can still get a degree of fov. Enough for most DSOs.

Not true, I don't think. It is enough for most DSOs with a size of a degree on the sky! What happens is that the general definition of DSOs becomes 'DSOs that will fit in your average amateur scope with a degree field.' However there are catalogues full of DSOs that are four, five six and more degrees across but they don't get included in most peoples' lists because they won't fit in the eyepiece. A circular argument. There are lots of large Sharpless objects. There's the Veil complex. Barnard's Loop. The Meissa Nebulosity. The Pleiades. Large clusters... The list is endless. A degree is not enough to view literally hundreds of catalogued DSOs.

For all its many virtues the Maksutov does have a slow focal ratio so its field is relatively limited compared with faster systems of the same aperture. I'm not saying don't buy one, just saying that there are very extended objects out there and that there is no magic cut-off at a degree. For me small telescopes do widefield well so I'd always like a small scope to have a short FL precisely because there are so many large objects. Or bins, of course, but they lack aperture. The entire Veil in a 4 inch F5 apo is quite a sight.

Olly

[earth titan]

For me small telescopes do widefield well so I'd always like a small scope to have a short FL precisely because there are so many large objects. Or bins, of course, but they lack aperture. The entire Veil in a 4 inch F5 apo is quite a sight.

Olly

Totally agree.

I have a C9.25 mounted in the obsy and for 'out in the field' work. Recently used this with 25mm and 15mm plossls for great views of M51 at SGL8 - but it's totally useless for the Pleides (even with a 40mm Aero). For bigger clusters, DSOs etc. the 80mm refractor is fabulous with the 40mm Aero. Both of these are DSOs but need different scopes to get the best. Hence the 80mm is also set up in the obsy - not just for imaging.

If only I could mount a big newt as well .

Not sure i would use a Mak as an all rounder though. Big central obstruction would cut down the light capture too much for me! Brilliant for planets though...

Not a lot of chance of seeing the veil though from my location, no matter what scope - worst luck.

[earth titan]

Just to give an idea, as I'm waiting for a client to go into a meeting, the following FOV by comparison using the eyepieces I use and the scopes listed previously (I've added in a 180 Mak for comparison):

25mm 1.25" TV Plossl

180 Mak 0.46 degrees

C9.25 0.5 degrees

ED80 2.6 degrees

40mm 2" Skywatcher Aero

180 Mak 1.01 degrees

C9.25 1.16 degrees

ED80 5.67 degrees

When you consider Pliedes is over 2 degrees in size, you can see the effect it has. Coupled with the large central obstruction of the Mac, the images will be dimmer.

Maks are also designed for bright objects and therefore DSOs are not always the best.

[SkyGibbon]

I suppose if talking standard products then on average it does appear that design affects field of view but it's just an illusion

I should add that I know quite a few people buy the small maks as travel scopes and are very happy with them. all I would say is that planets, double stars and moon are easy at home. if travelling then a wider field and more aperture might be better but I see the attraction of 'small'. make sure you have a mount that can handle the focal length though.

I am one of those smaller mak owners for travel. Have a 90mm. I also bought it for terrestrial viewing and my dedicated solar scope. It also does decently binoviewing sun, moon, planets. My newts always require a 1.5x - 2x barlow to get viewing correctly, unfortunately at a reduced field. DSOs will do appear decently in the mak, with a field reducer even better.

[tom33pr]

...so back to my initial question then... are you be able to fit M45 in 5" mac (FL1500, FR11 ) + Baader 17 (68' AFOV)?

[Moonshane]

1500 / 17 = 88x then 68 / 88 = 0.78 degrees so unfortunately not.

[John]

I think you need a true field of around 1.5 degrees to fit M45 comfortably into it. The 5" mak would give around .77 of a true degree, so not enough. A 32mm plossl would give 1.1 degrees so nearly, but not quite, enough.

[tom33pr]

Yea, I knew I was too adventurous with the 17... the BH 24 (with 68 AFOV) will give just over 1 degree (according to your formula)... just about to fit it... and it's going to be tiny...

Not that I am particularly bothered with M45... but if 5" mac can't manage it then I think I'll have to revise my plans.

The problem is I really need something small and quick and OTOH I love DSOs

[Moonshane]

unfortunately, every scope you buy is a compromise in one form or another. makes it more challenging and rewarding in the long run though.

[Moonshane]

maybe come at it from another direction: based on the eyepieces you have, what field of view do you want to satisfy your needs. if we take 1.5 degrees as an ideal and you have a 24mm eyepiece with a 68 degree field then you need a scope with a magnification of 68/1.5 = 45x which with your 24mm eyepiece equates to 45x24 = 1080mm or less (say 1000mm). therefore you need a scope with a focal length of 1000mm and the bigger the aperture, the faster the scope.

options might be a 6" f5 newt (750mm), a 100mm f10 frac, a 90mm f10 Mak, a 5" f5 newt (650mm). as I said, compromise is the key.

[Naemeth]

maybe come at it from another direction: based on the eyepieces you have, what field of view do you want to satisfy your needs. if we take 1.5 degrees as an ideal and you have a 24mm eyepiece with a 68 degree field then you need a scope with a magnification of 68/1.5 = 45x which with your 24mm eyepiece equates to 45x24 = 1080mm or less (say 1000mm). therefore you need a scope with a focal length of 1000mm and the bigger the aperture, the faster the scope.

options might be a 6" f5 newt (750mm), a 100mm f10 frac, a 90mm f10 Mak, a 5" f5 newt (650mm). as I said, compromise is the key.

Brilliant option Shane, I personally hadn't thought of it that way, but it makes perfect sense .

[Moonshane]

one thing I have found is that it's sometimes cheaper to buy a different/further scope to cover a certain type of observing than buy 1-2 eyepieces to achieve the same end. at least that how I justify it to myself!