Bhatinov mask - wavy line problem

[Uranium235]

Hiya,

Im just wondering why my B-mask for the ED80 at certain exposure lengths (short), no longer makes a classic diffraction pattern. Instead, the lines arent straight anymore (kinda curved) making it tough to judge when the focus is perfect.

Is it that the mask is of defective design?

Thanks for any help

[Bizibilder]

Are you using the reducer? If so the mask may not be set up for that focal length. I don't know what effect this would have but it may help?

[Uranium235]

No reducers, the only thing in the way is a filter. At first I noticed it only with the L filter, but I can get it to do it with the RGB filters also. However, it doesnt happen with NB filters.

[Psychobilly]

With filters I have seen the lines break down into discrete "dashes"...

[Uranium235]

Hmm... I will save a snap of it next time it happens. Its hard to describe exactly what it looks like. Probably the best description i can give is its the same pattern you would get if you placed an ED80 mask on an ST80.

Its a scopetechnix mask, so I at least expected it to do the job properly. Im just wondering if using subframes is a cause... it shouldnt be, but it always occurs once I go to 1x1bin using a subframe around the star.

I might be in the market for another mask then, one that lets in a bit more light (and works properly) would be nice.

[Uranium235]

Just been out and re-produced the problem. It was clear enough to see a few bright stars, so I grabbed a few snaps of Rigel with the mask on. The only thing in place is a blue filter.

A mix of 0.5 and 0.2s subs, 1x1 bin. Somethings not right....

Any ideas?

[Psychobilly]

The design calcs for the masks i used to use give you a slight "tolerance" to the FL and the same mask can be used at "odd" multiples and sub multiples of Focal length...

When I made these a few years ago I ended up with around 50 different designs... the largest being for a 36" f15 refractor ...

Peter...

[Uranium235]

Its original description was "80mm SFL (short focal length) refractor: One mask fits all 80mm including, Skywatcher ED80,Celestron ED80, Meade 5000 ED80, Revelation D80, Vixen 80 etc"... except that it doesnt fit all...lol.

Im gonna check out fleabay again for another one, theres a pretty good seller on there who makes masks (morris engraving), the one I got for the 150P from there was perfect.

[Psychobilly]

Purely out of interest whats the Slot and Bar widths?

For an ED80 First Order mask I would expect 3-4mm and for a Third Order one 9-12mm

Here's a PDF of the ED80 mask I used to make...

https://dl.dropbox.com/u/8191904/11_SkyWatcher_ED80_FO_V1-Model.pdf

Peter...

[Uranium235]

Just measured it:

Slots: 2mm

Bars: 1.5mm

Cheers

Rob

[Psychobilly]

I would have expected both to have been the same...

I am "playing" with soemthing different at the moment in an attempt to get a mask that works with short fl zoom lenses with a 3:1 zoom range.....

The width decrease with Focal length...

Peter...

[Uranium235]

Nope, just checked it again. The slots/bars are definitely different. But before I buy another I'll check with the seller in regard to the measurements used.

The slot count I have at the moment is:

10 angled slots per lower quad.

19 vertical slots in upper half

Yours looks a lot less complicatd with 6 & 13 slots, wish I had the materials to make (a good) one!