Dew Shield for Stray Light Reduction

[DanLXIX]

Is there any benefit to adding a dew shield like this one to my 150PDS to help reduce the chance of light artefacts from neighbouring lights?

I realise these are more for dew control on refractors, but do they help with light on a newt?

[John]

I use something similar on my 12 inch dobsonian. Its one of the half moon shaped ones to stop light getting onto the top of the secondary and inner end of the focuser. I've found it very effective. My light issues are from one general direction. For more all around problems a tube shape would be good.

Edited March 26, 2021 by John

[Barry-W-Fenner]

For a time I used an AstroZap Dew Shield on my then 8" dob. It certainly helped eliminate stray light from the neighbours. and I could see an improvemen in contrast also as I asked my wife to remove the shiled while I was at the EP.

The only downside I personally found was the length of the shield did make the front end of the scope heavy enough to unbalance it on certain occasions.

I have been looking around for something similar to @John for my 12" dob but I believe this Astro Essentials?? model is no longer produced. I feel that the half circular shiled would help with the balance more. there is actually a light shield on FLOS web page but it is a lit of money for what it is and I personally dont like it asthetically.

I was just yestreday looking for some dark mat black material and velcro to maybe try and make my own.

Cheers

[Barry-W-Fenner]

Here is a shield I knocked up out of some card and left over flocking material for the little fella.  Not the best but again certainly improved the contrast.

 

 

[CraigT82]

Key thing (for me at least) is for it to be as light weight as possible as adding weight to the very end of an already longish and heavyish tube is the absolutely last place you want to be adding weight.

I used thermal radiator foil and flocked it on the inside to make mine, extremely light and damp/water resistant too.

The branded shields you can get are nice and will last a long time but are just too weighty, not to mention expensive for what they are.

[Barry-W-Fenner]

6 hours ago, CraigT82 said:

Key thing (for me at least) is for it to be as light weight as possible as adding weight to the very end of an already longish and heavyish tube is the absolutely last place you want to be adding weight.

I used thermal radiator foil and flocked it on the inside to make mine, extremely light and damp/water resistant too.

The branded shields you can get are nice and will last a long time but are just too weighty, not to mention expensive for what they are.

Sounds like a great solution Craig. Do you have any pics?

[CraigT82]

6 minutes ago, Barry-W-Fenner said:

Sounds like a great solution Craig. Do you have any pics?

Thanks, only this one I'm afraid, it's quite short as I didn't have much of the radiator foil left. Not pretty but it works! 

[Tiny Clanger]

Random thought ... I went shopping in tesco earlier this week, and noticed they had folding car windscreen shades of aluminium type material very simlilar to the reflectix (I think that's the trade name) insulating stuff but with smaller 'cells' , and a sort of bound edge. Might be an alternative to buying a roll of rad. insulation, and the bound edge might make for a neat shield if you cut it to size leaving that existing edge on one side. 

Unless of course you want to insulate a radiator or something too ... 🙂

Heather

[John]

You can get thin foam foam sheets in hobby retailers that would probably have enough stiffness to make a suitable shield for a 150mm aperture.

I needed to keep the top end weight down on my dob as well because of balance so I'm pleased that my half moon shield (which is an Astro Engineering one) weighs very little.

 

[DanLXIX]

If you are making one, how long would you aim for? Is it a case of the longer the better? 

[AlexK]

It depends on the Newtonian construction and the nature of your local light pollution. Some Newtonian models have quite a serious distance from the edge of the OTA to the secondary and focuser "integrated" already. Some observing directions may expose the OTA internals to a discrete source of light in the background. Some observers don't get dark adapted enough to be able detecting the difference. So I would just make some makeshift shielding first and observe the change in the view. Then go fancy.

Keep in mind that the OTA size is engineered with the specific field of view in mind. Extending the edge without drafting it out may introduce vignetting.

For a small aperture like 150mm it is easy to 3D print a ultra-lightweight mathematically perfect full baffle or a segment of it. The 1D length extension is usually enough for a typical Newt. But up to 2D is still not totally insane.

On a side note, in a heavily light polluted environment and for bright targets (like the Moon and bright Planets) the OTA flocking on the inside is highly beneficial as well. The same is true for the inside of that DIY baffle. Thus "reflectix"-like and any other shiny polished/sealed materials are not good for its implementation or would require an additional resurfacing.

Edited March 27, 2021 by AlexK