Obsession 15 f4.5 vs. Obsession 20 f5

Two plus years ago, I bought my first serious scope, an Orion XT 10i 10” dob, a very good starter scope. A year later I had the chance to buy an Obsession 15 from a friend. A few months ago, I made the upgrade to an Obsession 20 (via the same friend).

There seem to be two rules for how big a scope to buy. One, aperture rules, as many have said. It’s certainly true. And two, don‘t buy a scope that is bigger than you can conveniently use. A big scope that does not get out at night because it is too much hassle to move, or set it up, is not a good choice, no matter how beautiful the views might be if only it were used.

The answer to the second rule really depends on your circumstances.  How big or how fit are you, and how fit will you (or I) be in 10 or 20 years. (OK, let's not go there.) How big a car you have, the place you have for storage, etc.

The Obsessions in general are very portable scopes, partly because of the included wheelbarrow handles. The Obsession 15 is a particularly portable with only 9 pounds of weight to lift at the wheelbarrow handles. With my 15, I could easily view objects at the zenith while standing on the ground (I  ‘m 6  ‘6╙), but my wife required a stool to do the same. The 15 was easily stored and I could wheel it in and out a sliding glass door fully assembled. If I wanted to do mirror cleaning, stick it in the car, add accessories, or whatever, I could pick up the mirror box by myself and put it where it needed to go.  It is the ideal scope if you are on your own.

I loved the 15, however, I was given a chance to upgrade to an Obsession 20, and was able to set the two scopes up side be side as part of the decision making process. The views with the 20 were better, no question about it. It was simply easier to see details, as the images were both brighter and seemed “larger”.

The 20 is, however a much larger heavier scope, with 25 pounds of weight on the handles, and a mirror box I would not dream of lifting by myself. With two people, it is not issue, but you will need the help of a second person occasionally with the 20.

To get the 20 mirror/rocker box assembly in and out the same sliding glass door, I needed to make some minor modifications, and getting it into a car now definitely involves ramps and some effort (by just me). With the 20, I need a ladder or tall stool/work platform for most viewing.

I did upgrade from the 15 to the 20 and am very pleased I did. For  me, I am willing and able to deal with the issues caused by the  larger size and greater weight in order to get the better views.  Right now, I have no real desire to go larger, unless, perhaps, I could place the scope in a semi-permanent setup/observatory. You have to decide what you can and will handle depending on your situation.

One thing I will add is that I added an Argo Navis and ServoCAT to both 15 and 20 inch scopes and can  ‘t emphasize enough how much these two well integrated accessories add to the ease-of-use and overall pleasure of using a Dobsonian scope. (I used both scopes with and without.)

Especially, if you are new to astronomy, leaning to star-hop can be a bit frustrating, although Telrads and Telrad charts really help. If you have the time, and you may well in retirement, leaning the sky the “hard way” is probably the right way to do things. On the other hand, one can lose interest if you are not patient and committed.  Having the computer (AN) guide the scope to the various objects really allows instant gratification and allows the viewing of many more objects per session.

Having objects the drift across the eyepiece, as is the case with an unguided scope, means that you will spend a fair amount of your concentration pushing the scope to keep the object in the field of view. The addition of the ServoCAT really means that you can concentrate on the details of the object rather than pushing the scope. Without the SC, if you are trying to show objects to someone else, the object may well have drifted outside of the FOV before they can view it, especially at higher magnifications. On the other hand, with the SC objects will stay right where they belong for extended periods of time.

It is a tough decision, but I think I would rather have a 15 with a SC and AN than a 20 without. Fortunately, you can start without either and add the AN, and then the SC, or just go for it and get both. You will also need to budget, perhaps $1000 for eyepieces and other accessories, so keep that in mind as well when you make your choice. I am sure you will get considerable enjoyment out of whatever Obsession you choose. They are really lovely scopes.