Which Gives The Brighter View of DSOs?

[BazMark]

Do I have this right?

Whilst a 5" f12 scope (127 Mak) will give excellent views of the planets and reasonable views of the more compact DSOs, will it give as bright a view of these DSOs as say, a 5" f5 scope (SW 130P)?

My feeling is that the shorter focal length scope will give the brighter view of DSOs even though the aparture/mirror diameter is the same.

Am I correct?

Does my question even make sense?

[rowan46]

the sw 130p will give the better view of dso's it will have a wider field of view but to be honest if youre looking mainly for dso's you need dark skies and a bigger apparture. I have a 5" sct and can only make out a few of them under urban skies. but to go back to your question i would choose the 130p

[great_bear]

For a specific magnification, the brightness will be identical in both scopes.

Views of the moon, planets, and double stars in the Mak 127 will be substantially better than the 130P

[MikeWilson]

Can I just say - I've got a Mak 127 and a 150P sat next to me right now. I've just been outside with them and I can confirm what great bear is saying. The views of Jupiter through the Mak show cloud patterns whereas through the much bigger 150P you just see shaded lines. Plus the contrast is better on the Mak too.

But navigating across the sky - the 150P is much much better. You have a much wider field of view.

I can totally see what people mean now when they say that different scopes are used for different things. The Mak delivers really good views of lots of things, but not DSOs.

[Rui]

Those are good questions and they do make sense.

I agree with Great_bear. For a given magnification, the views should be similar.

Now, suppose they both come with 2 EP's, lets say a 25mm and a 10mm Plössl.

On the 127Mak, you get 60X and 150X.

On the 130P, you get 26X and 65X.

The fields os view are also proportional to the magnification and so, with a given eyepiece, the view will be narrower.

With those EP's and considering they normally have 50º apparent fields of view, you get:

On the 127Mak a field of 0.83º with the 25mm plossl and 0.33º with the 10mm plossl;

On the 130P a field of 1.9º with the 25mm plossl and 0.77º with the 10mm plossl.

Compare that with an average apparent diameter of 0.5º for the moon...

Theses are just mumbers.

Something to consider also is maintenance. The mak will be more sensitive to dew, but will hold the collimation better, while the newtonian will have the mirror protected from dew but will need to be collimated regularly.

Decisions... decisions...decisions...

[sgazer]

when it comes to brightness, the focal ratio only comes into play when doing prime focus imaging (without an eyepiece).

For visual it's aperture which determines brightness, although other aspects of the type of scope will determine other visual qualities.

[Moonshane]

although not something to worry about unless you are buying a 'specialist' scope, the size of the secondary can also be an important factor. assuming the same aperture, a bigger secondary will give less contrast but reflect more of the light across the whole field, a smaller secondary will give more contrast and may miss some of the light reflected from the primary albeit concentrating on the central portion. most standard newtonians have a 'medium sized' secondary to provide a middle ground and be good all rounders.

eg my 6" f11 dob has a 1" secondary (17% obstruction) and therefore is 'ideal' for planets/lunar/doubles but not DSOs and hence the 12" dob too. I was seeing seriously good, virtually photographic, detail on Jupiter last night with this scope.

[ollypenrice]

Secondary size and contrast come into it, as has been said, but optical quality is key at high magnification. For the best of both worlds - fastish f ratio and unobstructed view - look no further than a five inch premium apochromatic refractor and a life in debt!!!

Olly

[great_bear]

The mak will be more sensitive to dew

People are always saying this, but it's really not an issue. I've got a stonking great Mak 180 Pro, and didn't own a dew shield when I went to Kelling Heath last year. Aware from previous experience that my observing would grind to a halt pretty darn quickly without a dew shield, I scanned the Kelling Heath Gift Shop for something suitable. I ended up purchasing a silver Holographic Foil Gift bag <click>, peeled open the bottom, and - with the help of some Sellotape - turned it into a simple dew shield.

Not only did it work - it worked superbly (and looks attractive if a little wacky...). My Kelling Heath Gift Bag Dew Shield is now an important part of my astronomy kit.

I've beefed it up with more Sellotape as well as a black paper lining, and it now feels like something that will last a lifetime. Furthermore I have found that even under the most extreme dew conditions, there's not a drop of dew on my Mak's lens. No need for proprietary dew shields or troublesome electrical dew solutions - just a shiny gift bag, that's all...

As for collimation, it's never needed on either of my Maks - let alone rarely.

Nope - the only extra consideration that's needed for my Mak 180 is to ensure it is adequately cooled before use. For that, I just leave it caps-off in the bathtub at dinner time with the bathroom window open.

(erm... with no water in the bath, in case anyone was wondering )

There's no reason at all for dew to be an issue with Maks.

[BazMark]

Thanks for the clarification everyone. Much appreciated

[gedmac]

oh dear more confusion:icon_scratch:

[great_bear]

Confusion about what?