720x!!!

[Rustysplit]

Wow the sky is good tonight. 5mm Radian barlowed to 2.5mm giving me 720x mag. Still sharp views, incredible. This is with the 360mm dob stepped down to around 110mm with an aperture mask. Not so good without the mask, so well worth making if you have large mirrors.

[Daniel-K]

i take it your on the moon ALAN?

[Rustysplit]

It felt like it. Yep, sorry, forgot to mention I was looking at the moon.

[Moonshane]

told you Alan. aperture masks work superbly well for anything 12" or more and possibly even 10"

[John]

Thats great Alan but surely you have the light grasp and resolution of a 11cm scope now ?

Or am I mistaken in that ?

Perhaps I'd better get my 120mm refractor out - the seeing with the 12" dob is decidedly variable here.

[John]

I've just knocked up a 10cm aperture mask for my 12" dob and I've got the scope alongside my ED120 refractor to compare the views. This might be interesting 

[Moonshane]

will be good to see if you agree with me about aperture masks John 

[Rustysplit]

I guess you are right about the resolution John. But to be honest the view was still breathtaking. There was a marked difference betwwen the steadiness of the masked down view compared to the full aperture. I feel this made up for the lower resolution. The only thing it did show was dust on my optics somewhere.

Yep, you were spot on Shane.

[John]

So far the masked 12" is doing an excellent impression of an excellent 4" apochromat refractor or and F/16 4" achromat. The views of Castor and Theta Auriga are very similar through both the ED120 at 257x and the 12" masked to 4" at 265x with the fainter companion to Theta Auriga a little brighter in the ED120.

The stars have that classic "refractor" look in the masked dob with a well defined central airy disk and a single diffraction ring around the brighter ones. 

The view with the unmasked 12" dob shows much brighter but less tightly defined stars on both binaries. Both are very clearly split but the view is not so satisfying as with the refractor and the masked dob. 

I need to wait for Jupiter to clear the trees and I'll see how it goes on that plus some objects in Orion when that gets up.

I won't be getting anywhere near 720x though I can see that !.

[Rustysplit]

The moon was at it's highest when I managed that John, certainly not happening now. I too am waiting for Jupiter and Orion to swing round a litle more.

Glad to hear you are liking the mask. It makes the simple dob that bit more versatile.

[Moonshane]

I'm busy sketching open clusters in Taurus. done ngcs 1647, 1809 and 1817 so far. nice! will have another butcher's at Jupiter myself before I pack up.

[John]

I think the mask is a neat idea if you have a big scope out and the conditions are working against the full aperture. It will give you some enjoyable viewing with little hassle and thats really worthwhile

[Stargazer_00]

Neat idea, any tips to make a mask?  I'd be able to get more out of my big dob this way

[Rustysplit]

Here you go Graham, this is my dead simple mask.......

It is made out of 2mm corrugated plastic, like estate agent boards with some sticky black plastic on the inside face. A centre cut out to allow for the collimation bolts to pass through and as large a diameter hole as you can fit between the spider vanes/secondary mirror edge. Simples! The only thing you have to watch is that you make sure the outer edge of your hole is within the actual diameter of your primary mirror.

[Stargazer_00]

That's great, cheers I'll try to knock something up

[Kirscovitch]

Being new this is the first time I've heard of one of these masks. Although I was wondering why the lens cap for my scope had 2 smaller round, removable caps. Had no clue what purpose they served but I think I found my answer!

[Rustysplit]

Hi Matt,

The smaller holes on your tube cap can also be used for adding a layer of Baader solar film, for a bit of white light solar viewing. Obviously, this needs to be well fixed and also the correct type of solar film.

Next time you are out looking at the moon, try it with the cap on through one of the smaller apertures. It will take the glare down. I think they work best on larger mirrors, but the principal still holds.

[Kirscovitch]

Thanks for the tip. I'll have to try it next time out.

[Stargazer_00]

Does the stop down shape have to be circular?  And must it only be one? 

I am wondering if an irregular shape, like a trvial pursuit cheese wedge shape, would introduce odd diffraction spikes that a circle would not.

Essentially I want to stop my 14" down to about 6" or 7" of aperture and wondered if this had to be achieved by a single aperture or could be achieved by 2 circlular apertures in a mask that equated to the same area as a 6" circle or an 7" circle.

[Rustysplit]

I am pretty sure the wedge will give odd diffraction spikes, a bit like when your light shroud or dew shield sags into the light path. As for 2 evenly sized apertures? there's only one way to know for sure....

[John]

If you have 2 or more apertures you are effectively creating a scope with thick obstructions (ie: the uncut areas of the mask) and therefore lots of contrast robbing diffraction.  I think it has to be a single aperture and a circular one will be the most effective.

[Stargazer_00]

Thanks John. I'll go and see what maximum diameter I can put into a single quadrant

[Moonshane]

as John said, you need one circular hole as anything not round will create problems with bright objects. also, just one for the reasons stated. you need to avoid the spider vanes and the secondary.

[John]

Thanks John. I'll go and see what maximum diameter I can put into a single quadrant

10cm was about the limit for my 12" allowing for the 63mm central obstruction and the gap between the primary and the tube wall. So I got a 4" with a focal length of 1590mm or F/15.6. Funnily enough the scope did then perform pretty much as I would expect a good refractor of that spec to perform 

Watch out that you don't start getting a taste for refractor type views though   

[swamp thing]

told you Alan. aperture masks work superbly well for anything 12" or more and possibly even 10"

Not sure about a 10" Shane. I've stepped my 10" down many times, with very questionable results. I just don't think its effective TBH. It does help if the seeing is bad, but the resolution is not enough through the mask to make it worthwhile IMO. I think at 12" you can get away with a decent sized aperture still, in a 10" it's just too small.

The last time I tried it was on Saturn. The rings became anaemic all detail was pretty much lost. The image was steady but......rubbish

Going back to full aperture resulted in terrible seeing. So kinda had the option of rubbish view or, rubbish view. I put the aperture stop back in the case and went after some DSO's instead.