Focussing through a DSLR.

[glowjet]

Sorry for not responding sooner, family arrangements for Easter Monday etc, Steve the article link was copied as quoted, when I tried it I had to put it in short cut and paste it back into Google, it was then ok, anyway its now sorted thanks, Mike MG1 the central diffraction spike moves in exactly the same way as the B-Mask, it translates either side of the central optical axis as you move in and out of focus, DP as yet there are no hard and fast rules about the constructional design and it may be best to use stiff black card or black plastic document folder cover etc, as with the B-Mask there will be a relationship between aperture and focal length which will dictate the width of the arms and leg of the Y, there was some talk with one of the US Astronomers that the side lobes to the X were not as bright which is why it was suggested Parvel B made the mask into a type of ronchi grating in order to build up multiple image orders, these are the little segments which make up the diffraction pattern, the brightest being in the center, this was, however, at the loss of light throughput, the B-Mask < 50%, the Y-Mask > 90%, in 2004 the Russian Astronomical fraternity were experimenting with the focusing issue and subsequently adopted Pavels design, it interesting to note, however, when reading through the threads that a number of Astronomers were having good success with a Y made out of string and wire, as Chris Lord has pointed out to increase the brightness of a spike there is only need to thicken the bar, this will shorten the the spike but give an increase in luminance, the thinner you make them the longer they get but lose brightens, bearing this in mind experimentation is the order of the day, if you come up with a good pattern then make it out of thin mild steel and spray it black, it is needless to say that if everything works out as the computer program ( Fast Fourier Transformation ) dictates it will, then it will be simple and cheap for most imagers to make their own.

John.

[Demonperformer]

Will report back when tested.

Well, as promised, reporting back on my experiences with the Y focussing mask. Attached are the two photos, one when out of focus, the other when in focus. Again I have used Antares, maximum sensitivity, low down, LP.

Clearly it works. I do, however, have two personal reservations that will probably mean I will tend towards the 2-triangle mask in future. Firstly, as mentioned earlier, I think it is more ubject to damage than the the other, due to the pointy-bit between the two upper arms of the Y. True, this could be resolved by making it of hardier material than stiff card, but to me that rather defeats the object of having something easy to construct in true 'Blue Peter' style. My second reservation is that I personally do not find the focussing approach so intuitive. To me, two objects merging into one when in focus is easier than judging whether the spikes are at their shortest and most symmetrical.

I am sure there are people who will disagree with me on both of the above points, and so I would emphasize that they are both personal opinion/preference and that the Y mask did the job adequately. I would also endorse what I understand to be the main point that was being made about this mask; that it is easier to construct than the standard B-mask and achieves the same result.

[glowjet]

Thanks for the feed back DP but I am a little puzzled by your description of the result with the Y-Mask the point of exact focus occurs when the vertical spike lies on the center of the X the X spikes being produced by the arms of the Y, the vertical moves to the left or right as you go in and out of focus the X remains stationary on the center of the optical axis, from what you have described with objects merging and you having to judge that the spikes are symmetrical, plus your comments about the pointy bit as you describe it, I am under the impression that you have cut a Y hole in your mask disk instead of constructing a solid Y to place across your telescope aperture, the Bhatinov Mask works on the principle of bars producing diffraction spikes, whereas the B-Mask has a lot of bars like a ronchi grating, which cuts down the light entering the scope, the Y-Mask allows over 90% of light through but works exactly the same, if you have time have a read of Chris Lords PDF file on the subject, mentioned in earlier threads.

John.

[Demonperformer]

I am under the impression that you have cut a Y hole in your mask disk instead of constructing a solid Y to place across your telescope aperture.

Yup ... OK, blue peter mark 3 coming up

[iamjulian]

Just wanted to thank everyone for their input, and especially Demonperformer for his dedication to the cause. I couldn't find my craft knife, so I made the two triangle version using a kitchen knife not tested yet but looks like it should work.

Eagerly awaiting the results of your Blue Peter MkIII !

[Demonperformer]

OK, the Blue Peter Mark III Mask is now complete and awaiting testing. Have to say it is by far the easiest of the three I have so far tried took about five minutes, considering I already had the 'tube' bit from my previous 4" attempt (where I cut the Y out). Forecasts all seem agreed that my next clear skies are going to be later on tomorrow evening, so will have to wait until then for the 'mintron test'.

Julian, please note my "dedication to the cause" is totally out of self-interest. I have lost more images through bad focus than anything else. They look all right on the screen, but when processed they are ghastly. I don't mind getting up in the middle of the night and standing around in the cold taking photos, but when they don't work (especially when it is my own fault) my enthusiasm takes a bit of a beating.

Would not want you all thinking it is altruistic .

[glowjet]

Just as a bit of interest many of the seasoned Astro Imagers who used variations of the Hartman Mask before the Bhatinov Mask appeared on the scene, which is essentially a disk with holes cut out, seemed to agree that the precise point of focus was not easy to achieve, once the B-Mask was born all things changed and there were reports of focusing to within thousandths of an inch and stars in focus down to a pixel, a point remembering about what Chris Lord said, the construction of the Y need not be uniform, thinner arms and leg make for finer but longer diffraction patterns as they are made wider the spikes become shorter but brighter, so with a bit of license you could make a variation with wide arms to the Y and a thinner leg, as I have said previously experimentation is the order of the day, have fun.

John.

[Demonperformer]

OK, MkIII-mask test results. Usual rules apply - Antares, 128x sensitivity, LP, but very little in the way of wind this morning.

Don't know about an 'X' - can't see one of those at all, but I can see the Gotham City Batsignal . This could be something to do with the sizes of the arms of the mask (14mm wide for a 102mm scope).

Have to say that achieving focus with this took me longer than with either of the previous two options. That could just be my ineptitude.

"Experimentation" may be the order of the day, and may produce much better results, but even though each one is so simple to construct, I'm not that motivated to go through twenty attempts to find the right combination. If someone comes back and says I should be using these measurements for a particular size telescope I might try again, but otherwise ...

I guess the bottom line is that focussing to 1 pixel is rather lost on the rest of my setup. My alt-az mount does not guide that well. For me, the important bit is achieving "acceptable" focus as easily as possible. With the two triangles offset by 60 degrees, it is not difficult to see when you have a achieved something akin to a 'star of david' shape as the two triangles merge, and I achieved this a lot faster and more intuitively.

Once again, I would emphasize that all of this is MY PERSONAL OPINION based on THESE LIMITED TESTS. I would welcome other members of the forum running comparative tests between these two mask types and posting their results, with photos to show how things ACTUALLY LOOK, whether or not they reach the same conclusion as I have.

As for me, I am now constructing a new 2-triangle front piece for the mask of my 102.

[iamjulian]

Thanks once again for sharing. I tested a very rough and ready two triangle mask cut from the back of a cereal packet last night, and it worked quite nicely.

[glowjet]

Thanks DP for your efforts so far, the width of the Y is on the large size at 14mm for a 102 mm scope, if the size of bar width is roughly based on Parvels formula, which is the focal length of your scope divided by 150 or 200 the product being divided by 2, you will have the bar and space width for the B-Mask, the 150/200 figures are to give some tolerance, if the resulting widths are to small to cut then a multiplying factor of 3 can be used to give a third order pattern, for example if your scope is 900mm fl dividing by each figure will give 4.5 and 6 then divide by 2 will give a bar width of 2.25mm or 3mm or 1/8" which is a first order pattern, having seen your results, try something in the order of 3mm to 4mm, if you have made a collar for the front of the scope out of cardboard you could make the Y out of strong black plastic strips glued together to make the Y, the Y part should have an angle of about 40 degs, you could then Cellotape it across the front of the collar to experiment, just some thoughts as guidance.

John.

[Steve 1962]

Last night was fantastic down here in Somerset...but b...y cold!

Continuing my theme of cheap and easy to make focussing aids, I bought 0.5m of 1mm black round elastic at our local haberdashery and with four knots made a Y mask.

Here's a picky of the stretchy mask (on my WO ZS70 but it's stretchy so it also fits my 100ED) and it's virtually indestructable too (unless you lose it when it pings off into the night I suppose) and the resultant diffraction spikes...on Mars.10 secs at ISO 1600...it only seems to work on really bright objects - I've also got some 2mm elastic - maybe try that tonight.

Steve

[Steve 1962]

Sorry - not "my" theme - I meant to say "the" theme!

[glowjet]

Steve,

That is quite a remarkable result from 1mm round material, and it at least proves what Chris Lord said that a Y mask will function the same as a B-Mask, I have the same scope as you but have been of ill health of recent times so have been unable to try out such a mask. I gave some directions in my last thread as to size for a Y- mask for your scope which should be in the order of 3mm, in width so your 2mm elastic may give a better result, what you have achieved so far is excellent.

John.

[Steve 1962]

Sorry to hear you've not been well John.

I've got the wife on the look out for some 3mm round elastic - the thing I was pleased with was that, being elastic, as well fitting both of my scopes, taking up no room in the kit box, and producing the desired effect, if was also easy to gently slip it on and off without "bouncing" the scope.

Thanks for the advice

Steve

[glowjet]

Steve, I think you must be the first to use elastic, round section, instead of flat material, I see no reason why this should have any different effect as a wavefront of light entering the aperture will only see this as a solid width, I assume point of focus is achieved by movement of the vertical diffraction spike until it lies on the center of the X. With a suitable width of material to match the FL of the scope you should be able to focus on any reasonably bright star and see the result on the screen

of your DSLR if it has the facility. Much more light is let through with the Y-Mask at least by one f stop, Chris Lord suggested that if the X diffraction spikes were not as bright, then there was only need to thicken the V part of the Y to improve matters, this has the effect of making the spikes shorter but much brighter, increases should be modest 1mm or 2mm at a time at the most, have fun.

John.

[Steve 1962]

Hi again John

You're right - the central "vertical" spike moves left and right as you travel either side of focus - which is very useful because I found that you could tell whether you're moving the focusser in the right direction.

Steve

[glowjet]

Hi Steve, This is why the imagers have adopted the B-Mask, it is fairly easy to split the X with accurate results, this is where it differs from the Hartman mask and its variations, where although blending of the illuminated shapes seems fine to focus, the precise point of focus is sometimes difficult to determine. The Americans proved this when experimenting with the B-Mask in the early stages, photos taken with Hartman type masks were shown to be ever so slightly out of focus on occasions when compared to the B-Mask, which achieves precise focusing every time.

John.

[Cath]

I have a question ..

If putting these different masks in front of the telescope creates these focus patterns, why doesn't the secondary mirror spider/veins create a focus pattern also?

[sanmatt]

I could have saved £17 if I'd seen this 2 days ago it looks like a grand idea

[Cath]

I could have saved £17 if I'd seen this 2 days ago it looks like a grand idea

Typical isn't it lol

[sanmatt]

It so is!

[1981suede]

The Bahtinov is so easy to use and achieve perfect focus I think it's £17 well spent. Especially when used with Bahtinov grabber software.