Bahtinov masks - hood/dewshield on or off?

[troypiggo]

Most of my experience is with an ED80, and I leave the hood/dewshield on it all the time. I've been putting the Baht mask on the front of the dewshield to date. The diff spike pattern is noticable, but have to zoom in to see it clearly.

For the first time at Astrofest I imaged with my C8, and put a mask on it right at the lens with dewshield off. The diff spikes were massive, very easy and clear to see.

Wondering if this is simply a function of the C8 collecting more light, or rather because I put the mask right at the lens rather than some distance in front of it.

It only occurred to me to ask this since I got back so haven't had a chance to test it myself. Plus, the ED80 hood is a little loose and I have it duct taped in place so removing it every time would require a slightly different approach.

[Dennis]

The difference is due to the C8 having a significantly longer focal length. Plug a x2 Barlow into the ED80 and the diffraction pattern will double in size.

Cheers

Dennis

[troypiggo]

I see. Thanks.

[Dennis]

Hi Troy

Its similar to terrestrial photography, a 200mm tele lens will give a larger image scale than a 28mm wide-angle lens, albeit at the expense of a narrower field of view.

So, the longer the focal length of your telescope, the bigger the image scale which will produce a more magnified view of the Bahtinov diffraction spikes.

If you plug say, a 20mm focal length eyepiece into your ED80 and then the same eyepiece into your C8, forgetting the differing fields of view, the image scale (magnification) will be bigger in the C8 due to its longer focal length.

On the issue of where to place the Bahtinov Mask, I usually place it on the naked optical tube ( i.e. minus the dew shield), although I did once do a quick test, placing it on the dew shield and it appeared to work/focus just as well. My dew shield is a homemade foam tube that easily slides off and my Bahtinov Mask sits more snugly on the naked OTA hence I focus with the dew shield removed.

If the ED80 has a rigid dew shield that will support your Bahtinov Mask, then I’d fit the mask to the dew shield to avoid fiddling around with moving/re-fitting the dew shield.

Cheers

Dennis

[troypiggo]

Cool, thanks for confirming all that.

[jjjnettie]

I thought you had to leave the mask ON when you image.:confuse3: At least that's what a couple of Mod's did at Astrofest. :poke: :lol:

[troypiggo]

LOL I can't talk. I did it too! - fortunately for me Geoff noticed before I did my second shot.

[jjjnettie]

Not exactly sure how long they left theirs on for. But I know it was quite a while. :doh::P:lol:

[renormalised]

That OTA should be ashamed of itself going without clothing in public!!!:P:P:D:D

[mithrandir]

It's such a pain to get my dew shield off the C8 that I don't. It works as well on the shield as it does on the front of the tube.

The dew shield on the ST80 isn't supposed to come off - but is loose enough that it sometimes falls off - and the mask works fine there too.

[citivolus]

I heard one "pondered" the issue for about an hour :lol:

[jjjnettie]

tee hee