Posts Made By: Albert Highe

It depends.

If you never want it to fall off, use epoxy.

If you want to get it off in the future, most people use silicone. I use clear GE Silicone Type II 100% Silicone Sealant. You can find it in large caulking tubes used for sealing around tubs and showers or windows and doors. In small tubes, you'll find it labeled as Household Glue.

Albert

Hi Justin,

First of all, don't believe everything you read on the Internet. That goes for articles on "Cloudy Nights." Quality of the articles vary and most contain limited, subjective opinions based on little or no data.

There are no perfect eyepieces, just as there are no perfect telescopes, cars, washing machines, or any other product. Each product has its advantages and disadvantages. It is a matter of personal preference which set of advantages and disadvantages you select.

I recommend you read all the other articles on eyepieces: http://www.cloudynights.com/premium2/eyepieces.htm They contain a lot more information than the article you cited. As you read, note what telescopes and types of objects were used to evaluate eyepieces.

What you'll find is that the "best" eyepiece depends on a lot of factors - your goals and preferences, telescope design, observing targets, observing conditions, observer's skill, your pocketbook, etc.

I own or have owned a variety of eyepieces (orthos, plossls, konigs, lanthanums, panoptics, naglers) from a variety of manufacturers. I've spent considerable time evaluating all of them over a period of months or years in a variety of conditions. The best advice I can give is use what you read or hear as a guide. But you'll have to use eyepieces in your scope for quite some time before you can decide whether they meet your needs. If you are a serious observer, you'll likely have specialized eyepieces for certain types of observing. If you are a casual observer, you'll likely notice little difference in performance, but you'll notice things like field of view and eye relief right away.

For example, serious planetary observers generally prefer simple eyepiece designs with exceptional on-axis sharpness and contrast. However, if you filled your eyepiece case with the "best" eyepieces recommended in an article on planetary viewing, you'll likely be very disappointed. The "best" planetary eyepieces usually have a narrow field of view and an even narrower "sweet spot" where the performance is optimal. You'd better have a tracking scope or you will rarely experience the best performance. In an undriven dob, the object will quickly drift outside the "sweet spot" and then outside the narrow f.o.v. In addition, with fast scopes, the off axis performance of basic designs is quite poor. Star clusters will look far better in Naglers that are designed for fast scopes.

Simple planetary eyepieces generally also have very short eye relief (in the focal lengths used for observing planets). If you must wear glasses, you might as well not even bother evaluating them. In the longer focal lenghts, the eye relief of orthos and plossls is comfortable.

That being said, I prefer the new Nagler T6's for observing star clusters and hunting faint fuzzies in my scopes. I generally use 12-1/2" - 18" reflectors with f/# around 4.5-5. The old 9mm Nagler was a superb eyepiece. Except for the smaller size and weight, the performance of the new 9mm T6b seems the same. However, the 5mm and 7mm Nagler T6's have noticeably better contrast than their previous counterparts (4.7mm and 7mm) as well as better eye relief. Most of the time, they even provide very good views of the planets. It takes some rare, exceptional seeing to distinguish the views between these Naglers and simpler eyepiece designs. The 7mm Nagler T6 is in my focuser at least 90% of the time.

For seeing really gets good, I bring out the simpler orthos and plossls for planetary viewing.

In summary,
1) Be clear of what YOUR goals and constraints are.
2) Eyepiece performance can vary greatly among scope types and focal ratios. Consequently, pay the most attention to opinions voiced by people who are using the same scope as you have.
3) Eyepiece preference is very subjective. What different people think depends a lot on what they value - what their goals are. So, don't pay too much attention to other people's opinions. Only yours matters. Consequently, try (in your scope) before you decide which eyepieces to keep.

Albert

Justin De Guire said:

I saw this article on cloudy nights and I am just wondering what you guys thought of it. I found it to be interesting and just wondering if most people agreed with it or not. thanks

http://www.cloudynights.com/eyepieces2/performance.htm

Hi Craig,

That's my mirror cell in the Sky and Telescope article. I've attached scores of mirrors (as large as 18") to mirror cells (3, 6, and 18 points) using silicone. Here is my procedure.

Prepare a clean padded surface for the primary mirror. Place the primary face down on this clean, non-abrasive surface. Make sure that the mirror is level.

The surfaces of the support pads and back of the primary mirror must be clean and free from grease, dirt, etc. Acetone can be used to remove grease, but it is not effective for removing silicone and other contaminants. The best surface is one that has been abraded to expose fresh material. Be careful not to get any sanding grit under the mirror. It will do great, irreversible damage to your mirror.

With the primary mirror face down, center the mirror cell on the back of it. Mark the location of the support pads. Turn the mirror cell pad side up and abrade each pad with 60 grit sandpaper. Sanding the location of each pad on the back of your mirror will also create a better bond between the silicone and the glass.

I use the GE (General Electric) 100% Silicone Type II outdoor sealant to attach mirrors to the support pads. It is used for filing the voids around tubs and tiles (their Kitchen and Bath variety) or around windows and doors (sold as their Windows and Doors variety). I suspect they are the same. These sealants are sold in a variety of colors. I have used clear, white, black, and gray. They all seem to perform the same, but I prefer the clear Windows and Doors variety. The same material is also available in small tubes under the name of 100% Silicone Type II Sealant/Household Glue.

Apply a thin coat of silicone sealant to each pad. Invert the mirror cell so that the pads come down onto the alignment marks you made on the back of the mirror. Allow the weight of the mirror cell to apply the correct pressure. The silicone sealant should wet out the entire surface of the support pads and ooze a small amount beyond their edges. The thickness of the silicone is less than 1mm. Double check that the mirror cell is centered. If your mirror is not level, the mirror cell can float out of position. Allow the silicone sealant to cure fully – usually overnight – before moving.

After waiting 24 hours, carefully test that the silicone has completely cured. Then install the mirror and its cell into the mirror box.

I design the mirror cells so that the total area of all the support pads glued to the back of the mirror is approximately 2-3% of the mirror area. That doesn't sound like a lot, but it does the job real well.

By the way, you shouldn't be too concerned if your mirror cell isn't the optimum design. The key question is whether the cell will do the job.

Albert

I see the most detail when the exit pupil is around 1.5mm. I believe that more detail is visible with exit pupils as low as 1mm, however seeing rarely permits the higher magnification with my scopes. When seeing permits, the smaller exit pupil is usually better. That translates to magnifications approximately 150X - 250X for the 10" and approximately 225X - 350X for the 14". The 7mm Nagler T6 is by far my most used eyepiece (285X in 17.5" f/4.5). M82 is stunning at 285X, but loses a lot of detail at lower magnification. The proof is in the results. Using 1-1.5mm exit pupils with the 17-1/2" has allowed me to observe almost 400 galaxies in the Coma Cluster and over 300 galaxies in the Perseus Cluster.

Bottom line, experiment with a wide variety of eyepieces. Use magnifications higher than observing lore suggests.

Linton,

I've owned a lot of dobs from 6" to 18". I've had the 6" the longest, and will always have one. Its perfect for carrying outside for those quick observing fixes. But when I go to a dark site, I invest the time to set up the 18". I also really enjoy a 12-1/2" scope, especially my ultralight.

With a 12-1/2" scope, you won't run out of observing targets in your lifetime. Larger scopes will provide an even longer list of objects you'll never see. Showpiece objects are marginally better.

If you don't get out to a truly dark site almost every New Moon, you probably are kidding yourself. Keep what you have and don't buy anything else. But if you are serious about getting out often to remote dark skies, then consider buying a larger scope.

If you didn't already own a scope, or are planning to sell the 12-1/2", I'd recommend buying the largest you can easily transport and use. Don't underestimate the importance of size and weight. Astromart ads contain a lot of larger scopes that were too much a challenge for their owners.

Despite the Zambutto optics, consider that Starmaster scopes are larger and heavier than others of the same aperture. Don't be fooled by the Zambutto hype. Carl's mirrors are good, but so are those of plenty of other manufacuturers who charge far less money. You won't see a difference.

But, if want to keep the scopes you have, I recommend the next step up be the square root of 2 times the diameter (e.g. from 12-1/2" to 18"). Any smaller step doesn't provide enough difference.

I wouldn't agonize over the decision. If you get a good deal on Astromart, you'll probably get your money back when you sell. The danger lies in paying new price for the wrong scope. Buy used, try for a while, and sell what you don't find useful.

Albert

I recommend you buy the 7mm Nagler no matter what. With your f/4.5 scope, the 7mm will give you an exit pupil of 1.5mm. You'll find that will give you improved contrast on most dim objects like galaxies and nebula and improve your view of globulars as well. It's my favorite eyepiece on my 4.5 dob. You'll undoubtedly use it more than your 9mm.

There are few reasons to use Naglers for planetary viewing. The only reason to consider them is if your dob is undriven. The object will stay in view longer than if you use a narrow-field eyepiece. However, don't expect too much out of your scope. Best viewing is only in the center of the field anyways. You are not likely to get any better views at high power if the planet doesn't stay centered, no matter what eyepiece you use. If your scope is driven, you can use less expensive, narrower field eyepieces that will do a better job than the 5mm Nagler.

Albert

Hi Bill,

The 75% limit at the edge of the field is an assumption NEWT makes for simplicity. What you want the edge illumination to be depends on what you plan to do with the scope, what eyepieces you use, and how much you care. It is more important to have the size of the 100% illumination be large enough to suit your goals.

Let's discuss the limits.
I would never design any scope where the area of 100% illumination is zero. You might as well have bought a smaller mirror to begin with. In my opinion, its a bad decision to give up center illumination to chase a little extra contrast. The improvement for using a secondary smaller than 0.2 * primary
diameter is insignificant. The best choice is to step up to a larger scope.

I would never design any scope where the area of 100% illumination completely fills the f.o.v. For photographic purposes, you want to approach this limit, but it isn't possible to achieve it. For visual use, it isn't practical or even necessary.

My personal design guideline is to make the center half of my most used (for critical galaxy hunting) eyepiece be at least 100% illuminated. The selection of 1/2 is arbitrary, but at least I have a reason for making the selection. If I'm hunting down galaxies at the limit of detection, a good size of my field better be fully illuminated. Asking it to be fully illuminated across the entire field is an option, but I figured I couldn't
take in the entire field at one time anyway. I use the periphery for recognizing star patterns where illumination isn't key, and it might cost me too much elsewhere in the design.

Although I now mostly use a 7mm Nagler, I selected the 9mm Nagler as my "standard" eyepiece. The size of the full field of the 9mm Nagler is 9mm * 82/57.3 = 13mm. Half this diameter is 6.5mm, or about 0.25". Planetary viewers could get by with less. Those interested in detecting the faintest objects across a wider field can select more. By the way, in the above formula, "9mm" is the eyepiece focal length, "82" is the apparent field of view, and 57.3 converts degrees to radians. You can use this formula for all eyepieces.

For the reasons stated above, the diameter of 100% illuminated field should be at least 0.25". In practice, the size of 100% illuminated field of my personal scopes turns out to be 0.3" - 0.5".

The edge illumination should be determined by your lowest power eyepiece, how you use it, and how much you care. 75% at the edge of your lowest power eyepiece is a guideline, not a hard-and-fast rule. Some people use 50%. It is OK to ignore when NEWT flags that the edge illumination drops below 75%. On the other hand, you won't go too wrong if you use it.

NEWT doesn't tell what the center is when its not 100%. It also doesn't tell you what the edge illumination is of any particular eyepiece. These are understandable limitations and the selection of 100% and 75% limits will result in good scopes. Providing more versatility in NEWT would probably be confusing to most people.

For a more detailed representation of what the illumination distribution looks like, I created my own spreadsheet. You can use the program SEC made available by Alan Adler to see how the illumination varies for any eyepiece, diagonal size, scope, etc. You can download it via the Sky and Telescope website.

Albert