Moon Filter vs. Aperture Reduction

[spaceslug]

Greetings 

I've been a lurker for quite some time and have found this to be a fascinating and useful website. I freely admit that the recent (and entirely understandable) policy change on the classified section has encouraged me out into the light (dark?) and to ask one or two probably daft questions. 

Having found some of the technology associated with astronomy irritating at times, I've recently acquired a Skyliner 250 manual dob to experiment with going back to the basics of the hobby. Very impressed with the optics for the money, but last night I was looking at the moon and it was the brightest I'd ever seen it in a telescope view; far too bright in fact. I put the end cover on and removed the cap on the small aperture to reduce the amount of light coming in, which seemed to preserve the detail although gave the rising moon a bit of a yellow tinge. I was wondering if this was the best way to reduce the brightness, or whether a decent moon filter would do a better job?

Thanks in advance

Mike

 

[Ricochet]

In my opinion the best option is to invest in binoviewers so that both eyes receive the same brightness. 

The best filter to buy is the Baader neodymium, but you can also get a good neodymium direct from china quite cheaply. 

Only removing the small cap will significantly reduce the resolution of your telescope and is not something I would do. 

[johninderby]

The Baader is my favourite but I have compared it with a Svbony moon & skyglow filer and  was surprised at how usefull the Svbony was. Yes it’s a poor second best but still worth having if you haven’t got the Baader.

Edited September 25, 2021 by johninderby

[John]

In my view it's better to filter but retain the full aperture of the scope. Reducing the aperture reduces resolution so the fine details that a 250mm scope can show will be lost. The aperture in the dust cap of the  250PX is only about 52 mm in diameter I seem to recall so that is a lot of lost performance potential. Not to mention a rather bulky 52mm aperture scope !

Personally I don't use a filter to observe the moon even with my 12 inch dobsonian but I know that some find it preferable to use one. There have been some good suggestions made on which filters might serve well.

 

[Louis D]

I agree that using binoviewers is your best option.  Remember, the full moon is only as bright as asphalt at noon on a sunny day, so your eyes should have no issues with its brightness if you use both at the same time.  I find viewing the full moon with binoviewers to give very rewarding, highly detailed, non-blinding views.

[Carbon Brush]

Hello Mike.
First of all, there is no such thing as a daft astronomy question.
Good advice in the earlier posts.

As the moon is very bright, you can afford to share the light between two eyes, which makes for very pleasant viewing.
Keeping the full aperture allows high magnification.

If you go for a big lump of binoviewer sticking out of the top, you might want to look at a balance weight somewhere below the pivot point, rather than stiffen the mount.
Being a skinflint I can suggest a magnet on steel parts or can of baked beans and bungy cord.
OK, being serious, after spending on a binoviewer and eyepieices, there is no need to spend on expensive weights when low cost solutions work just as well.

Keep asking, David.

[Kon]

You could also get a cheap Variable Polarising Moon Filter; I do not use it since I am happy to look even at full moon directly with my 8" Dob. On the other hand, my wife prefers looking at the full moon with the filter on and the fact you can filter how much light comes through is a nice option.

https://www.firstlightoptics.com/moon-neutral-density-filters/astro-essentials-variable-polarising-moon-filter.html

 

[spaceslug]

12 hours ago, Kon said:

You could also get a cheap Variable Polarising Moon Filter; I do not use it since I am happy to look even at full moon directly with my 8" Dob. On the other hand, my wife prefers looking at the full moon with the filter on and the fact you can filter how much light comes through is a nice option.

https://www.firstlightoptics.com/moon-neutral-density-filters/astro-essentials-variable-polarising-moon-filter.html

 

Interesting. I like the idea of a variable polarising filter, I've used one on my DSLR camera previously for standard terrestrial shots.  I may be about to challenge the "no daft question" comment though; if this filter fits on the inside base of a 1.25" eyepiece like other moon filters, how do you control the variability when it is attached to the telescope?

Thanks in adv.

[spaceslug]

 

Thank you everyone for the useful replies and info, much appreciated.

I will look into the variable polarising filter as I quite like that idea and it looks to be a very cost-effective option. 

I had toyed with the idea of a binoviewer as well so this may be the excuse to invest in one!

Thanks again.

Mike

[Ricochet]

8 hours ago, spaceslug said:

Interesting. I like the idea of a variable polarising filter, I've used one on my DSLR camera previously for standard terrestrial shots.  I may be about to challenge the "no daft question" comment though; if this filter fits on the inside base of a 1.25" eyepiece like other moon filters, how do you control the variability when it is attached to the telescope?

Thanks in adv.

Option 1: use two 1.25" filters on the bottom of the eyepiece, the lower of which is able to freely rotate. You have to remove the eyepiece to adjust the angle of the filters. 

Option 2: buy a 1.25-2" reducer that has 2" filter threads at the bottom of it, for instance I have the Baader Clicklock. You then put a 2" filter on the bottom of the reducer and a 1.25" filter on the bottom of the eyepiece. You can then adjust the filter by turning the eyepiece. If you do this with binoviewers you have to loosen off the clamp on the reducer and the clamp holding the reducer so that you can rotate the reducer as you cannot rotate the binoviewer. You also have to ensure that the barrel of anything you insert into the reducer does not protrude far enough to come into contact with the lower filter. 

However, I found that reflections created by a pair of polarising filters scrub detail from the image and so they no longer have a place in my eyepiece case. 

[Kon]

9 hours ago, spaceslug said:

if this filter fits on the inside base of a 1.25" eyepiece like other moon filters, how do you control the variability when it is attached to the telescope?

Yes it does filter at the 1.25" barrel and unfortunately you have to remove the eyepiece (not the filter) every time you want alter amount of light and you just rotate it. When i setup for my wife, as a trial and error, i look at the moon holding the filter either with or without the EP and roughly adjust it and then put it in the EP/focuser.

 

9 hours ago, spaceslug said:

I will look into the variable polarising filter as I quite like that idea and it looks to be a very cost-effective option. 

For ~£20 it is worth trying and see how you get along with it.

Edited September 27, 2021 by Kon

[etsatlo]

I use binoviewers a lot and the moon is still very bright when near full. 

Recommend picking up a 0.96 Moon filter from FLO, only £9 and makes it far more comfortable. 

Can't quite imagine not using a moon filter as I find it equivalent to having the optician shining the light into my eye to examine my retina!

[Tiny Clanger]

10 hours ago, spaceslug said:

 

Thank you everyone for the useful replies and info, much appreciated.

I will look into the variable polarising filter as I quite like that idea and it looks to be a very cost-effective option. 

I had toyed with the idea of a binoviewer as well so this may be the excuse to invest in one!

Thanks again.

Mike

I've found the variable  pol. a faff in my dob, you have to rotate the two filters with respect to each other, which means removing and replacing the eyepiece to twiddle the filters . In a 'scope which has a diagonal you can separate the two polarisers, remove the diagonal and screw one filter to it, put the other on the eyepiece, then just turn the eyepiece to rotate one filter, which is convenient, but you can't do that in a dob .

So I'd suggest either a Moon filter(some of which have a greenish tint I don't much like) or an ND which is neutral as far as colour goes.

Heather

[johninderby]

Photophobia (light sensitivity)

Photophobia is a symptom of light intolerance, where the eyes are very sensitive to certain levels of light. Also known as light sensitivity, it can involve an uncomfortable or painful feeling in response to sunlight, or indoor lighting like fluorescent or incandescent light bulbs.

Probably explains why some need a moon filter and others aren’t bothered at all and don’t need one.

Edited September 27, 2021 by johninderby

[spaceslug]

Thanks again for all the replies.

Despite it being a bit of a faff to adjust, I think I'll try the variable polarising filter to start with and see how I get on.

Mike