Why chose an APO for visual?

[Mr Spock]

It's a difficult choice and there are reasons for choosing each. I had the same choice this year - 120mm apo or 250mm Dob. I went for the Dob, mainly for the extra resolution when conditions allow and for better contrast on the moon.

But wait, don't apos have better contrast? No, they have better MTF. Contrast is the difference between the brightest and darkest parts of an image; MTF is how contrast is applied to detail. This is why a lot of people prefer refractors for planetary observing.

Take a C14 as an example. That 350mm gives it huge amounts of contrast compared to a 120mm apo. However, the large central obstruction gives it a poor MTF curve. Practically the C14 will resolve a lot more detail on Jupiter for example, but the apo will show darker belts and could give the impression of more detail; the comparatively 'washed out' views through the C14 could disappoint, especially in moderate seeing. However, imaging overcomes this and is why you see so many excellent, high resolution planetary images, from the C14.

I'm aware the 250mm I have shows Jupiter with slightly 'faded' belts compared to the 120mm apo I was considering, but, this isn't evident on the moon which is my main target. 

Of course, with unlimited funds, it's wise to have one of each...

...or, with unlimited funds, a 600mm f4 Newt with a sub aperture mask of around 240mm to avoid the central obstruction. Just imagine a 240mm f10 apo and what it is capable of on planets, as well as a 600mm f4 for deep sky.. You can see where my lotto funds are going 😜

[Peter Drew]

For me, eyesight and seeing conditions play a much larger effect than contrast.  I have a 4" Vixen fluorite with a lens made by Canon, the same company that makes the ones for Takahashi.  I don't know whether the Tak ones are made to a higher spec but the Vixen is considered by many users to be on a par in most circumstances.  If a Tak 4" could show me the images reported by those fortunate to have excellent eyesight and conditions I would buy one in a heartbeat, as it is I need at least a 6" to compete.  Some of my best planetary views have been through my Celestron 8SE, having it stationed in Tenerife might possibly make the difference!      🙂

[paulastro]

18 hours ago, Buqibu said:

I know apochromatic refractors are considered the best optical telescopes out there, and I get that they are great for astrophotography because of their lighter weight, lack of diffraction spikes, pin point stars etc... But for visual, why buy a 2000 dollar 4 inch apo when you can get a 14 inch dobsonian for the same amount of money? The dob will have better resolution and light gathering capabilites, so why go for the APO? (we are assuming in this scenario that storage is not an issue). Would love to know and hear some opinions, I'm quite new to this. Thanks in advance

I can't carry a 14 inch dob in one hand on it's mount for grab and go.

Also,  if I had a 14 inch dob the seeing and transparancy I usually have at home would almost never allow it to perform to its capacity 😉

 

[jetstream]

8 hours ago, neil phillips said:

There is a lot to the question of contrast.

👍

8 hours ago, neil phillips said:

And a lot that is also very misleading. 

Big time.

This subject is misunderstood by many IMHO.