Which of these two for planets...

[Steve]

one thing I don't understand is this. my experience with newts seems to confirm that a longer tube and slower focal ratio equates to a smaller secondary and that it is largely this latter point that generates the increase in contrast?

We are generalizing but, yes.

there also seems to be more 'depth of field' (i.e. more range of focus where the image is sharp).

Yes, the longer/slower telescope will have increased depth of focus.

with a faster newt, the contrast is reduced I think due to the larger secondary and possibly more scattered light due to the shorter tube? the range of sharp focus is also much smaller.

Yes, but the reduction in contrast is not as much as some might have you believe. The difference is subtle. (Light scatter has more to do with the mirror's surface accuracy and finish).

what I don't understand is how a scope like a Mak or SCT with a long focal length and large secondary obstruction can have better contrast/detail than a slow newt of the same aperture?

The Maksutov will have a smaller secondary obstruction, also see my previous post about Maksutov optics.

HTH

[Steve]

Would a SW ED100 beat the 150 Mak on lunar and planets?

I e-mailed a respected retailer and they said that it would.

That isn't true.

The Skywatcher ED100 will give very nice views of planets and be more convenient (it will also accept an optically matched 0.85x reducer for a faster, wider field) but the 150mm Maksutov's increased focal length and aperture offers greater light gathering and higher definition.

HTH

[Ags]

I am not looking to buy anything at this moment, to clarify my intentions a bit... I am trying to understand the optical differences. Obviously in the medium term I will buy something, and I suppose my problem is I am trying to cross the chasm between my basic equipment and a nice setup in small steps... :-)

Maks may have a small obstruction compared to an SCT, but they have large COs compared to even a fast Newt. The PDS is simply an extreme example, where the scope is both fast and it has an oversized CO for AP.

I think the answer has to boil down to the quality of the optical surfaces, as well as control of spherical aberation. But if the Pro series Maks are really 1/6PV or better, why doesn't SkyWatcher trumpet this fact like any other supplier would?

One good thing about Maks - here in Holland they come up regularly for sale second hand at dump prices.

[Nexus 6]

One good thing about Maks - here in Holland they come up regularly for sale second hand at dump prices.

that looks worth checking Ags, do you mind if I ask where you look?

[Steve]

... if the Pro series Maks are really 1/6PV or better, why doesn't SkyWatcher trumpet this fact like any other supplier would?

Skywatcher don't offer figures for any of their telescopes. (They do quote maximum payloads for their mounts but I think it wise to take those with a pinch of salt).

One good thing about Maks - here in Holland they come up regularly for sale second hand at dump prices.

You can quickly and easily check their collimation by looking down the front of the tube from a distance of around 5-10 feet. Stand at whatever distance enables you to see a series of rings. If they look concentric the collimation is probably okay. It is only a rough-and-ready test but better than nothing.

HTH

[Ags]

You can quickly and easily check their collimation by looking down the front of the tube from a distance of around 5-10 feet. Stand at whatever distance enables you to see a series of rings. If they look concentric the collimation is probably okay. It is only a rough-and-ready test but better than nothing.

HTH

Thanks for the tip! I just tried it on my little Mak.

[spaceboy]

You can collimate MAKs easy enough anyway can't you ????

[great_bear]

Not all models unfortunately.

[Mr Spock]

The other thing to consider is where are you keeping the scope. If it is indoors the Mak will have a horrendous cooldown time; you may be able to use a Newt in an hour, but the Mak could take 3-4 hours - or never if the outside temperature is dropping.

A Mak is best kept in a cold place

[great_bear]

My old trick of putting the Mak in an empty bathtub with the bathroom window open in late afternoon to early evening, is an excellent shortcut to more usable Mak cooldown times.

Now I'm in a new house of course, it lives in the shed inbetween sessions.

[Ags]

I was just looking at the Orion Optics 1/10 PV Newts today. If the difference between the newt and the Mak comes down to the optical surfaces, how would a 1/10 PV OO F5 Newt compare to a Mak of similar aperture? Bearing in mind the OO Newt has a smaller CO, even at F5...

[johninderby]

There's an Orion Optics UK 1/12 PV 150mm f/11 newt OTA on Astrobuysell at the moment at a very good price considering the high spec.

http://www.astrobuysell.com/uk/propview.php?view=48192

It's a real specialist planetary scope and would be even better than a 180 Mak for planetary observing.

John

[Ags]

"Yes, but the reduction in contrast is not as much as some might have you believe. The difference is subtle. (Light scatter has more to do with the mirror's surface accuracy and finish). "

So I am thinking the fast newts can win if they have high spec mirrors...

[Alkaid]

It's a real specialist planetary scope and would be even better than a 180 Mak for planetary observing.

John

Is that because of the higher grade optics? Would the extra apeture of the 180 not matter here?

[johninderby]

Is that because of the higher grade optics? Would the extra apeture of the 180 not matter here?

The OO newt has a much smaller central obstruction compared to the Skymax 180 so that makes up for a lot of the difference in aperture. And yes the better optics of the newt also make a difference.

The Skymax 180 is a very good planetary scope indeed, but the OO is a hand built custom scope with great optics. Then there's the cooldown time.The OO newt cools down in minutes whereas the Skymax 180 really needs to be kept somewhere cold as otherwise it can take hours to cool down.

John

[michael.h.f.wilkinson]

One problem with fast Newtonians is their high sensitivity to miscollimation. Furthermore, though theoretically the image is perfect at the centre, it degrades quite rapidly off centre. A slow Newtonian is far more tolerant, has a smaller central obstruction, and requires less of the optical quality of the eyepieces. A fast Newtonian requires more, and will need to be barlowed far more if you want to do planetary photography. This reduces image quality. One 2.5x or 3x barlow in an F/8 will get you an almost perfect match to the pixel size of ccds for planetay work. An F/5 requires a 4 or 5x barlow (or two stacked).

A few points about resolution versus contrast: aperture affects resolutions MUCH more than does the central obstruction. Central obstruction mainly affects contrast, and less than most people think. Resolution-wise the 180 Mak beats a 150 Newtonian. Contrast-wise the slow Newtonian will have the edge. All this assumes similar quality optics. I had a 6" F/8 newt with 1/12th wave mirrors (or so they claimed). I later got my C8 (old black tube version). The C8 shows more detail on Jupiter, but the 6" had the better contrast. In the C8, like the Mak 180, you need to get used to the slightly lower contrast to tease the extra detail out, but it is definitely there.

Ultimately, for a given aperture, the quality of the optical surfaces has more impact on performance than the exact design.

[Ags]

Ultimately, for a given aperture, the quality of the optical surfaces has more impact on performance than the exact design.

Too true!

...TS are trying to sell me a newt with an OO 1/10PV mirror, steeltrack focusser and carbon tube...:-) I'm sure it would be lovely, but it would cost more than a Mak 150 and a 150PDS together!