Double images on handmade Dobsonian telescope?

[reggieana]

My husband and I are just finishing up our 6" scope. Images are clear, but we are seeing double. Anyone know what the problem could be? Here's the specs:

6" F8
Focal length = 48"
1.5" secondary

Collimation seems good.

Thanks!
We purchased the mirrors from Hands On Optics. Primary has 1/6 peak to valley wave. Secondary has 1/10. Didn't do Faulcult test. Eyepiece is a 25mm taken from a 3" Bushnell scope that functioned fine.

[busted]

Eyepieces come in all shapes and sizes with all sorts of lens arrangements. The most important feature of an eyepiece is its focal length, which is usually, etched on the eyepiece somewhere. The focal length of an eyepiece determines the magnification when used in a telescope. To calculate the power of a given eyepiece you will need to know your telescopes focal length in millimetres as well. The magnification is given by dividing the telescope's focal length by that of an eyepiece, eg. a 600mm focal length telescope when used with a 12mm focal length eyepiece, will give a magnification of 50X…. 600/12 = 50)

It might seem from this that one should buy a range of eyepieces with relatively short focal lengths as they will give better views because of the high power. However, before buying your eyepieces seek advice of other users. A 2mm eyepiece in a 600mm focal length telescope gives 300X, but there are problems here. A telescope has a maximum magnitude beyond which, although you can buy the eyepieces, it is useless to go. If you try to magnify beyond the practical limit of a telescope you will simply get poor images. To calculate the maximum magnification of your telescope multiply it's aperture in millimetres by two. For example a telescope with a 80mm aperture has a maximum useful magnification of 160X. There are several reasons why you can't use the theoretical limit of magnification for your telescope. Of these the unsteadiness of the night air (in astronomical terms “seeing") is the biggest problem. Too much power will cause objects to jump around in the eyepiece. Reducing magnification also reduces the effect so you can continue viewing. This unsteadiness in viewing is also evident when a warm telescope is taken outside on a cool night. It is particularly noticeable for owners of Newtonian telescopes as convection currents caused by heat radiating form the mirror spirals up the tube.

There is also a lower limit to the magnification you can use. It is calculated by dividing the aperture in millimetres by eight. The 80mm telescope above has a minimum magnification of about ten (80/8 = 10). If you try to use lower than the calculated minimum you will be wasting some of the light gathered by the telescope. You should calculate the maximum and minium magnification for your telescope and bear these in mind when choosing eyepieces. Three eyepieces will usually give you a good start. Choose one that is about 75% of the maximum magnitude, one that is about twice the minium magnification and the third halfway between the two.

The next most important feature of an eyepiece (after magnification) is Field of View (FOV). FOV describes the size of the circle you see when you look through the eyepiece. The larger the FOV of an eyepiece, the better is the view you will see. A large field of view makes viewing more interesting than a narrow one. Some eyepieces have an FOV as large as 70deg, although 40 - 50 is the norm.

Another consideration for an eyepiece is eye relief. This describes the distance you need to hold your eye from the eyepiece when viewing. This is important as it affects your comfort; holding your eye very close to the eyepiece for any length of time can be very tiring. Eyepieces with large eye relief that allow you to hold your head back are much better especially if you want to show the view to people who are not used to using a telescope. Generally low power eyepieces have better eye relief than high powers ones.

When choosing eyepieces always try to pick eyepieces that are coated. Anti-reflective coatings give superior viewing to uncoated optics, with fully coated, multi coated, and fully multi-coated indicating the increasing degree of coatings applied to eyepieces, and, increasing cost.

ED Eyepieces
Recently Orion Optical introduced a new range of budget priced eyepieces that have very good qualities and would normally cost considerably more . “ED” stands for “Extra-low Dispersion” optical glass, a special grade of glass that boasts superior refractive properties compared to standard optical glass. In fact, ED glass is the same type used in many expensive camera lenses and high-end refractor telescopes costing several thousand dollars!

ED 1.25" eyepieces feature a six-element lens design in which two of the elements are crafted from ED glass (hence, ED-2). This allows each eyepiece, from the 22mm down to the 3.7mm focal length, to provide a luxurious 20mm of eye relief with remarkably reduced chromatic aberration and excellent sharpness. Long eye relief means that even if you wear eyeglasses, you can take in the full, unconstricted field of view. Which is a generous 55°—a wider apparent field than that of Plössl eyepieces! Notice also that even the short-focal-length EDs sport nice, wide eye lenses—not small peep-holes! Another advantage of the EDs’ unique optical design is they are Fully Multi-Coated and Parfocal. ED-2 eyepieces are fully multi-coated (every air-to-glass lens surface is multi-coated) for maximum image brightness, and the lens edges are blackened for improved contrast. The series is parfocal, so little or no refocusing is needed when switching from one ED eyepiece to another. Currently these can be purchased from some Aussie suppliers for just $79 each.


Sizes
Three standard eyepiece diameters are available; they are 24.5mm/. 96ins, 31.8mm/1.25ins, and 52/ 2ins. Prices vary considerably depending on size, focal length, type, and manufacturing quality. Prices for mid quality, mid focal length range, .96 " eyepieces are generally between A$30 (modified Huyghenian) and A$80 (orthoscopic). For 1.25" eyepieces there is an enormous range of prices. They vary from $30 (Meade modified achromat [MA] and Kellners), and depending on quality, Plossles can range up to $100+. Orthoscopics can still be obtained from about $80 - $135 The general trend in 2" eyepieces is to produce as wide a FOV (65+deg) as is possible and therefore these are usually of more exotic multi element design. Prices can range from $280 (modified Konig) to the 31mmTeleview Nagler type 5 (82deg FOV) which is $900+.

[injanier]

If the collimation is good, check the mounting of the mirrors. Both mirrors should float in their mountings. A tight clamp or other restrictive mounting can distort the mirror and cause a variety of problems. Geoff's suggestion is a good one, too - find someone who knows telescopes to look it over.

[Geoff G]

It could be a number of things...hard to say without actually seeing the scope and the image it produces. Did you make the primary mirror yourself? What did the Foucault test look like? It could be that there are optical problems with the primary or the secondary. What sort of eyepiece are you using?

[edit] The best thing to do is to contact your local astronomy club and get someone there to take a look at it. I don't know why someone marked my answer thumbs-down. I was just trying to get enough information to give an answer.