The Aperture Ailment

Jack Kramer

You've read observing reports or equipment ads and start to feel as though you're stuck in near space ... sort of closed in. It's the deep space experience that is beckoning you, and it's saying that your present telescope isn't quite up to the task. You crave more light grasp. It's an affliction called aperture fever. The AMA has yet to find a cure.

Before you succumb, there are two issues that need to be resolved: How large a telescope will satisfy you? Are you willing and able to go through the added effort required to use it?

The second issue is based purely on factors unique to you - such things as the size of your vehicle, available storage space, and perhaps most important of all, your physical condition. So we'll table that question and instead look at the philosophy of increasing light grasp.

Aperture fever is sometimes equated with "twoinchitis" - a belief that if you had a telescope just two inches larger, that would be all the light grasp you'd ever need. Of course, it's a death spiral. After you've had that bigger scope awhile, you start thinking ... well, twoinchitis all over again. Putting aside the matter of the instrument's bulk, how much is necessary to net a significant increase in light grasp? Notice I didn't say a meaningful increase. Any increase in light grasp and resolution may be meaningful, but will it be significant enough to warrant the cost and added effort involved in a new scope?

Many experts contend that to get a significant increase, you need roughly a doubling of light grasp. A common way to compare light grasp is to divide the aperture squared of the larger scope by the aperture squared of the smaller scope. That rule says that going from an 8" scope to a 10" will net you 56% more light grasp. But to see a significant improvement, you'd need to go to a 12"; that would give you a 125% boost. This is sometimes referred-to as the "two sizes rule" - skip the next size up and go at least to the one beyond that. The strict logic of this rule breaks down, however, once you get into the realm of light buckets where successive percentage increments in light grasp become smaller.

Others suggest ignoring telescope size per se and instead aim to go up by one full magnitude. This will open a vast expanse of the universe to you. Let's continue our example of starting with an 8" scope. Depending on whose figures you use, the 8" has a magnitude limit of 13.3; with its limit of about 14.3, a 121/2" would net you that full magnitude. But getting to the next full magnitude step beyond that requires going to a 20", with its 15.3 magnitude limit. However, I don't put a lot of stock in most of the published magnitude limits. Under good observing conditions, I've often pushed my scopes well beyond their stated limits.

A more novel approach was followed by Christian Luginbuhl and Brian Skiff in their Observing Handbook. They claim that a 6" telescope will show a thousand galaxies, a 10" will show 10,000 galaxies, a 12" will show 20,000 and so on, exponentially. I don't know exactly how they derived those figures, but that's more galaxies than most of us could ever hope to observe! This also in a way confirms what some have said: a 6" telescope is as much as many observers will ever need.

So is there any sensible criterion? About twenty years ago I went from an 8" to a 10", which by most accounts wouldn't be classed as significant. Yet I could see an immediate improvement in the appearance of deep sky objects, and for me it was meaningful. Then too, other considerations may enter the picture, such as a change in telescope design or upgrade in quality, and these can wield more influence than the size of the optics. Finally, there's that nagging issue of how easy it will be to handle and transport the new telescope.

Perhaps there is no real logic to aperture fever. We just want a better view of what's out there. A more apt term might be "telescope fever", wherein a certain instrument captures our fancy for its perceived benefits, regardless of size. Maybe it's even smaller than what we currently have. It could be the reason why some people collect all kinds of different telescopes. They just like the darned things, and they revel in whatever part of the universe any good telescope can lay out before us. Ultimately, maybe Yoda was right: "Size matters not".

Published in the August 2005 issue of the NightTimes