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Dickenmeyer
2003-May-20, 04:43 AM
I was trying out some new eyepieces and accessories the other night during the lunar eclipse and had some problems getting a good focus with my two highest power eyepieces. Would replacing the focus knobs with bigger ones help me fine tune the focus? The knobs on my scope (Meade LXD55 SN8) are plastic and kind of small, almost the only gripe I have with the scope. On the user boards I have noticed that a lot of folks recommend replacing the knobs but no one really says why, are they having the same problem I have?

kilopi
2003-May-20, 04:52 AM
What are your high power eyepieces? Perhaps they are too high power for your aperture.

Pinemarten
2003-May-20, 08:35 AM
I have had the same problem adjusting RF tuning knobs. Instead of trying to fit a bigger one, I just attach a stick to increase the diameter.

Glom
2003-May-20, 12:24 PM
There is a limit to how high you can go with magnification before the imperfections in the mirror or lens prevent the image from being sharp. Don't forget, HST is not a particularly big telescope, it's just extremely high quality so it can be use for high magnifications.

Pinemarten
2003-May-20, 12:54 PM
I found with my 'Roebucker' that when I tried the greatest power eyepiece, and the Barlow 2x, the problem wasn't focus; but low light to the eye. Is the image still 'bright' at max magnification?

Glom
2003-May-20, 01:06 PM
In The Backyard Astronomer's Guide Revised Edition, one thing Dickinson makes clear is that any telescope sold on the basis of magnification is highly likely a CTS (Christmas Trash Scope). Magnification is obtained by plugging numbers.

Magnification factor = Telescope focal length Eyepiece focal length.

This clearly does not give an adequate description of the abilities of the telescope at hand, because theoretical magnification, according to this equation, is independant of telescope quality and aperture size. And of course it is, but that doesn't mean you're going to get anything good out of it.

Telescopes should be sold on the basis of aperture. The larger the aperture, the more light is collected and so the higher the magnification you can use before the light becomes too diffuse.

The table 'Telescope Performance Limits' on page 55 of BAG details the maximum viable magnification for according to aperture.

Aperture/inches Aperture/mm Highest usable magnification
2.4 60 120
3.1 80 160
4 100 200
5 125 250
6 150 300
8 200 400
10 250 500
12.5< 320< 600

(bloody software :x )

kilopi
2003-May-20, 01:38 PM
Telescopes should be sold on the basis of aperture. The larger the aperture, the more light is collected and so the higher the magnification you can use before the light becomes too diffuse.
It's not just a matter of not collecting enough light. At those theoretical limits, the Airy disks start to overlap, due to diffraction by the finite aperture.


The table 'Telescope Performance Limits' on page 55 of BAG details the maximum viable magnification for according to aperture.
Rule of thumb, and that table follows it closely, is 50x for each inch of aperture, or 2x for each mm of aperture.

Glom
2003-May-20, 01:47 PM
Airy disc?

kilopi
2003-May-20, 02:02 PM
Airy disc?
I think that spelling is also acceptable. :)

Glom
2003-May-20, 02:05 PM
Good. What is it?

kilopi
2003-May-20, 02:20 PM
Sorry, I guess I'm sensitive about spelling criticism (http://www.badastronomy.com/phpBB/viewtopic.php?p=76742&highlight=spelling+kilopi#76742).

Here is a link that discusses diffraction and the Airy disk (http://www.astrosurf.com/m111/Scopes.html#AIRY DISK BRILLIANCE). There are more technical explanations out there.

RichField
2003-May-20, 02:32 PM
I found with my 'Roebucker' that when I tried the greatest power eyepiece, and the Barlow 2x, the problem wasn't focus; but low light to the eye. Is the image still 'bright' at max magnification?

My understanding is that the same amount of light is coming through the eyepiece at all magnifications for a particular object, assuming the entire object still fits in the field of view. However, the light is being spread over a larger area of your eye and will therefore appear dimmer as magnification is increased.

Rich

Glom
2003-May-20, 02:36 PM
Cheers, kpi.

Grey
2003-May-20, 02:43 PM
Good. What is it?

It's the central region of the diffraction pattern of the stellar image (even though it might be a point object, you see it as a disk due to diffraction; this is caused by the finite size of the telescope). At the point where the Airy discs (or disks :) ) of nearby objects begin to overlap, there's simply no way to resolve them, regardless of how good your optics or how steady the atmosphere that night. You're limited by the diffraction effects of light.

http://www.mellesgriot.com/glossary/wordlist/glossarydetails.asp?wID=253
http://astro.isi.edu/notes/magnify.html

Glom
2003-May-20, 02:45 PM
If the inside of your telescope is shiny, can you get interference patterns?

Dickenmeyer
2003-May-20, 11:48 PM
What are your high power eyepieces? Perhaps they are too high power for your aperture.Yeah, I thought about that but the highest power I used that night was a 6.4mm super plossl which yields 127X on my scope. Or actually 254X because I had the barlow in, but using the 50X per inch of aperture rule of thumb I should be good up to about 400X in steady seeing so even with a little upper atmosphere turbulence 250X shouldn't have been that bad. This isn't a department store trash scope, the optics are pretty good. I may not have been patient enough though, by the time I worked my way down (or up) to the 6.4mm it was pretty late and I was hopping around a little to Jupiter and Coma Berenices and things.

Hale_Bopp
2003-May-21, 12:20 AM
Here's a thougth...did you let the telescope cool down? A warm telescope can have bad seeing due to the heat currents and radiational cooling going on inside. It's similar the the "heat waves" you see in the summer coming of hot asphalt. You might think they would be too small to see, but remember, you are magnifying them 254x! I frequently see them in my telescope at 50x early in the evening if it has not cooled down to air temperature.

I don't know how long the Meade 8" LXD55 takes to cool, but it is probably about 45 minutes depending on temperature difference.

Rob

Dickenmeyer
2003-May-21, 04:09 AM
Here's a thougth...did you let the telescope cool down? A warm telescope can have bad seeing due to the heat currents and radiational cooling going on inside. It's similar the the "heat waves" you see in the summer coming of hot asphalt. You might think they would be too small to see, but remember, you are magnifying them 254x! I frequently see them in my telescope at 50x early in the evening if it has not cooled down to air temperature.

I don't know how long the Meade 8" LXD55 takes to cool, but it is probably about 45 minutes depending on temperature difference.

Rob
Rob you may be on to something. My scope was plenty cooled down, it had been out over an hour before I couldn't get it to focus BUT I had it out in the driveway looking down the street over half a dozen houses (because that's where I could see the moon best) and I know roofs shed heat like crazy. Couple that with me being tired and impatient by that time (after 11:00 local) and it's no wonder I couldn't get a good focus. Regarding bigger knobs though, would the bigger diameter help with fine focusing? Or do some people just like big knobs better than the factory set?