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Focussing through a DSLR.

Sierra Golf Mike

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Sierra Golf Mike

Sierra Golf Mike

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Apologies if this is a stupid question. I have just started telescope astronomy and have bought a Skywatcher Explorer 150P which came with a camera adaptor so I just need to get a T-Mount. As my Canon 30D does not have "Live View" am I correct in assuming that to focus you just do it through the camera eyepiece as normal?

Cheers

Steve

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brianb

brianb

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am I correct in assuming that to focus you just do it through the camera eyepiece as normal?

Yes... but it takes a lot of care to get the focus reasonably accurate.

A focus confirmation type adapter is helpful, a replacement focus screen designed for manual focus telephoto lenses more so. Not sure you can get the latter for the 30D.

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MG1

MG1

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I found the canon c angle adaptor really handy with my 30d as it magnifies the view enabling a more accurate focus. It also enabled a more comfortable viewing angle.

I'd also recommend making a bahtinov mask as this seems to be by far the easiest way to confirm that focus is spot on.

If you search the forum you'll find plenty of links on how to nake one, or if you don't have the time, you can buy them for around £30 I think.

Mike

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MG1

MG1

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a replacement focus screen designed for manual focus telephoto lenses more so. Not sure you can get the latter for the 30D.

Unfortunately this can't be changed on the 30d (unless there is a third party mod I'm not aware of?)

I've just checked the manual and it shows the angle finder c on p186. But as suggested third party ones are significantly cheaper and I guess just as good.

Hth

Mike

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Steve 1962

Steve 1962

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Hi there

I use a Hartzman mask - see the attached link for a video.

Andy's Shot Glass - Hartzman Mask, Focus on Deep Space Objects

I use two triangular holes rotated 60 degrees relative to each other instead of circles and I good results.

I made mine from an cereal packet covered it in sticky back plastic - so you can try it for free. I left three tabs on the outside and just hold it onto my scope with an elastic band

If you want to see what mine looks like - I'll post a picture

Hope this helps

Regards

Steve

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glowjet

glowjet

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Another alternative to the Bahtinov Mask is the Y-Mask, a simple solid Y will give you the same precise focusing as the B-Mask, the thicker you make the arms and vertical of the Y, will give you brighter but shorter diffraction spikes, the advantages are it is easy to make and has more light throughput > 90% compared to the B-Mask which is < 50%, a difference of about an f-stop, the B-Mask being similar to a ronchi grating,

HTH.

John.

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Demonperformer

Demonperformer

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I use two triangular holes rotated 60 degrees relative to each other instead of circles and I good results.

Steve

What sizes did you use? Diameter of scope? Length of triangle side? This could cut out (if you'll excuse the pun:D) a lot of trial and error for me.

Thanks

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Steve 1962

Steve 1962

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Hi DP (and all)

I use Hartzman masks on both my scopes - pictures are attached.

The mask for the 70mm is about 95mm dia. (because it fits over the dew shield and each triangle is 30mm on the side and set 10mm from the centre of the mask.

The mask for the 100mm is 122mm dia. with triangles 40mm on the side and set 12mm from the centre.

Although they are differing proportion - both work a treat and if you cover then in sticky back plastic they're quite robust.

I tend to delete out of focus images so I haven't got any to post - but if you zoom into a star - out of focus and focus is pretty obvious. Out of focus gives you two seperate or displaced triangular stars and focus gives you one star shaped star (i.e. two overlaid triangles) with uniform diffraction spikes.

Hope this is useful.

Best wishes

Steve

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Sierra Golf Mike

Sierra Golf Mike

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DP,

No problem, I was intending to ask for more detail myself.

Steve,

I take it that the mask is used for focussing, then removed for imaging? It may be stating the obvious but being a newbie it is best to ask. Also do I just scale up for a 150 scope?

Cheers

Steve

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Steve 1962

Steve 1962

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Crikey - it's like a scene from Blue Peter out there!

Steve - yes - you'll need to remove the mask after focussing - and yes - you can scale it up

Ian - I'm with you on the ease of cutting - I did mine with an old modelling knife - I also think the Hartzman is more robust in use

Steve

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Demonperformer

Demonperformer

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I guess these two pictures speak for themselves. Conditions not good, the wind was having serious effect on scope stability. Decided to give it a really extreme test, so used Antares (bright and low down), using 128x sensitivity.

'Focussed' image may not be wonderful, but the important thing is that it is obvious when the two triangles become one splodge. Star looked a lot better when (a) sensitivity reduced, and (:( mask removed.

All in all, I would classify it as a success.

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simon84

simon84

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Wish I'd found this thread sooner. I just made a b-mask for my 80ed and the cutouts are 1.7mm and there were 39 of them, I made it from laminating the prinout on photo paper, cut the slots, then covered both sides with sticky backed black velor so had to cut all the slots out again. Took ages.

The triangles look great!!

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glowjet

glowjet

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With humble apologies I do get the impression that the Y-Mask that I have mentioned on a few occasions in the past is being overlooked in favor of more established masks, however, whilst researching for information on the Bahtinov Mask for an article in my clubs magazine, the Cloudy Nights web site revealed some quite startling facts supplied by an English Astronomer in June of last year which detailed information on the Y-Mask and I am indebted to Chris Lord for his PDF primer on Fruanhofer Diffraction spikes and the Bahtinov Mask, http://www.braybrookobservatory.org/BrayObswebsite/HOMEPAGE/Forum.html

If anyone wishes to read the paper it explains how and why the Bahtinov Mask functions and the fact that any solid bar placed across the aperture of a telescope, in any position, will produce a diffraction spike perpendicular to the bar but, resulting in a spike that always lies on the optical axis, the few USA astronomers on the web at the time agreed with the findings, even one to the point that the Y-Mask was an improvement as it was, of course, far easier to make, furthermore the Y-Mask produces a brighter image by an f stop for those of you thinking about making a mask for your telephoto lens, if you find the diffraction spikes not bright enough then the rule here is any parts of the Y can be made wider as this will produce shorter but brighter spikes, if your resulting image has a bright fine center spike but the side lobes are not, then thicken the two arms to suit, they might not be as long but should have the same luminance, the finer spikes that the Y-Mask produces will enable absolute precise focusing.

John.

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MG1

MG1

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Hi John,

Does the y mask produce a moving diffraction spike that enabled fine tuning in the same way as a b mask?

If so then it's a no brainer really. I'll have to try this one out.

I'm assuming that the y shape is the obstruction? If so, how come thicker obstruction makes brighter diffraction spikes? Apls if this is explained in article, am having trouble opening that on my phone.

Cheers,

Mike

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Demonperformer

Demonperformer

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Never let it be said that I am not prepared to experiment. Following on from Glowjet's post, I have now produced a Y-mask for my 4" and with clear skies promised for tomorrow morning (DOH! - that's blown it!), will try it out then.

It was just about as easy to produce as the two triangle mask, but my concern is the durability. It is made it out of two sheets of stiff black card stuck together. However, the pointy-bit, between the two upper arms of the 'Y' strikes me as its weak point. I can see this getting bent, broken, torn. OK - no real problem making another one, but unless it performs much better than the two triangles, I suspect the replacement will be one of those as/when needed.

But maybe I should see how it performs before making that call. Will report back when tested.

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