Bhatinov Focus Mask

[Star-Gazer]

I know this is really good for DSO and Stars from what I have read but does this mask work for planetary viewing and imaging?

If so then how?

Thanks.

[Bizibilder]

Yup. Just focus on a star and then move the scope to the planet or Moon without touching the focus.

[Star-Gazer]

Wow - I thought the answer would be no.

So can I use my webcam and the mask together to get the best focus for the moon on a particular night for imaging?

[callisto]

Yes, but make sure you take the mask off while you image

[great_bear]

You don't need to buy a mask either,

A "Y" shaped bit of cardboard is equally as good and costs nothing.

[great_bear]

Here's why:

http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/Fraunhofer-primer.pdf

[Star-Gazer]

Go then great-bear what does the y shaped bit of cardboard do and how do I make it?

[great_bear]

Go then great-bear what does the y shaped bit of cardboard do and how do I make it?

Explained here:

http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/Fraunhofer-primer.pdf

[Star-Gazer]

lol - had a quick look and will read the rest when I finish the engineering degree I haven't started yet!!

[great_bear]

I think the last three sentences of the article are all that most people need to know.

[JamesF]

If I understand the pdf correctly, I could basically make a usable mask by twisting a couple of pieces of wire together and hooking them over the dewshield? (Where by "twisting together" I mean to create a "Y" shape.)

James

[glowjet]

After some research I did an article for my club on the very subject, Astronomers who image or otherwise are quite entitled to buy a Bhatinov mask, but the widening consensus of opinion, after Chris Lord`s PDF primer, is why go to that expense when three strips of black plastic in a Y shape will achieve the same or slightly brighter results, one difficulty is how wide the strips? I have explained this on occasions in the past and you need to go back to friend Bhatinov`s original concept. Simply take the focal length of your scope and divide by 150 or 200 this will give you a max and min value. Example : 1200mm fl divide by 150 will give 8 and by 200 will give 6, without going into the details for the design of the B mask you need to divide these results by 2 so you end up with a final width of 4mm or 3mm, so you experiment with your strips cut to, or including anywhere between the two dimensions. For those with very short focal lengths who may be reading this, to divide by the given figures would result in impractical widths, in which case it is turned into what is called a third order mask by simply multiplying the fl by 3 and continuing from there. The only visual difference is wide strips tend to give shorter brighter diffraction spikes and thin ones longer and slightly fainter, HTH

John.

[Star-Gazer]

I now know the mask will work on planets and stars but does it work with the moon?

[Psychobilly]

To produce the clean diffraction spikes you need a "point" rather than "extended" source... in fact if you use in on the inferior planets you will get multiple phases shown its a cool effect..

SO for the moon you really need to focus on a bright star first and then slew back...

Billy...

[Freddie]

Hi, the point is that the mask will allow you to setthe correct focus for distant objects. All astro objects are the same in terms of focus distance so it is more about using an object that will allow you to easily see when you are in focus rather than needing a different focus for moon, planet, stars etc. Hope that makes sense.

[Star-Gazer]

so how do I know what star to focus on if I want to get focus for the moon or will any star do?

[Psychobilly]

any star...

[Star-Gazer]

Hi Freddie

Thanks I obviously didn't appreciate that but do now!!

So the focus is the same for all but needs to be set up once properly?

I thought that you had to pick a star near the object you want to view but you are saying that just one source of focus is necessary for the whole viewing - whether planets, the moon or dso's - correct?

[Psychobilly]

One of the reasons you tend to pick a star close to the object you interested in is to minimise the "slew" after focussing.. some types of scopes can suffer from mirror flop as they track across the sky...

[Star-Gazer]

Thanks and what is "Mirror Flop" if I might ask?

[Psychobilly]

In some scope designs (sct's & maks) the primary mirror is moved for focusing... and its possible for it to shift slightly... although it may be an urban myth...

[great_bear]

Thanks and what is "Mirror Flop" if I might ask?

If you look at this diagram, it's not too hard to see how any play in the Mirror Slider can cause the mirror to "flop about" when it crosses the point staring straight upwards whilst tracking.

[AndyUK]

(I missed this... and just asked the self same question in another section...!)

John - In your description on Y mask design, I understand what you've written... but is there any rule on the angles in the Y? I think I read in that link that it should be 20 degrees inclined... but (and I'm sorry for being a bit obtuse), does that mean that each of the top arms of the Y need to be angled 20 degrees from the vertical, or there's a 20 degree angle between the two top arms?

[Digz]

...and I'm sorry for being a bit obtuse...

No pun intended?

This is a great read and as someone who was about to source a Bhatinov mask Im gonna give the Y mask a ago.

Andy, Im guessing the angle would be the same as for the Bhatinov i.e. the spalyed slots are 40deg apart, 20deg each from the vertical. Im actually in the middle of drafting some CAD templates for the various masks for my scope, when they are done I can post a link.

[Ad Astra]

Andy, Im guessing the angle would be the same as for the Bhatinov i.e. the spalyed slots are 40deg apart, 20deg each from the vertical. Im actually in the middle of drafting some CAD templates for the various masks for my scope, when they are done I can post a link.

That sounds excellent, Digz!

I'll look forward to seeing it.

Dan