3 Apo vs. 4 Acro

Just a quick poll. Given equal quality and prices and assuming a mix of deep sky and planetary observing, would you prefer a 3 inch APO or a 4 inch acromat?

Jeff Thomas said:

Just a quick poll. Given equal quality and prices and assuming a mix of deep sky and planetary observing, would you prefer a 3 inch APO or a 4 inch acromat?

If the scopes are of equal quality I would opt for the 4" scope. It will go a little deeper.

Clear skies,
Richard

If the 4" is F15 or slower, I'd go with that, assuming you
are OK with the long tube and have a good mount. If
grab-and-go is important, maybe I'd get the 3" APO.

Jeff,
I have a Stellarvue SV85S doublet apo (f6.9) and a Stellarvue SV102EDT triplet achro (f6.2). Choosing between the two would be tough, since many would call the 102EDT a "semi-apo," but if I had to choose I'd probably go with the 85S on the strength of its better contrast and color performance.

Jim

Jeff Thomas said:

Just a quick poll. Given equal quality and prices and assuming a mix of deep sky and planetary observing, would you prefer a 3 inch APO or a 4 inch acromat?

I think I'd go for a 4" apo.

Seriously, the 4" will gather 77% more light than a 3" - that's significant.

But to get a very nice image from an achromat, you need a relatively long tube, and then portability starts to become a factor.

If I remove the dew shield, I can fit my TMB 115 into carry on luggage. My astronomical society's Unitron 102mm f15 is a little awkward to transport in a typical car, especially when you factor in a suitable mount. (The mount may need to be scaled up to handle the length and weight of a longer tube.)

How you factor image quality, light gathering, and convenience is up to you - but if it was me, I think I'd see if I could find a star party with a good selection of scopes similar to ones I was considering so I could look them over, and compare them head to head to make my best decision.

Good luck.

Mark

I have owned both and would take the 4" achro as F/12 or slower, especially for planets. It will not be a "grab and go" scope especially with an adequate mount. My current grab and go-a TV85. An old Jaegers 4 1/8" F/15 is an all-time favorite too!

Jeff Thomas said:

Just a quick poll. Given equal quality and prices and assuming a mix of deep sky and planetary observing, would you prefer a 3 inch APO or a 4 inch acromat?

To my mind, the big advantage of an APO is the ability to get good correction in a short focal length. A 3" APO is generally short enough to provide nice wide, low power views, yet is able to handle high power planetary viewing and yield about as much as any 3" can show.

You can get a reasonably fast 4" achromat that will also give low power wide field views, and secondary color is not much of an issue at low powers. Things will get pretty nasty for high power planetary viewing, however.

A quality 4" f/15 is going to be a nice planetary scopes, and it will give fine views of deep sky objects, but will be incapable of low power, wide field views.

Properly mounting a 4" f/15 OTA is not trivial, especially compared to a short 3" APO, and even a 4" f/15 shows some color (an f/20 would be essentially color free).

I was able to use a 4" f/15 Clark for a while, and I really enjoyed it. Although I'd love to own such an instrument, I'd consider a 4" f/6 APO a better and more versatile choice.

Clear skies, Ala

Jeff Thomas said:

Just a quick poll. Given equal quality and prices and assuming a mix of deep sky and planetary observing, would you prefer a 3 inch APO or a 4 inch acromat?

What sources have been found for 4-inch achromats in the f/12 to f/20 range - and what is your take on the quality of the lenses?

Being that I own a Takahashi FS 78 and the Astro-Physics 92mm Stowaway and having compared these two scopes with larger sized Achro's 4 inch 5 inch and 6 inch in each case the Achro's suffer from contrast and since they are not figured all that well lack in Resolution as well. Yes they got more light grasp OK but its all smeared across the image its not in focus. When you look through a APO such as a Tak and or an AP you will know what contrast and sharp optics are. Achro's only bring two wavelength's to a common focus the other wavelengths and those around the other two wavelengths are simply smeared across the image. APO''s bring essentially all the wavelength's to the same focus. Since my Takahashi is an FS series doublet it brings all the wavelength's to the same focus except for those wavelength's far into the blue end of the spectrum. This renders an object such as Jupiter slightly yellowish or warmer The AP Stowaway brings everything in the same focus. The result on Jupiter is stark white very simliar to what you would get with a longer focal length Newtonian but without the secondary in the way robbing you of contrast.You can try longer focal length's but even if you had a F90 Achromat you will still never achieve the color correction that an APO such are what an Astro-Physics scope can deliver.

Clear Skies
Dwight L Bogan

Well, I have one of each. I have a Vixen 102mm achromat and a Takahashi 78mm APO.

They are both great. The 4" Vixen goes much deeper than the 3" Takahashi. On M13, for example, it is NO CONTEST. The Vixen resolves many more stars. Similarly, on M42, the Vixen shows much, much more nebular detail.

On Jupiter, however, the Takahashi has an edge. It cannot handle quite as much magnification, but shows finer belt detail. For instance, I watched a beautiful shadow transit a few nights ago in the Takahashi. Inky black obvious shadow clear as crystal in the tiny 3".

I suppose the choice depends on what you are going to be viewing primarily and whether portability is a factor. For DSOs and even general observing, go with a quality 4". For portability and lunar and planetary, the 3" APO would be my choice.