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How bright is too bright? and Miras in moonlight?

mdrapp's picture
mdrapp
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Joined: 2011-12-20

Hi all,

I've been reading on the forums and in other places about using an appropriate aperture for a variable; that is, not using a telescope that renders the variable too bright.  The problem is, I'm having trouble deducing what an appropriate magnitude range is for a given telescope.

While I am fortunate to have access to several telescopes, due to sheer convenience it seems that my 66 mm f/6 refractor is becoming my variable star scope of choice -- its light weight (I can move around the yard with ease during a session to avoid trees) and gives a generous field of view.  In some sense it's really one-half of a really high quality pair of binoculars.  Indeed, I'm probably going to use it for the AAVSO binocular program and I think its safe to say that the variables in that program have been preselected for typical binocular apertures (50-80mm).

However, I also routinely use my 4" f/10 refractor and 8" f/6 Dob, often on weekends.  For these scopes, how bright is too bright?

On my second topic, I seem to remember reading --- somewhere -- that Miras shouldn't be estimated when the Moon is up as the moonlight combined with their red color can make estimating difficult, especially for beginners.  Is this correct?   If it is, is there a certain class of star I should seek out on those nights between new moon and first quarter?  (I tend focus on the moon and planets after first quarter as there aren't enough stars left to star hop!)  My NELM is 4 on a dark night, so this might not be doable with my skies and equipment.

--Michael (RMW)
Dickinson, Texas

How bright is too bright?
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MDAV
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Joined: 2010-07-23

Mike- A complex question. Although there is no iron clad rule there are a couple of rules of thumb.

The first is to target variables that are a couple of magnitudes brighter than the instrument limit. It gives you star images that are bright enough to see comfortably but not so bright as to dazzle or saturate the eye. 

The second is to avoid having stars so bright that colors "color" your observations. Color is a red flag and cause for caution. Ideally-which is rarely the case- the comparison star and the variable should be comparable in color. This is one of the factors the sequence team deals with in selecting comparison stars. But especially with a star with an intense red color it may not be possible to find those.

The star R Leo is a good case for both.

Just as an exercise- download some photometry tables for R Leo in the LCG and look at the B-V color indexes. The higher this number the redder the star. R Leo has a B-V of 1.3 which itself changes during the cycle. The 57 and 92 stars on your 13318BNA chart are the only ones even near R Leo in terms of color index.

As you saw- right now it is near minimum but even then you noticed how red it was and how much contrast there was between it and the comparison stars. At maximum it not only saturates my eye but can be so intensely red that getting a reliable measurement using my 12" dob's full aperture is virtually impossible. Absolutely a gorgeous sight visually but that's not what we are after here. I go to the finder or use a stop-down mask to bring the apparent brightness down to a reasonable level. 

My other suggestion is that you use the LCG or the search option in WebObs to check your own observations against others. R Leo is a very well observed star and you won't have any trouble finding recent observations. Uncheck all the observation types but the visual box. 

Have Fun

Dave M

bright moon
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SET
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Joined: 2010-07-23

Hi Michael,

I'm glad to see you are visually observing variable stars. It is a great hobby and a wonderful way to contribute to science. I have always observed variable stars whether the moon is bright, or no moon at all. You will loose some dimmer magnitudes due to bright moonlight, but it's no problem at all to observe long period stars that tend to be redder.  Good luck with your observing, and if you need any assistance, you can email me directly at     rcobservatory@gmail.com


I am on the Mentoring Team, so I would be glad to mentor you.

 

Chris Stephan    SET

Robert Clyde Observatory

Wooster, Ohio

Instrument and Moon
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Sebastian Otero
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Joined: 2010-09-19

Hi Michael,
I've always found the rule in Levy's Observing Variable Stars book to be confirmed while observing: the best range for me is 1 to 4 magnitudes above the limiting magnitude.
E.g. From Buenos Aires I have the following limiting magnitudes and best mag. range:

Naked eye  5.5 = 1.5 - 4.5
7x50 binos  8.8 = 4.8 - 7.8
8" reflector 12.8 = 8.8 - 11.8

Of course, this varies slightly with seeing conditions but I can say that the most difficult stars for me to observe are those falling inside the gaps between instruments:

~4.7 mag. stars too faint for easy naked aye observation and too bright for binoculars.
~8.3 mag. stars too faint for binoculars and too bright for my telescope.

This also has to do with the availability of comparison stars since for bright stars you need wider fields or you might lack proper comp stars.

About the presence of the Moon and red variables, I have to agree that it plays a role. It has to do with the background sky, which will be much brighter when the Moon is up. This also happens in a bright city sky in hazy nights when you have more street lights being reflected in the haze and the sky also becomes brighter. Red stars will appear brighter by contrast with the white sky. The problem is that whiter stars will get lost among that background light, so the different effect depending on color will cause a larger deviation from V the larger the color difference between comp stars and the variable is.

I wouldn't go as far as saying that red variables are banned on moonlit (or hazy) nights but at least be careful with these effects and try to use the reddest comp stars available to minimize it. Also try to avoid the stars very close to the Moon. And as usual, beware of the Purkinje effect by using quick glances to the target.

Cheers,
Sebastian

Observing manual
lmk's picture
lmk
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Joined: 2010-07-23

mdrapp wrote:

I've been reading on the forums and in other places about using an appropriate aperture for a variable; that is, not using a telescope that renders the variable too bright.  The problem is, I'm having trouble deducing what an appropriate magnitude range is for a given telescope.

On my second topic, I seem to remember reading --- somewhere -- that Miras shouldn't be estimated when the Moon is up as the moonlight combined with their red color can make estimating difficult, especially for beginners.  Is this correct?   If it is, is there a certain class of star I should seek out on those nights between new moon and first quarter?  (I tend focus on the moon and planets after first quarter as there aren't enough stars left to star hop!)  My NELM is 4 on a dark night, so this might not be doable with my skies and equipment.

-

Hi Michael,

Your questions are fairly common ones. Most of them can be easily answered by reading the appropriate sections of the AAVSO Visual Observing Manual. This can be found from the main dropdown menus on the top of the AAVSO website "Observing" -> Observing Manuals -> Visual Observing Manual. Chapter 3 "Making Observations" in particular addresses your questions.  The direct link is here -

http://www.aavso.org/sites/default/files/publications_files/manual/engli...

Now, I can understand scanning through a large detailed manual can be somewhat daunting, but keep in mind, this manual represents the best compilation of experience and recommendations of many, many experienced observers over the hundred year history of the AAVSO! It should be considered the best source of information for visual observers. And, it's sometimes wise to be wary of "advice" given in various other forums, as you never know where that "information" is coming from.

That said, I would like to point out that typically too much fuss is made over what size telescope to use. Obviously, you need as much aperture as you can get to see what stars you want to observe under you particular conditions. Bigger is better! Probably the best technique to use to cover a wide range of brightness is the defocus method. Bright stars can be rendered faint enough by sufficiently defocussing them to disks, instead of sharp points, as is necessary when you are working at the faint end of your instrument limit. Actually, comparing brightness of disks is easier than small points! So, using defocus, I can easily estimate 7th magnitude stars using my 20" reflector. I know some seasoned observers cringe at this suggestion, but in fact it works perfectly well. The real issue with estimating bright stars with large apertures is the field of view is typically too narrow to get the comp stars in the same field. Thats the main and only real problem. By defocussing sufficiently, even bright stars can be made faint enough in a large instrument, that you no longer see color, and the Purkinje effect is removed.

Regarding Miras under moonlight, I dont think that would be a particular problem. Moonlight is similar to light pollution in reducing contrast and whitening the background, but you are still comparing stars to other stars, of fairly similar brightness. And this is a very important point - You need to choose your comp stars to be fairly close to your variable in brightness, and bracketing the variable, one brighter, the other fainter. If the spread is too much between the comps, the estimation gets difficult and less accurate. And absolutely, avoid "extrapolating" outside a range of comps. Estimation becomes quickly inaccurate if you are not bracketing the variable with your comps brightnesses.

So, don't worry so much about your aperture. Use the biggest scope within reason, defocus as needed, and read the Observing Manual!

Best of Luck.

Mike LMK

size of scope and bright moon
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SET
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Joined: 2010-07-23

Michael ,

 


I agree 100% with everything Mike has said. I use the out of focus technique, it gets rid of all the red in stars. Just don't defocus to much, just slightlyly so the stars are small blobs. I have used a 14" telescope to observe 7th and 8th magnitude stars, all the way down to close to 15th mag.

Again, with the real bright moon, I would pick stars that are not close to the moon. You have 75% of the sky left to observe when there is a full moon up there, so stay in those areas.

 

Chris Stephan    SET

Thanks all!
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mdrapp
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Joined: 2011-12-20

Thanks all for the detailed replies!  I'm going to re-read the Visual Observing Manual.  I had read it, but it has been quite some time.  It'll be a great refresher.  I actually had forgotten about it!

I'm also happy to know that I won't have to pack it in on moonlit nights (of course, finding stars does get harder the closer it is to full moon).  

Tomorrow, which is forecast to be clear, I'm going to try for Z UMa and perhaps one or two other stars in or near the Big Dipper.

--Michael (RMW)
Dickinson, Texas

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484