Silly question about Focusing Masks......

[Craney]

Evening everybody,

This could be a dumb question...... but here goes.......

Are focusing masks Focal Ratio dependent ?? 

What prompted this question was that recently a couple of second hand masks came up for sale where the sellers made a point of detailing the Focal Ratio of the scope it should be used on.   

At first,  I thought it was an irrelevance,  SURELY it is only an aperture dependence, ...... or is it ??  ..... you can never be quite sure in this game.   Looking at the FLO  website  made my uncertainty even more acute.   There are ... in fact  6 different ones for the humble 80mm refractor...     Oh dear, have I been getting this wrong all along,  or is the range of model sizes to compensate for the different size of dew shield.

Would anybody like to shred some light on the matter.....

This is the type of stuff nobody ever tells you.....

 

Cheers,

Sean.

 

[geordie85]

As far as I'm aware they are only aperture dependant 

[Alien 13]

Not silly at all, I have no idea having never used one, I did make a Y mask once almost as good and can be used on any scope or lens.

Alan

[souls33k3r]

Well firstly you will most definitely see masks that you can find on fleabay for your aperture and they will work just fine.

Then you get companies like "StarSharp" who sell masks that are fine tuned to the telescopes focal ratio. FLO sells such masks 

[Tim]

It is a good question

I guess in an ideal world you'd have one dedicated mask for every telescope. However, I go through lots of telescopes for review and for personal use, and only have a limited selection of Bahtinov masks, 4 to be precise. In practice you can get away with using slightly bigger or smaller masks, eg a 6" mask on a 4" scope and so on, as long as the pattern is centralised in the aperture.

I guess an optimised mask may give the sharpest results?????

[Craney]

Phew !!..

I thought my reputation was astro-toast with that question...

..... yes  @Tim   That's exactly what I do....  use a bigger mask on smaller scopes.   A crafty rubber band stretched between two of the three 'feet'  on a StarSharp design causes the mask to hang centrally over the smaller aperture. ....

[johninderby]

In theory a mask should be optimised for the focal length for perfect focusing but in practice you don’t have to be that precise. 

Wonder if the seeing has an effect? Would you need a dark site and great seeing to take advantage of an optimised mask?  ?

http://astrojargon.net/maskgenerator.aspx?AspxAutoDetectCookieSupport=1

[Pompey Monkey]

I have noted difference in focus between different masks on the same scope. I don't know if this was due to incorrect focal ratio or improper centring.

 

[carastro]

I have a mask which was made for a previous scope with 72 aperture, I use it on my 71mm and I made an outer "skirt" for it so it fits my ED80 as well with two sets of bolts on it to rest on the tube making the Bahtinov sit central on each scope.

I have a larger Bahtinov for my ED120 and it also works on my SW130PDS sitting inside the OTA in front of the spider.

Carole