How to Get Optimum Performance

By Daniel Mounsey

One of the things I’ve always admired about Bryan Greer is his quest to help observers realize that reflectors CAN produce aesthetic planetary images which are even capable of surpassing some of the largest and finest apochromats money can buy. I strongly believe the failure to recognize this comes from a series of mistakes during set up which I’m going to cover here. I can personally attest to these claims. Having access to some of the largest and finest apochromats in the world today, I have seen it with my own eyes and I’m 100% convinced that Bryan is correct. I am posting this as a check list of issues which should be examined a bit more carefully. Some people may generalize about what the issues are, but this is going to be a step by step procedure you can refer to. You do not need a lot of skill to do this and no one is going to talk over your head.

When I was at RTMC this weekend, I noticed a series of errors in reflective setups which I believe contribute to the publics never ending belief that the refractor is the best way to go and put false blame on central obstructions. Even if you own a small reflector just 6″ or 8″, it can be possible for it to deliver great planetary images if you check just a few things. It’s really not that hard. If you own a Hardin, Discovery, Obsession, Starmaster, Orion, Celestron, etc. you can use this procedure. During a particular observing run, there were numerous scopes pointed at Saturn, yet I was surprised at the amount of people who were lining up to look through a scope I set up in this manner, asking what I did. The truth is that I did very little, which means you can do it too. Let’s get started. The first thing you need to find out is whether or not you have a good enough optics. Always remember that there is no optical system without flaw, not even mine. Every scope has some sort of abberation because of the nature of its design, but we will start by conducting a star test. If you do not know how to conduct the test, I’ll explain some basic things after the next procedures are fallowed first, so put the star test aside for now.

If you are planning to observe planets with your reflector, check the internet for jet streams, seeing conditions and general weather. You can use the clear sky clock as well. I have determined that it is not always that accurate during a careful study we conducted, but it’s better than nothing. Personally, it’s better to become familier with your own surroundings to determine the seeing on your own, but for now let’s just keep it simple. None of what I’m going to share is of any use if mother nature doesn’t cooperate. If you feel there’s a possability that the seeing may be good, then set up your scope as early as possible and give it a chance to settle down an cool. You never know, it may end up a pretty good night.

Before you start, consider the spot from which you plan to observe. Cement is one of the worst surfaces to set your scope on because it holds heat for hours. The heat currents which you can not see are actually there, crawling right up the tube. imagine the heat you see down a hot road. Watch out for warm cars or heating vents, etc. Sometimes I’ll rinse my area down with a garden hose and let things dry before I set up. I now use a $16 blue moving blanket from Home Depot to set my scope on over the cement. It tends to work better than cement itself. Setting up on grass is even better if you’re able but I’ve gotten good enough results with my insulated blanket.

Don’t worry about collimation until after the scope is set up and has cooled down for a couple of hours first. If you live in a light polluted sky, don’t worry. Light pollution has ZERO effects on planets. Think of a reflector like this. It costs about $10,000 for a good 6″ apo set up, if not more. A decent reflector can cost you about 1 to 2 thousand dollars. For an extra $200, you can make a couple of VERY important modifications and you’ll still be about $8,000 dollars ahead.

You need to buy yourself a simple home fan. A 10″ or even smaller one is fine. Place this behind the mirror at low speed for a couple of hours while you’re at home. If you are observing from an obscure location, then you’ll need to get a DC fan behind the mirror. I don’t care how you do it, but just do it. Never collimate a scope first and let it cool and assume its still collimated. Wood and metal can bend and change shape during temperature drops, thus changing the collimation. Always let the scope settle down first and then collimate. Let’s just briefly talk about focal ratio because this is HUGELY important. Many beginners question whether or not they should have a 10″ F-5, 8″ F-6 or a 6″ F-8 for example, thinking that the longer focal ratio will yield the tightest images. That is absolutely not true. Get the largest aperture you can, however, I don’t suggest anything more than 14.5″ for most applications but don’t be blind about focal ratios.

What I’m gonna share with you is where the failure begins and is HUGELY underestimated. I have shared a few numbers From Nils Olof’s collimation site. Imagine this in your mind. All the information from the primary will be reflected on to the secondary mirror. When you look down the focuser, you will see the mirror that looks like a circle from your perspective. In a fast optical system like an F-4, there will be a very tiny field of perfect optical definition at the center of that mirror. No matter what the aperture is, at F-4 it’s only 1.4mm which is about half the width of a Q-tip stick. That means the stars should look like tiny points and anything beyond that, coma or distortion for a simpler word starts to kick in. If the optical system is F-6, the size of that little field of perfect optical definition increases to 4.8mm. That’s less than the width of a typical Bic writing pen.

When you collimate, I STRONGLY suggest you stop skimping, spend a $150 and buy one of Howie Glatters 1.25″ barlowed lasers. This idea was invented by Nils Olof Carlon, a collimation guru. Put the old collimation tools to rest and save yourself the headache of dealing with them in the dark and the thousands of dollars people are fooled into believing they need to spend on an apo. The differences in optical performance floored me. Never have I seen planets look so tack sharp and absolutely incredible in a reflector. When you place any kind of laser in a truss system and shift the scope at different angles, assuming you have a tiny dot or donut on the central axis of the primary, watch how the laser dot behaves on the primary. On very fast optical systems like F-4 to even F-5, just the slightest shift will cause that tiny field of perfect optical definition at the secondary to be replaced by the coma or distortion of the primary.

My Starmaster has only 4 truss poles and tends to flex a bit. I can easily varify this by moving the scope up and down. It can be caused from numbers of things, a slightly loose mirror cell, a weak spider or flexture in the tube itself for example. Because of this, I collimate the scope based on the angle it will be pointed at the sky. Since planets shift so slowly, the collimation lasts most of the night. Truss scopes with 8 poles are more rigid, so there tends to be less shift. Some people may find this a hassle, but that’s the price you sometimes pay for having such a fast scope.

If the system were F-6, it would be surprisingly far more forgiving because the field of perfect definition at the secondary is larger. If the scope comes out of collimation a bit, you have a better chance of keeping the central axis of the secondary diffraction limited. My advice is that a cheaper cheshire collimator is fine for an F-6, but when you’re working with these very fast optical systems, F-5 and below, I STRONGLY recommend the barlowed laser, it takes no prisoners when properly used. My buddy John Hawk also uses the same one for his 18″ F-4.5 Obsession and the images were incredible in it. So, the point of this matter is make sure you collimate for the region of sky you plan to observe the planets if you are using a fast scope. Scopes with tubes are usually more rigid and less likely to flex, so use your best judgement, but remember more than anything, don’t assume your system is properly collimated without some careful inspections. I am very careful about this and believe me, it makes a very important difference. Too many individuals are careless and don’t get the desired results they expect.

We have to assume that the central axis of that primary has been figured to where it should be. With that said, look very carefully at the primary dot to be sure the laser dot is centered on it. I also do not recommend a standard laser collimator, in fact I am convinced they can cause miscollimation just as Nils mentioned in his wonderful January 2003 S&T article p. 121. The reasons are technical and may not need to be covered here. Once the reflection of the donut on the primary hits the face of the laser, this is the most accurate you can collimate the scope.

Now let’s talk very briefly about fallowing the ambient temperature drop. Sometimes we can all get very technical about the methods used for cooling down the scope. Today, let’s just put that aside. I’m not telling you this because it’s not important. It is important, but not for this check list. Any fan will increase the performance of a Newtonian. The longer you splash ambient air against the back of the primary, the quicker the primary will reach ambient temperature. Once you’ve cooled the optics down, pull the fan away for this experiment. Now, point the scope at a bright star and rack the image way out of focus with a farely high magnification eyepiece and look to see if there are little waves moving around. Remember that seeing conditions usually run in one direction, while the boundary layer looks like a series of various waves moving at different angles. Train your eyes and learn which is which. If the boundary layer exists, then throw the fan on the primary a bit longer and check it again 10 or 15 minutes later.

There is one problem you may face. If for some reason the ambient temperature drops rapidly every hour, this is where more technical and strategic fan placements need to come into play, because the fans may need to be left on at all times. If they’re pushing too much air, then turbulence will fill the optical image and cause it to degrade. The key is to get the optics and the scope thermally stabalized.

Once the scope is cooled and collimated, you can now conduct the star test to see if any of the issues I’ve mentioned even matter. Test the scope on the best nights of seeing. Star tests can get terribly complicated, but let’s try to keep it simple here. Some observers don’t even know what they don’t know. All I’m attempting to do, is to get you to know what you don’t know. Think of the star test like this. Imagine a very still pond. If we dropped a pebble into the pond, a series of ripples would appear. The key is to see if these ripples exist on both sides of focus when the star is racked in and out. When you conduct this test, you should use a pretty high magnification eyepiece such as a Nagler. Use a bright star and keep it on axis as closely as possible to see if the ripples exist on both sides in a Newtonian design. If they don’t, then don’t be too quick to judge the optics harshly. There are numerous things which can cause this appearance but it’s more technical than we need to be at the moment. The rings should basically look very much the same on both sides of focus under good seeing. Using just these simple procedures should help your scope perform much nicer. Be careful and critical about your set up. Most of all, point your Newtonian at a planet and see what happens before you get too technical. There are so many things invloved with this subject, it would blow your mind.

SUMMARY:

1. Pick the proper environment from which to observe.
2. Set the scope up one to two hours early and allow the optics to stabalize using a fan.
3. Carefully collimate the telescope after it has stabalized with the scope set in the direction of the planet if neccessary.
4. Conduct the star test.

We could all get very techinical about this stuff, but this is just a basic start guide. If there are issues you feel strongly about, then share them and try to help others understand more.