Using one from my Stupid Question Quota

[Size9Hex]

For once I've reached the end of the week with out exhausting my Stupid Question Quota. As they don't roll over, I may as well use one...

Why does the main dust cap on my telescope (10 inch Skywatcher Dob) come with a (purposely made) 2 inch circular hole in it please? The hole is offset from centre and covered with a removable cap of its own. There is another similar looking 2 inch circular feature on the opposite side of the main 10 inch cap, but this one just looks like a bump on which to pop the cap of the first hole.

I'm guessing something to do with reducing aperture, or aiding ventilation, but I'd love to know for sure!

Thanks as always everyone.

[PeterCPC]

http://stargazerslounge.com/topic/142165-dobsonian-dust-cap/

Peter

[Size9Hex]

Thanks Peter, that's very helpful. My own search of Google and this forum didn't find much unfortunately.

[Dave In Vermont]

Good thread there, Peter. There's one thing I can think of that's not mentioned: Venus. If you've been looking, Venus is up and VERY bright these mornings - along with Jupiter and Mars (not so bright). So stepping-down the aperture with the smaller cap for Venus would also work.

Dave

[Stu]

Bear in mind that using the small aperture turns your 10" scope into a 2" scope, so as well as brightness you lose a lot of resolution (and thus detail)

For the moon I would just use full aperture and if you find it too bright, just use a neutral density filter.

For solar, a similar thing applies although it's often the case that an aperture of 4 or 5" shows the best detail given seeing conditions during the day. In that case, a full aperture mask with a 4" hole covered with solar film fitting between the spider vanes would be a good solution. Always make sure you understand all the risks and how to safely observe the sun before attempting it though.

[Charic]

I've use this feature quite often for just viewing the Moon, reducing its brightness, then when the scope is back indoors, the 2" aperture is left exposed to allow the scope to breathe. Back at room temperature the aperture is closed, the scope is covered and placed back in its cupboard, ready for the next session. Its spent a lot of time under wraps this season, due the the pp? weather!

Note that the 2" aperture is off-set due to the position of the secondary mirror, which would block the incoming light if the aperture was central ie. In-line with the secondary .

[Size9Hex]

Thanks for the latest replies.

I did try the reduced aperture (without knowing whether that was the intended purpose of the cap) on a near full moon recently, and it did seem less detailed to me (and still surprisingly bright). Overall I found the moon in the full 10 inch to be bright, but not so bright that it was a problem.

The thought occurred to me that if it was for reducing the aperture, it was an expensive way of buying a very unwieldy 2 inch scope :-)

[Stu]

The thought occurred to me that if it was for reducing the aperture, it was an expensive way of buying a very unwieldy 2 inch scope :-)

Exactly [emoji3][emoji3]

[Paul73]

If the moon is too bright. You aren't using enough magnification!

For me, too much detail is lost to make this one worth while on the moon and brighter planets. I have had some luck using this method to split bright doubles. And it works well with solar film.

Paul

[ollypenrice]

OK, but suppose you have two opposed two inch holes on each side of the main aperture? My (not very recent  ) studies in interfermometry seem to tell me that you can have big scope resolution at small apertures. All of this was in radio astronomy and, to be honest, I spent most of the course a bit bemused...

Olly

[John]

OK, but suppose you have two opposed two inch holes on each side of the main aperture? My (not very recent  ) studies in interfermometry seem to tell me that you can have big scope resolution at small apertures. All of this was in radio astronomy and, to be honest, I spent most of the course a bit bemused...

Olly

I don't think that works too well, for visual anyway. Your aperture mask ends up behaving like a very large central obstruction with very thick support vanes.

Edit: actually, there is an aperture mask with 2 holes in it - the Hartmann Mask. This is used as a focussing aid when imaging I believe. The 2 holes create an effect rather like the old rangefinder cameras so when the images merge, you are in focus.

[Charic]

Paul73..........If the moon is too bright. You aren't using enough magnification!

For me, too much detail is lost to make this one worth while on the moon and brighter planets. I have had some luck using this method to split bright doubles. And it works well with solar film.

Paul........

......and too much magnification reduces the field of view, nothing is clear cut in this hobby, and on the Moon, not really practical.

There is no hard and fast rule on how to use the smaller aperture, its down to the individual in the end. I even use mine on occasion to suck the warm air currents out, and before you go there, a pipe and fan is used!

Ive seen several reports on the web, regarding the brightness of the Moon, and ways to reduce the glare and the issues caused from irradiation. For me, the 2" aperture only gets used on the Moon.

A supporting link here...https://www.astronomics.com/aperture-mask_t.aspx

[Garethr]

I just remove the small cap and fit it onto the dummy cap used as a holder,

Works for looking at the moon and letting the scope "breathe" when there a bit of dew on the mirror at the end of a session.

Gareth  

[Paul73]

Paul73..........If the moon is too bright. You aren't using enough magnification!

For me, too much detail is lost to make this one worth while on the moon and brighter planets. I have had some luck using this method to split bright doubles. And it works well with solar film.

Paul

........

......and too much magnification reduces the field of view, nothing is clear cut in this hobby, and on the Moon, not really practical.

There is no hard and fast rule on how to use the smaller aperture, its down to the individual in the end....https://www.astronomics.com/aperture-mask_t.aspx

I couldn't agree more re. the "down to the individual" comment. And, I really must remember to put smileys after tongue in cheek statements such as the magnification line above.

It is true that piling on the magnification does dim the image enough to make for comfortable viewing. However. There are times when only a full disc will do. For this I have a Baader ND 0.9 filter which works a treat.

The 2" hole is just another option.

Paul