Planetary scope

[marxy]

Hey all.

I have found that I spend a lot of time on planets since I live in Manchester and it gets quite Light polluted where I live.

I used to own an Antares 127/f9.4 refractor on an unpowered eq5 mount.

I sold it because I found the scope too unwieldy.

I am now looking for a new scope. I love the planets, and don't want a dob really since I want to do some webcam astrophotography.

Does anyone have good suggestions?

I don't want a massivly long achro - since that was the reason i got rid of the antares.

Maybe a SCT is the best option for me?

Budget is roughly £750

Thanks a million,

Jay

[GazOC]

I'd go for a ED refractor or a Mak/Cass, the Mak would be better for webcamming.

[Kaptain Klevtsov]

The Skywatcher MAKs come in various sizes up to 180mm within budget, the TAL200K MAK is within budget - I'll let you know how it works when mine arrives and a second hand OMC140 is within the price range. That's assuming the budget is OTA only.

A 150mm Mak. on an HEQ5 mount would be in budget if you need the mount also.

http://www.firstlightoptics.com/proddetail.php?prod=promak150ota

Captain Chaos

[geppetto]

Have you already got a mount?

[Macavity]

Perhaps you might consider a MAK(sutov). At higher F numbers (f>1500) also anecdotally (practically!) suitable for the planetary observation. Of inherently small-ish FOV, but no more so than any scope of long focal length - And all compressed into a short OTA. Often capable of taking a 2" diagonal / eyepieces and giving >1 deg FOV ...

For that budget, perhaps a 140mm OTA: http://www.orionoptics.co.uk/acatalog/OMC140GPE.html

[marxy]

Thanks for all these replies.

I don't have a mount at the moment.

I was thinking of maybe a heq-5 (non goto), or the noiseier, but goto enabled cg-5.

I've seen the skywatcher 8" reflector on heq-5 for £530, but then again, the celestron 8" reflector on cg-5 goto is only gonna be £550 with discount.

Thing is, I'm not sure I want to be constantly fiddling with the collimation - unless someone knows where to get catseye tools from in the uk?

Maybe something like the c6-s from FLO?

[Whippy]

I wouldn't worry too much about collimation, it isn't rocket science. Steve from FLO brought a laser collimator with him to the star party and collimated 2 or 3 scopes in about 10 minutes. Purely anecdotal I know but the larger the newt, the more it has to be collimated. My old 6" got collimated once in 4 years! While my old 10" had to be done almost every time I went near it. My 8" I have currently holds it's collimation pretty well. I've only had to do it once since I've had it and it's been in the back of a car to Wales and back.

Saying all that, my understanding is that if you want a planet-killer, go for a refractor! But I'll leave that to people with better opinions than me...

[GazOC]

Gregorian Maks (ie the cheap ones) hold collimation very well, year to year practically.

[marxy]

Would a mak be better than the celestron c6-s?

on cg-5 goto btw

Also, do these type of scopes show CA?

Mak?

Sct?

[GordonCopestake]

Maks tend to have smaller central obstructions than SCT's of the same aperture making them a little sharper. For planetary viewing though the most important thing is aperture to get the resolution up for those small details. The best images i've seen have been taken with a C14 but a 6" Mak will probably do very well

[GazOC]

As Gordon says for imaging aperture really counts and inch for inch SCTs are cheaper than Maks but for visual Maks give far better planetary views than SCTs even allowing for the smaller aperture. I'd take a 7" Mak over a 8" SCT for purely visual planetary views......

<puts tin hat on>

[GordonCopestake]

I must admit I would like to see a direct comparison between a C6S and a MAK150

[marxy]

Does anyone have a price for a setup for a mak?

I kow that FLO has the c6-s goto for roughly£750 (minus the 10% discount etc)

Thanks

[Kaptain Klevtsov]

Check the link in my previous post, 150 Mak. on a HEQ5.

Captain Chaos

[tom.yates]

But they do say the Celestron 9.25 is very good and i must say that i never thought a sct could be so good but since i bought this scope i seem to be using it more and more

[KaStern_Former_Member]

Hello Marxy,

first of all:

Do you know the different types of telescope,

their typical behaviour under nightsky conditions,

the collimation perfection required for excellent performance,

wich eyepieces are suitable for faqst f/ratios ?

There are so many points that one could write a book about...

First of all I would say that 5" is minimum aperture for observing the planets when they

are your main objects (my personal opinion, I own an 8"f/6 ATM Dob and 6"f/12 ATM Yolo).

You can observe planets with smaller apertures than 5" but I think that the resolution is too

limited and you will soon want to have a bigger aperture scope.

Besides the resolution the modulation transfer function of the scope is very important.

The MTF depends on the aperture and on how much an how strong contrast lowering

factors are present.

Decollimation is one of the most important factors, but it can be solved.

Seeing is another big factor, but cannot be solved, you have to wait for good seeing conditions.

Tube seeing is the third big factor and all open tube telescopes with a mirror (= heating plate)

at the bottom suffer from it, and closed tubes with a mirror at the bottom too.

Colour aberration is a factor with achromats. It can be more or less solved by making

the focal length very long, for example 5"f/20. Then there is only a little purple fringing left.

But too expensive for your budget, and they need a very stiff mount (= expensive) too.

Modern 2-lens ED refractors does not need to be so long.

But they are too expensive for your budget.

Reflectors without a refracting element do not suffer from colour aberration.

The simplest one is the newt and it is the cheapest too.

It needs good collimation because the diffraction limited field is narrow

(how narrow depends mainly on the focal ratio).

In all types of reflectors besides the TCTs there is a central obstruction.

This leeds to a lowered MTF curve (in the middle parts of the curve,

the MTF for the middle spatial frequences is lowered. See the pages of

Thierry Legault, he has some nice diagrams on it.)

How much? It depends on the sice of the central obstruction vs the aperture.

Below 20% linear obstruction the effect is negliable, 25% is no problem,

but more than 30% is not good for a planetary scope.

A 6"f/8 Newt or a even better a 8"f/6 will be fine. Needs a sturdy mount.

The Cassegrain-Type telescopes have a big advantage: They are compact!

Therefore they are suitable with smaller mounts.

And you can sit behind them.

But central obstruction is bigger than with a visul Newt.

The secondary mirror size does NOT tell the whole story,

you must keep in Mind that the baffles increase co.

And I have to oppose to Gordon`s statement:

The co of a Mak and a SC of equal aperture, equal f/ratio

and equal primary mirror f/ratio is equal too.

But in real live the SC has some disadvantages:

- primary mirror is very fast, about f/2 inthe 8" SC telescopes ==> collimation is very critical

- most Mak have slower primarys, about f/3 or so. And the C9,25 too.

- most classical Cassegrain have about f/4 primaries.

- the SC are not fully collimatable by the user.

The primary mirror has to be collimated by the manufacturer.

And focussing by moving the primary mirro causes mirror shift (= decollimation).

In real live I have seen relatively good planetary views with Maks,

despite of their big central obstructions. Most of them were russian Maks

(system Rumak, with separate secondary mirror radius) of 6" and 7" size.

One good thing is that an f/12 or f/15 Mak does not need short focal length

eyepieces to reach high magnifications.

And even Kellner eyepieces work fine at f/12 or slower.

But the russian maks are too expensive for your budget.

The chinese 5" or 6" Mak or 6" SC might fit into your budget.

But will they give better views than your old 5"f/9,4 achromat?

There is no ca like with the achromat, but the big co loweres contrast

on the planetary surfaces too. I do not like ca, it is very bad when you observe Jupiter.

A cheap 6"f/8 Newt will suffer from some problems too:

- they include a spherical Mirror, they can be only slightly below the diffraction limit

- the tube is not blackened good enough inside (= lowered contrast due to straylight)

- the tube is too short (= lowered contrast due to straylight)

- the tube needs a cooling fan to prevent tube seeing

Same is true for the 8"f/6 Newt, but that one has a paraboloid mirror,

wich should be preferred. The 8"f/6 Newt needs to be collimated very well.

All in all there are not very much alternatives when the budget is limited.

I think it would be best for you to meet some other stargazers and try their scopes

until you know what you want to buy. For example tom.yates seems to live in your town too,

so there will be possibilities for you to compare different types of scopes.

Regards,Karsten

I have edited some typing errors....

[marxy]

Well Karsten, I THINK I understand what you're saying

I've had a look at the skymax pro 180 and it's slightly out of my budget

[KaStern_Former_Member]

Hello Marxy,

I enjoy planetary views and globulars

I found the above in another thread from you.

If globulars are what you want to see things get very easy.

You need enough resolution and enough light to resolve more than just M13.

I depends too on where you observe. The darker the sky, the better resolved are the globulars.

In May 2002 (or 2003?) a friend and me made a "globular contest" mith my 200/1200mm Newt and his 114/900mm Newt.

We drove to the region called "Sauerland", wich gave us very dark skies.

He could easily see resolved stars in M13. I do not remember if in other globulars too?!

I could resolve stars in M2/3/4/5/9/10/12/13/14/19/22/28/56/80/92.

But out of the 50.000 inhabitant Town where I live in I can only resolve M13 and M92 (too depending on the skies).

So if globulars are really that important for you get the biggest aperture you can afford. 10" or even 12" dobsonian.

Regards,Karsten