Whitish glow in eyepiece

andreas.jordan (Andreas) · #1 19-01-2010, 09:38 AM

Hi All,

I was looking at the new moon and Jupiter last night and noticed that within the eyepiece there was a whitish cast over the entire field of view. I am certain this was some sort of optical affect (vs dirt or fingerprints on the lenses).

Any ideas?

mental4astro (Alexander) · #2 19-01-2010, 11:04 AM

If the mirror of your dob is clean, then the most likely cause is internal reflections within the eyepieces. Good quality eyepieces are marked with "Fully Multi-Coated". They will also have the edges of the lenses blackened. All this goes to reducing these reflections. If they only state "fully coated" or make no mention of coatings, then this is the most likely cause.

There are three solutions:
1, Use a Neutral Density filter to reduce the amount of light from bright objects

2, Invest in better quality eyepieces. These are not necessarily expensive. Andrews Communications has a range of GSO plossl eyepieces that would do the job admirably until you are prepared to spend a little more.

3, You might also like to make a mask for the dob, reducing the apeture of your scope, there by increasing its focal ratio and reducing the glare on bright objects. It will also improve the image quality with the increased focal ratio. Make the mask out of waxed cardboard that you can find at fruit and vege shops as these are water resistant- good for when the night begins to dew up. You can start by making the apeture 4".

Mental.

andreas.jordan (Andreas) · #3 19-01-2010, 11:36 AM

Hi Alex,

Good call. I plan on investing in a couple GSO eyepieces shortly. I just want to work out exactly what I need insofar as filters go for cutting down on Light pollution.

The majority of my viewing will be done in my back yard which is only about 3-5km's out from the city so lotsa light pollution. Another thread mentioned that instead of "standard" light filters that there may be others meant for something else but work excellently for light pollution.

(I'm waiting to work out what eyepieces I need so that I can mail order them from Andrews)

mental4astro (Alexander) · #4 19-01-2010, 12:18 PM

There is already a thread talking about filters, but I'll follow on from it.

I've never used GSO filters. The only one I've used is my Lumicon 'Deep Sky Filter'. Light pollution filters are used to reduce the light continuum but selectively allow specific wavelengths through. In this case the ones at which emission and reflection nebulae shine at. You can also get filters that are even more selective to either one of these two types. What they don't do is allow easier viewing of anything else as everything else shine at ALL wavelengths, including galaxies. In light polluted areas, and dark-sky sites, these filters also help the same way, and so are used in both sites.

I too live in a very light polluted area in Sydeny, but I may or may not use my deep sky filter. Depends on what I'm looking at for the reasons mentioned above.

Coloured filters are best used with the planets to bring out specific details. No good with anything else.

Look at the Lumicon filter site to see what each type of filter does before you buy any. With colour filters, limit yourself to two, no more than three, as you really will not use them verymuch at all. Really. One I can suggest is a No.80A Blue to help bring up the Great Red Spot on Jupiter as it quite hard to see right now. It is a pale salmony-pink colour.

http://www.lumicon.com/astronomy-acc...d=1&cn=Filters

Eyepiece wise, the GSO Superview's are excellent for their cost. I have the 30mm (2") and 15mm (1.25") which I nearly exclusively use. If your focuser is only for 1.25" EPs, the GSO 32mm super plossl is still OK. For high power, unless you are prepared to spend more than $100 on a single EP, I'ld suggest a 9mm super plossl and a 2X barlow from the GSO range. Any EP shorter than this, in this price range, starts to become really difficult to use. I don't really like barlow lenses, but they have a place. For me they just put more glass between me and what I'm looking at and hence reduce the precious light, just my humble opinion.

I've underlined my suggestions. Note that the Superview 30mm is a 2" barrel size, so you might need to take the 32mm instead.

andreas.jordan (Andreas) · #5 19-01-2010, 02:01 PM

Thanks Alex. I have read through the at least 2-3 other forum threads regarding eyepieces and I must admit it hasn't quite sunk in properly. I have a 6" dob which has a 2" inch focuser with a 1.25" adaptor so I could go either way.

I'm looking at purchasing the following(at least they're on my wishlist):

GSO SUPERVIEW 68° - GSO Superview 30mm at $69.00 ea
SV 15mm (4-el) $49.00 ea
#80A Medium blue 2" filter(AD116) $25.00 ea
Moon filter (mid green) (AD068) $12.00 ea

...and I was thinking about this filter --->
NEW! UHC filter - narrowest passband of GSO filters, for best results when viewing ring nebulae and most planetary nebulae, 500nm centred $59.00 ea

Note: All prices quoted from Andrews Communications as at 19/1/10

rmcpb (Rob) · #6 19-01-2010, 06:02 PM

One comment on the UHC filter. You may not be too happy with it on a 6" scope. They really cut down the light levels and are usually better in larger scopes. If you can try borrowing one at an observing night before parting with the hard earned.

As for the eyepieces try some at the same time as the filter and see how they work in your scope before buying. They really add up if you are not careful.

Cheers

mental4astro (Alexander) · #7 19-01-2010, 09:05 PM

I agree with rmcpb, try before you buy. If there is an astro club near to you, or try seeking out other IIS members where you live.

I would suggest a 1.25" blue filter, not 2". Most 1.25" focuser adaptors don't accept 2" filters, and the shorter focal length EPs that you need for the planets are all 1.25" barrels.