Yet another eyepiece question
Posted 24 February 2013 - 11:19 PM
It would be used on an 8 inch SCT...so what do you think or do you have a counter suggestions on how to spend this money
Posted 25 February 2013 - 05:57 AM
$100? Anti dewing equipment ... or an extra battery to feed the existing stuff.
Posted 25 February 2013 - 11:34 AM
Posted 25 February 2013 - 01:25 PM
The image is still brighter using the 40mm; a significant advantage for deep-sky observing under anything other than the most light-polluted skies. Even though the actual field of view is limited to slightly less than 1 degree using this combination, the contrast and brightness is remarkable for a C8.
As an expansion to what I had said regarding field of view, the actual fields of view are dependent upon the eyepiece design, and is limited to the field stop diameter, which is the maximum inside diameter of the 1.25 inch eyepiece body.
I have also obtained very beautiful views with my C8 with only slight vignetting (darkening at outer periphery of view) using a 35mm TeleVue Panoptic (2 inch) eyepiece and a 2 inch optical back and diagonal. This combination yields 57 power, a 68 degree apparent field of view, a 3.5mm exit pupil, and a 1.19 degree actual field of view. Unfortunately this combination exceeds your $100 budget by a substantial margin; the cost of a new 2 inch diagonal alone exceeds $100. But the 40mm yields a very nice low-power image and is well within your budget; you'll be impressed.
Posted 25 February 2013 - 01:55 PM
I've used one of these with my C8 to get even an even wider field of view with my 40mm eyepiece; the exit pupil is then about 6.3mm, and while that's too big for my old eyes it would be fine if you're young. With your 25mm eyepiece the f/6.3 reducer would yield a 4mm exit pupil and 50X, the same as if you were using a 40mm eyepiece without the reducer.
Posted 25 February 2013 - 04:22 PM
Posted 26 February 2013 - 07:36 AM
Exactly the wrong answer IMHO ... focal reducers reduce the illuminated FoV and introduce aberrations of their own ... as for brightness of the image, it's irrelevant for point objects like stars at low magnification. The brighter sky background (especially at light polluted sites or in moonlight or twilight) can be an issue & the larger exit pupil can be a real issue with scopes like SCTs that have large secondary obstructions.
a focal reducer. That might be exactly the answer.
Posted 02 March 2013 - 11:38 PM
Posted 07 March 2013 - 12:46 PM
Posted 12 March 2013 - 01:32 AM
BrooksObs - Yes it is specifically for variable star observing. My theory was the wider field would allow more comparison stars. Clearly the trade off with brighter background makes this less attractive than I thought.
lee14 - Before the snows came I was experimenting with the idea of higher magnification to bring out fainter stars, and the results had been promising - I was getting about a magnitude deeper with a jump from 81x to 140x. When I get some scope time I want to try and figure out an optimum magnification for darkening the field before seeing begins to play a role
Posted 12 March 2013 - 05:08 AM
This definitely depends on the seeing. Sometimes I've known it to be so bad that stars were fuzz balls in an 80mm refractor (of excellent optical quality) at only x24.
When I get some scope time I want to try and figure out an optimum magnification for darkening the field before seeing begins to play a role
FWIW I find that x200 beats x140 for limiting magnitude on a 11" SCT about two nights in three, but going past x200 is hardly ever worthwhile. If you're not parked under Jet Stream Central or you have exceptionally bright skies you might find it worth pressing the magnification just a bit further.
Posted 14 March 2013 - 02:02 AM
Posted 15 March 2013 - 04:13 PM
Posted 16 March 2013 - 02:50 AM
And Lee as I think about it - Seeing really is not a factor. All the stars in the field will be affected the same, so an estimate will still be consistent.
Posted 16 March 2013 - 03:36 AM
When you're close to the magnitude limit, defocusing or seeing blurring turns the "ideal point" image of a star into an extended object & it will start to become less visible. Same happens with excessive magnification. It's when seeing is good that the point image of a star stays compact for longest as the magnification is increased ... the intensity of the point remains the same whilst the extended background is darkened by spreading it out further, increasing the contrast & making fainter stars visible.
Seeing isn't really much of a factor with higher mags though. Purposefully defocusing stars for brightness comparison is an acceptable and very effective method for making an estimate, so a lttle fuzziness due to poor seeing shouldn't affect one's ability to compare nearby stars.
Once you get the magnification above about 20 times the aperture in inches (0.8x the aperture in millimetres) the Airy disk image of the star becomes a factor & increasing the magnification further no longer helps to see fainter stars. But with larger instruments seeing usually blurs the star into a disc before the Airy disc should be resolvable.
As you say, the blurring caused by bad seeing or focus errors doesn't affect estimates; deliberate defocusing can be helpful when trying to estimate stars which are really too bright for the instrument (more than about 3 mags above the threshold) or when trying to estimate the magnitude of a cometary nucleus ... but getting the focus critically accurate is essential if you really want to dig as deep into the faint object realm as your scope can manage, and with larger scopes you're going to need the seeing to cooperate as well if the star is going to remain point like when sufficient magnification is used.
Posted 16 March 2013 - 06:42 AM
Posted 16 March 2013 - 10:45 AM
Defocusing also helps with color induced error, ie. red stars tend to appear brighter as you continue to gaze at them. As the light is spread out over a larger area, this effect is diminished.
Small differences in brightness are more difficult to detect with stars near the threshold of visibity. As the seeing varies, the stars tend to fade in and out. It's often difficult to decide which is brighter, as each varies independently. Defocusing can be useful here as well, but in a different way. For example, take a 13.1 comparison star, and a variable that appears to be very close in brightness. If you consistantly just lose the 13.1 by slight defocusing, but the target variable hovers in and out of visibilty, you can be sure it's the brighter of the two.
Different techniques work best for various observers. The one sure way to know how accurate you are being is to compare your estimates to an AAVSO light curve that includes your data points.