Small section of dust cap removed for planetary ?

[Red Dwarfer]

Hi all , 

Just wondered how many people removed the small section of the dust cap for planets or lunar observation ? See photo below from google images ...

Just out of curiosity I tried it tonight on Rigel and Mintaka double stars at high magnification and found that they were cleaner and more spherical using this technique , then took the cap off altogether and they were back to the blurry , watery mess at high magnification ( 5mm barlowed to 2.5mm in a 90mm refractor ) 

Any thoughts ? Anyone else tried this ?

Clear sky

Aaron 

[LukeSkywatcher]

Remove the whole front cap. It will allow more light in making images brighter. The only purpose of the inner cap (kind of obsolete) is for when viewing the Moon (or the Sun with the use of proper safety filters). For all other objects, you want as much light as possible.

Sound like your scope needs collimating, or it could be down to local seeing conditions where you live.

[mikeDnight]

Some mass produced low cost refractors from China can suffer from spherical aberration which can play havoc with the image quality. By reducing the aperture you may be using the better figured central region of the lens. 

[LukeSkywatcher]

What scope is it?.

[Moonshane]

You have effectively changd the focal ratio of your scope to a much slower one but with less aperture. This will make stars tighter in general but you lose resolution on e.g. the moon and planets and light gathering on faint objects. 

Try a star with a fainter secondary like Polaris with and without the main objective cover. You'l see a difference I expect.

[Red Dwarfer]

33 minutes ago, Moonshane said:

You have effectively changd the focal ratio of your scope to a much slower one

That was the theory - the original f6.7 effectively doubled with 50% less light - bit like a X2 converter on a prime lens in photography slows the speed of the lens and doubles the f number .

[Red Dwarfer]

2 hours ago, LukeSkywatcher said:

What scope is it?.

One of my smaller refractors , a Meade 90/600/6.7 with a 5mm ED BST , tried on the Moon beforehand and found absolute zero CA at the rim and could see the four peaks inside Theophilus crater tack sharp .

[Jimtheslim]

I did this to view the first quarter moon with my 10" reflector recently and found it quite pleasing. Unfortunately I was so engrossed I forgot that most the cap was on and was going crazy that I couldn't find the Orion Nebula afterwards. I'd forgotten that I'd stepped down the aperture from 10" to about 2! 

I guess whatever worked for you, just go with it. 

[Charic]

On 21/02/2018 at 20:05, Red Dwarfer said:

Any thoughts ? Anyone else tried this ?

I regularly use the 2" aperture on my 8" scope when the Moon is full-on and intensely bright!
For everything else, the full 8" aperture is used.

I also discovered one night, to my surprise! ( thinking that there was just too much cloud overhead - especially for Stargazing) just how good a layer of cloud worked,  which effectively filtered down the brightness, yet still allowed me to see the Moon's surface detail, the results were good.