Aperture Mask: Yes. Pluto: Not so much.

[Whistlin Bob]

Got home from an evening out last night to a glorious clear sky. On a Monday? Really?

I made an aperture mask for my dob over the weekend and was itching to try it out. The idea behind the mask is that on bright targets cutting out a bit of light and diffraction from the secondary struts should improve clarity, even though the aperture is reduced (350 to 160 in this instance).

And so after failing to resist temptation I was setup by 11:20:

Jupiter: Disappeared behind neighbours house. Need to catch it in the gap between house and apple tree 1.

Saturn: Seeing dreadful without mask: boiling away with no clarity at all. With the mask: same but dimmer. Hmmm.

Pluto: Spent ages looking for this. Definitely in the right place. Pluto formed a triangle asterism with two other faint stars. I upped the magnification to dim the sky glow and there was definitely something there. Wobbled the scope – that helped. Averted vision- didn’t make much difference. So- I’ve looked at Pluto but not seen it! Beginning to regret doing this on a work night…

Jupiter again. Behind apple tree 1. That moved quick! Damn!

Izar: At last- some success. Successively improved views moving from Baader zoom to binoviewers to adding aperture mask. In the final view the stars were pinpoint sharp and well separated with the companion showing a lovely blue.

Double double: the same experience. The 2 pairs were easily separated in all 3 configurations, but the binoviewers plus aperture mask gave the best view.

M13: Too dim for the mask, the best view was in the binoviewers- resolving all the way to the core and seemingly spherical, even though at that distance you don’t really have depth perception!

Jupiter again: Gotcha! Just before it snuck behind apple tree 2… Definitely a better view with the aperture mask- slightly dimmer but with much more clarity. 6 bands plus the GRS were clearly visible, with some detailing on the bands, plus the moons spread out as clear disks 3 to one side and one on the other.

Well worth the fatigue today!

Very pleased with the aperture mask- it’s not often an astro upgrade is almost free. It’s only really good for bright objects and with the binoviewers I had to velcro 4kgs onto the bottom of the tube to balance it- but well worth the hassle!

[Stu]

Good stuff Bob. I've seen first hand the effect as of an aperture mask and they can be very effective.

[Paz]

Great report. I have an aperture mask for my dobsonian and it gets used at some point on most outings.

[Stu]

I guess also at mag 14.2 you need all the aperture you can get to see Pluto.

[John]

4 hours ago, Stu said:

I guess also at mag 14.2 you need all the aperture you can get to see Pluto.

Indeed. Allowing for it's low position in Sagittarius it's probably fainter than mag 15 from the UK currently due to atmospheric extinction.

 

[Stub Mandrel]

22 hours ago, Whistlin Bob said:

For the uninitiated, this is Rob's incredibly fast f1.8 16" newt.

[Whistlin Bob]

7 hours ago, John said:

Indeed. Allowing for it's low position in Sagittarius it's probably fainter than mag 15 from the UK currently due to atmospheric extinction.

 

Hmmm- mag 14 is about the best I've got at home, and my Southern horizon is the worst for light pollution so I was probably always onto a loser but hey ho- you don't know unless you try!

[John]

6 hours ago, Whistlin Bob said:

Hmmm- mag 14 is about the best I've got at home.....

Is that at full aperture or masked ?

Masking obviously reduces the effective aperture available and therefore the faintest stars you can see.

With my (unmasked) 12 inch dob I've got down to mag 14.7 stars so far. These were close to the zenith - the limit would be lower near the horizon.

 

 

[Whistlin Bob]

5 hours ago, John said:

Is that at full aperture or masked ?

 

 

That's unmasked- it'd be interesting to see what the limit is with masking as effectively it becomes a 160mm scope. 

I'm guessing the difference between a 12 and 14 inch scope wouldn't be huge. Last year, when it was a little higher, I was trying Saturn's moons and found Hyperion (very faint at 14.8) but got stuck on Phoebe (16.8!!)

[John]

Here are some stars around the Ring Nebula with their magnitudes marked. The faintest I've managed to get are the ones I've marked with red arrows, so far. The Ring is alomost overhead from the UK around midnight currently so it's quite a good testing bed !

[alan potts]

I have done all of those and a few more John with my 12 inch Meade SC, but I believe this is easier with firstly a driven scope and secondly, most likely better skies. I am sure I saw the centre star once at X358 (8mm Delos) with this scope after 30 hours  of looking, I have since done it with the 18 inch. It is fairly easy with the 115mm apo and a camera but that's talk of the devil here.

Alan.

[John]

I think I may have glimpsed the central star on a couple of occasions Alan but nothing concrete enough to feel sure about so there is no red arrow on my chart to that one.

Re: imaging things that are very hard to see visually, I agree that the central star pops out quite readily from efforts that I've seen from my society friends who dabble in such black arts. The Horshead Nebula also seems to be relatively easy to pick up on an image but fiendishly difficult (for me from my back yard) to actully see visually.

 

 

[Whistlin Bob]

On 04/07/2019 at 09:47, John said:

Here are some stars around the Ring Nebula with their magnitudes marked.

Thanks @John- that's really handy. I'll have a go at that and see how far I get 👍