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Observing >> Variable Star Observing and Radio Astronomy

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MG1962
sage


Reged: 08/13/11

Yet another eyepiece question new
      #5699106 - 02/24/13 11:19 PM

I have been gifted a $100.00 to spend on an astronomical related item. I have a good range of eyepieces from 25mm down to 9mm. So for variable star observing is there any value going up to something around 40mm for no other reason than to get a wider field of view to work with

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


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brianb11213
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Reged: 02/25/09

Loc: 55.215N 6.554W
Re: Yet another eyepiece question new [Re: MG1962]
      #5699323 - 02/25/13 05:57 AM

Probably not ... a 40mm EP in 1.25" fitting will not give a wider field than most good 25mm EPs and, though there is some mileage in going to a 2" visual back & diagonal, a 8" SCT will not fully illuminate it.

$100? Anti dewing equipment ... or an extra battery to feed the existing stuff.


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Peter D.
sage
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Reged: 02/09/12

Loc: Central New York
Re: Yet another eyepiece question new [Re: brianb11213]
      #5699708 - 02/25/13 11:34 AM

While I agree that a 40mm eyepiece will not give a wider field of view using the stock 1.25 inch optical back and diagonal, there is another advantage: a brighter image. I use a 40mm University Optics eyepiece on my C8 routinely; it gives a 5mm exit pupil at 50X power instead of the 2.5mm exit pupil at 80X that I get with my 25mm eyepiece. The apparent field of view with the 40mm eyepiece may actually be somewhat smaller than with the 25mm, depending on the characteristics of the 25mm, but the brightness (and maybe contrast) will be noticeably better with the 40mm.

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Peter D.
sage
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Reged: 02/09/12

Loc: Central New York
Re: Yet another eyepiece question new [Re: Peter D.]
      #5699847 - 02/25/13 01:25 PM

I just noticed a math mistake in my previous reply; the exit pupil using the 40mm eyepiece will actually be 4mm rather than the 5mm I had previously stated.

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.


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Peter D.
sage
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Reged: 02/09/12

Loc: Central New York
Re: Yet another eyepiece question new [Re: MG1962]
      #5699890 - 02/25/13 01:55 PM

Another idea to increase both brightness and field of view using any of your existing eyepieces would be to get an f/6.3 (0.63X) focal reducer; these are available new for well under $100, and can often be purchased used on Cloudy Nights classified.

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.


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MG1962
sage


Reged: 08/13/11

Re: Yet another eyepiece question new [Re: Peter D.]
      #5700172 - 02/25/13 04:22 PM

Thanks for the feedback. Especially Peter with the suggestion of a focal reducer. That might be exactly the answer. And like you I have old eyes as well lol

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brianb11213
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Reged: 02/25/09

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Re: Yet another eyepiece question new [Re: MG1962]
      #5701097 - 02/26/13 07:36 AM

Quote:

a focal reducer. That might be exactly the answer.



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.


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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Yet another eyepiece question new [Re: brianb11213]
      #5709758 - 03/02/13 11:38 PM

Is your intended purchase specifically aimed toward helping in variable star observing? If so, you want to maintain the broadest fully illuminated field you can and not push your scope to such a low magnification that significant vignetting may come into play. With very low power you also lose a good percentage of the field's fainter stars (and comparison stars) to the brightness of the sky background.

BrooksObs


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lee14
super member


Reged: 12/19/09

Re: Yet another eyepiece question [Re: BrooksObs]
      #5718369 - 03/07/13 12:46 PM

The 40mm is a good choice with your scope for locating a target variable in it's field. A focal reducer might give a wider FOV, but it wouldn't really be money well spent. For faint stars (once you've identified the correct field) you'll want to use higher powers, they will improve contrast by darkening the sky background. The reduced FOV eliminates non-relevant field stars, as well as revealing fainter comparison stars.

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MG1962
sage


Reged: 08/13/11

Re: Yet another eyepiece question new [Re: lee14]
      #5727360 - 03/12/13 02:32 AM

Sorry guys been away for a while.

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


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brianb11213
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Reged: 02/25/09

Loc: 55.215N 6.554W
Re: Yet another eyepiece question new [Re: MG1962]
      #5727498 - 03/12/13 06:08 AM

Quote:

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



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.

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.


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MG1962
sage


Reged: 08/13/11

Re: Yet another eyepiece question new [Re: brianb11213]
      #5731585 - 03/14/13 03:02 AM

Well I have reasonable quality eyepieces to push up to 185. So with the chance of better skies as spring approaches I will definitely test the idea out

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lee14
super member


Reged: 12/19/09

Re: Yet another eyepiece question new [Re: MG1962]
      #5734517 - 03/15/13 05:13 PM

MG1962- Higher magnification is defintely needed for faint stars. The trade off is a decreased field of view and the loss of the outer comparison stars. (AAVSO charts are perfectly suited to accomodate whatever level of magnification you're using) We're getting similar results, I use 63X and 163X and gain about a magnitude as well in my 8" SCT. 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.

Lee


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MG1962
sage


Reged: 08/13/11

Re: Yet another eyepiece question new [Re: lee14]
      #5735497 - 03/16/13 03:50 AM

Yes I had an unexpected observing night last night and was noodling around in Ursa Major. I had a variable that I could not see at 81. Kicked it up to 180 and it was very obvious.

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.


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brianb11213
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Re: Yet another eyepiece question new [Re: MG1962]
      #5735518 - 03/16/13 04:36 AM

Quote:

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.



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.

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.


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jgraham
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Reged: 12/02/04

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Re: Yet another eyepiece question new [Re: brianb11213]
      #5735617 - 03/16/13 07:42 AM

Interesting comments. There was a conversation a while back in the deep sky forum about observing the central star in M57. The consensus was that when trying to observe faint stars on the limit of visibility high magnification and good seeing were both important. My own experience is consistent with this approach. Once you've found a specific faint star then you can soften the focus a bit if needed.

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lee14
super member


Reged: 12/19/09

Re: Yet another eyepiece question new [Re: jgraham]
      #5736087 - 03/16/13 11:45 AM

Of course if a star is at the threshold of visibility, defocusing will lose it. Perfect focus and optimal seeing will always enable one to reach as deeply as possible. With high magnification though, stars are no longer point-like anyway, the light is starting to spread out. When you defocus brighter stars at lower powers, you're simply making it easier to discriminate small differences in brightness.

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.

Edited by lee14 (03/16/13 11:46 AM)


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