step down aperture mask on 14" dob

[Rustysplit]

I have been meaning to do this for ages after my post about a yearning for a long focus refractor, and the tip from Shane about trying it out.

I roughed up in much haste an apprx 120mm diameter mask, giving me an effective f15, rather than my native f5. I have to say the result is quite impressive. It really evened out the seeing on the moon at 300x, although with a much dimmer image. I would not say that is gave more detailed views, but certainly steadier with a bit more contrast.

I also tried it out on the double double, I could split it at 150x with the full aperture, though the stars were showing the effects of the spider diffraction. With the mask they were a fair bit dimmer, but a cleaner split.

Which view did I prefer? both! It has proved to me that I absolutely love the full aperture, diffraction spiked view of the dob, but also the uber black, pin sharp stars of the psuedo frac view with the mask. What a 'scope a good dob is! You can step down a newt, but you can't step up a 'frac. Cheers for the tip Shane. I shall be making up a decent mask v.soon.

This is not meant as a dig at 'fracs by the way. I still have a yearning for one. I think it is " horses for courses", the 'frac does was it does superbly, but cost dose limit it's aperture for most mere mortals. I think for lunar, planetary and double stars, they're hard to beat.

[John]

Interesting report Alan

I've yet to try this with my 12" F/5.3 dob. Mine has curved secondary supports though so I don't get diffraction spikes. There is diffraction still of course but I guess it must spread itself more evenly across the field.

I still hanker for a long, large refractor though, totally irrational though it is

Maybe it's an "itch" than I'll need to "scratch" before long

[estwing]

i'm goin to do the same with mine Alan...think I can get away with a 180mm or 190mm hole in my mask...does anyone know what this will give me... is it F8?

[Naemeth]

i'm goin to do the same with mine Alan...think I can get away with a 180mm or 190mm hole in my mask...does anyone know what this will give me... is it F8?

By my rough calculations (using 457.2mm as original aperture, exactly 3.9 as focal ratio to work out the focal length, then dividing by 180mm and 190mm), you will get F/9.38 or F/9.9 . Easy way to do it:

Focal Ratio = Focal Length of Scope / Effective Aperture

[estwing]

thank-you Jonathan....should be ok on planets..

[John]

It's a strange hobby at times. Here, we are looking at ways to remove / minimise diffraction effects and perhaps create "refractor-like" views, by creating an off-axis aperture mask. In the imaging section theres a thread running on how to add diffraction spikes to stars imaged with a refractor either through software editing or through physically adding wires across the aperture of the refractor !

We are never quite happy, are we ?

[Naemeth]

It's a strange hobby at times. Here, we are looking at ways to remove / minimise diffraction effects and perhaps create "refractor-like" views, by creating an off-axis aperture mask. In the imaging section theres a thread running on how to add diffraction spikes to stars imaged with a refractor either through software editing or through physically adding wires across the aperture of the refractor !

We are never quite happy, are we ?

If we were always happy with our scopes, there would be no reason to have more than one .

[estwing]

Tinkering...I think they call it!

[Rustysplit]

Too right Calvin, Tinker on......

[AlexB67]

It's a strange hobby at times. Here, we are looking at ways to remove / minimise diffraction effects and perhaps create "refractor-like" views, by creating an off-axis aperture mask. In the imaging section theres a thread running on how to add diffraction spikes to stars imaged with a refractor either through software editing or through physically adding wires across the aperture of the refractor !

We are never quite happy, are we ?

... and rather amusing how people buy the bigger light buckets only to cover them up to make a smaller aperture scope. There is an amusing way of carrying out certain tasks in this hobby to achieve the same end goal

My mad idea of the day, put the dob on a spinner and that will cause some interesting smoothing out of the spikes, there again, much more easily easier done in photoshop afterwards anyway

[Moonshane]

As Alan says, this is in no way meant to replace 'fracs as a lot of people still will buy and like using them. what it does is provide your larger aperture scope which is great in itself, a way to convert the views to the contrasty, small exit pupil, pin sharp views provided by refractors for a cost of about £3 (on top of the scope of course). no mount to worry about, no weights to worry about and you still have a superbly stable image. personally, I have yet to see Jupiter in any scope with as much detail, sharpness and contrast as with my big dob masked off.

I am sure such a scope would exist but I'd never afford one even if I wanted one.

[Rustysplit]

Could an aperture mask make a large dob the elusive "scope for all seasons"?? If you discount advanced imaging and portability, they get damn close.

I was really pleased with my views of Saturn a few weeks back without the mask. I am looking forward to comparing the views of Jupiter when it rises a bit higher.

[estwing]

your not the only one Alan!!