Quote:
Originally Posted by refractordude
Hello There
Thanks for all the replies. Using the four inch mask allows me to get high magnifications. Sometimes I can go high as 300 mag. Without the mask objects breakdown a little after 100 mag. Which filter will allow me to up the mag with full aperture? I once tried a #12 yellow but it did not work. By the way I have a six inch aperture F8 DOB. However, I do not like the diffraction spike on Jupiter and Mars. Also with the DOB I can not change the angle of the focuser to stop Crater Illusion when viewing the Moon.
Thanks to you all
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You need to step down the aperture to increase the f/ratio to reduce chromatic aberration. Seeing always has a certain degree of scintillation, but reducing the aperture also makes that scintillation smaller than the resolution of the scope.
If diffraction spikes bother you, continue to use a refractor, though you might like a Maksutov-Cassegrain, which doesn't have diffraction spikes or chromatic aberration.
Great seeing will allow you to increase the magnification. Great collimation will as well. And, the third "C": cooling of the optics.
Cooling:
Your scope will get better image quality at high power after cooling the optics to the ambient temperature. Some time outside will improve the images, especialy with a 6" lens.
Collimation:
The use of high quality collimation tools is essential for higher power use. If you ever want to use 300x in a scope, good collimation is critical. Collimation tolerances are loose at low power, but very tight at high powers.
Fortunately, refractors are usually collimated, though it is something you should check.
Conditions:
High powers demand good seeing. You can control seeing to a small degree:
--observe objects when more than 30° above the horizon, and, preferably, 45° or more.
--don't observe objects directly above a roof, as roofs leak heat all night long and that causes turbulence in the air.
--try to avoid setting up the scope on asphalt or concrete, as they lose heat all night long under the scope. Instead, choose dirt or grass.
--don't set up anywhere near a bonfire or barbecue at night due to the heat rising from them.
--if you can, set up in the middle of a valley, not at its edges. Air rises and falls at the edges, but the air in the center of a valley is more stable.
--observe at high powers 2-3 days after the passage of a front, not immediately after. Slow air movement, even stagnant air, leads to more stable seeing and the ability to use high powers.
--observe after midnight. Quite often the morning hours have the calmest skies.
I am not sure what you mean by "crater illusion". If you mean that craters appear as domes and vice versa, this goes away with time and familiarity of viewing the Moon. More moon viewing will help.
If by that you mean the appearance of floaters: dots, amoebas, bacilli, lines, and squiggles against the moon when viewing, know this: it is a universal human problem, and the older we get, the more of those are in our vision.
They seem to appear more at small exit pupils, like when the focal length of the eyepiece equals the f/ratio of the scope or shorter (8mm and shorter in your scope). By the time you get to an eyepiece whose focal length is 1/2 the f/ratio of your scope (~4mm), virtually everyone sees these "floaters in the eye". They are small concentrations of opaque material in the vitreous humor of the eye. The cure is to use a lower power or to ignore them, though ignoring them is hard to do when they settle in the center.
But, your original question was about a filter, and, sadly, there is no filter that will improve seeing (turbulence). Some have said that by cutting off the violet and blue they see less light scatter in the eyepiece, but that is not the same as improving seeing.
What is true is that the more you observe, the more likely you will be observing when the seeing is great and high powers can be used. Seeing varies from night to night and even hour by hour.
maybe this will help:
6" scope at
low power--22-60x usable every night it's clear.
medium power--60-120x Usable most nights.
high power--120-180x Usable fairly often, but not every night.
ultra high power--180-300x. Not frequently usable.
The Seeing scale:
https://www.damianpeach.com/pickering.htm