10 inch dob stopped down to 2 inch - Details on Mars!!

[Victor Boesen]

A couple of weeks ago I was out after a long time and I tried observing double stars with my dob. I found that using the "aperture mask" built into the dust-cap really made it easier to separate most of the stars. Two nights ago I was out observing Mars(probably the last time this "Mars-season") and it was almost like observing a ball of fire. I wasn't surprised that I could see any details, because I observe from a balcony in the middle of one of Denmark's biggest cities.

I then tried to put on the aperture mask, and I could now see the icecap at times and a couple of times, I could just about see some dark features, which skysafari later confirmed. What I'm trying to say is, that I'm really surprised with the difference and I want to hear your opinion on using aperture masks instead of using a smaller aperture high quality refractor or mak.

Victor

[Charic]

The planets are poorly placed for my needs, or I'm poorly placed for them ( trees, woods, buildings )  but the 2" aperture on my 8" scope works well when I view the Moon. 

[Victor Boesen]

2 minutes ago, Charic said:

but the 2" aperture on my 8" scope works well when I view the Moon. 

haven't actually tried it on the moon yet. Maybe I should try tonight if it's clear again. Sorry to hear about the planets.

[Ben the Ignorant]

Good idea but a 250mm scope allows a much larger off-axis aperture, around 100mm. Make your own mask to exploit the system to the fullest.

[Victor Boesen]

29 minutes ago, Ben the Ignorant said:

Good idea but a 250mm scope allows a much larger off-axis aperture, around 100mm. Make your own mask to exploit the system to the fullest.

Thanks for the advice! I will try this.

[PaulM]

Interesting - I will try this on my 12" dob as Mars is just a bright disc at the moment 

[Ben the Ignorant]

1 hour ago, Victor Boesen said:

Thanks for the advice! I will try this.

I forgot: the aperture should be in the lower part of the cap because warm air currents are mostly in the upper part of the tube. If your scope has no closed tube maybe a shroud will help prevent ground turbulence from disturbing the light. It goes through the scope twice so it will be disturbed twice. But then again the shroud itself might be a disturbance depending on its thermal properties, try a little experimenting with cheap materials you already have?

For the same reason the off-axis aperture should be on the side opposite the focuser, so body heat from the observer won't do as much harm. Given that you also have to avoid the vanes, its position is not random at all, only a small quadrant of the cap will do.

[John]

This opposition, with Mars being so low from the UK and with the effects of the dust storm masking the finer details, I've found that my smaller aperture refractors have put up better images of the planet than my 12" dobsonian has. Even the 70mm TV Ranger shows some markings and the southern polar cap quite nicely.

This years opposition has some unusual circumstances though - in years where Mars has been high in the sky I've found that larger apertures do pay off.

I would have tried an off axis aperture mask with my 12" dob (I can get around 110mm of unobstructed aperture with that) but I have refractors of that aperture which are somewhat more convenient to use and being on taller mounts getting Mars in their sights is easier as well. My dob is not well designed for peering over hedges and walls !

Next opposition (2020) Mars will be 30 degrees higher in the sky but not as quite large. If there is no dust storm it might well be a better opposition for observing the red planet than this years has been.

 

[Paz]

I use an aperture mask often and have a set up that makes it easy to use it. It is helpful for a cleaner image of bright stars, planets, and the moon. My reflector is 350mm and the aperture mask is 137mm.

I find that as magnification goes up full aperture eventually becomes preferable for me, with its advantages of brightness and resolving power becoming more dominant.