200P. What's this bit for?

[doctortim]

Just a niggling query. The big black OTA cover of my 200P has 2 small circular things that look like lens covers, one of which is removable exposing a hole.

Why?

thanks,

Tim.

[ronin]

The moon is bright, just use it with the small cover removed for this as yoiu do not need the full aperture. Also as it is offset it means that light entering misses the vanes and secondary so reduces the spikes - think you could still have one in the way but better then all 3 or 4 and the secondary.

Some use the small aperture for solar viewing, again the sun is fairly bright, when we see it. If you use the scope for any solar viewing remove or add a filter to the finder. Toio damn easy to cover the main scope then try to look through the finder.

Always use a purpose-made solar filter when observing the Sun. If you do not you will suffer irreversible eye damage!

[Cornelius Varley]

The moon is bright, just use it with the small cover removed for this as yoiu do not need the full aperture. Also as it is offset it means that light entering misses the vanes and secondary so reduces the spikes - think you could still have one in the way but better then all 3 or 4 and the secondary.

Some use the small aperture for solar viewing, again the sun is fairly bright, when we see it. If you use the scope for any solar viewing remove or add a filter to the finder. Toio damn easy to cover the main scope then try to look through the finder.

Always use a purpose-made solar filter when observing the Sun. If you do not you will suffer irreversible eye damage!

Remember to use a solar filter over the opening in the cover !

[RichieP]

For lunar viewing reducing the aperture like this will also reduce the resolution of the image, not just the brightness, so

I wouldn't recommend it. You are effectively turning a nice big scope into a little scope. Use a moon filter instead.

It may, however, proove useful when you have really bad seeing, when a small aperture can produce better images than a larger aperture.

[JKB]

Also the one that isn't removable is to allow the removable one to be kept safe. Not sure that made sense. Take off one and put it on the other

As for reducing Resolution: A smaller aperture decreases the Angular Resolution but increases the Depth of Field. This in turn can make images appear sharper. I generally use my scope (reflector) with a slightly smaller open end than the mirror. Home made ring that is placed at the tube opening to reduce aperture by 15mm.

As said certainly with poor seeing conditions smaller can make a big difference. The only time I have ever used the small cap hole is during solar viewing (ALWAYS WITH A SOLAR FILTER!) just because I am a little scared

HTH

Cheers

Jamie

[doctortim]

thanks everyone for the informative replies!

[John]

For lunar viewing reducing the aperture like this will also reduce the resolution of the image, not just the brightness, so

I wouldn't recommend it. You are effectively turning a nice big scope into a little scope. Use a moon filter instead.....

I agree with this 100%. I really can't see the point of creating a 55mm aperture scope that is as bulky and expensive as a 200P is !

The eye soon adjusts to the brightness of the moon and brighter planets and you need the aperture to achieve the resolution and contrast.

I regularly observe the moon with my 12" scope and use no filter or stopping down. The detail is breathtaking

[LeeB]

If you view the moon without a filter, you are limited to smaller exit pupils (higher power) as your iris contracts in the brightness and you end up having to move your eye to see the whole field. I struggle as it actually hurts to view with my 10 without a filter at low power, I use a uhc as a minimum or a high density moon filter. Mind I do like the lower power views. At higher power it will dim naturally without filter. I also can't see the point of dropping aperture unless the sky is awful. I have never used the small whole except on a frac when the increase in fl will help CA

[John]

If you view the moon without a filter, you are limited to smaller exit pupils (higher power) as your iris contracts in the brightness and you end up having to move your eye to see the whole field....

I've been observing the moon for many years and I can honestly say I've never noticed this, even when I view the moon with my 100 degree apparent field eyepieces.

Perhaps different peoples eyes react in different ways ?.

[Stu]

Not sure about the exit pupil issue John, but I am sure people's eyes behave in very different ways. My eyes each are quite different so I only really observe with my left eye which gives me detailed images which are just right in terms of brightness. My right eye however seems much brighter (almost painfully so on the moon) but actually seems to have far worse resolution so I just cannot pick out the detail, particularly on planets. If both eyes had been like my right eye I would probably have given up visual astronomy because it would not have been very rewarding!!

I guess my point is that whilst I know you find the moon fine without filtering, I can totally see why some find it too bright, it's just we are all different! It makes sense to me to filter if necessary, rather than stop down the scope so as to preserve the resolution.

Hope that makes sense!

Stu

[swamp thing]

Looking at the moon through any scope is no brighter than walking outside on a summers day. If you have to wear sunglasses every time you leave the house in summer because your eyes hurt. You'll be needing a filter.

[Stu]

Maybe not, but your eyes will normally be at least partly dark adapted the first time you look so it can be pretty dazzling to start off with. To be clear, I normally observe it unfiltered, but can sympathise with those who find it too bright.

Stu

[planet15]

I was watching the full moon through SW200p straight about 3 weeks ago. It was beautiful image - lots of detail and true colour, but too much brightness hurt my eyes.

I put on some filter on the eye piece, then it was OK, but the view was not as realistic or nice as straight through the EP.

[Matt Scunthorpe]

I use full apperture on the Moon and dont use a Moon filter. Its so beautiful, i dont want to dim it down.

I have a solar filter on the inside of the dust cap to make use of the hole.

[planet15]

Next time gazing the full Moon I will try to dim SW200p down with the cap on but the small cover open. I was not even aware of the existence of the covers on the cap .... thanks.

[Moonshane]

I use both full aperture and an aperture mask with the moon and my 16" dob. as John says, the view of the moon naked eye is actually brighter than the view through a scope showing less than the full disk. there's only so much light than can be reflected. when using the aperture mask with my f4 scope I can :

• use lower magnification as the focal ratio is higher and exit pupil not a problem
  
• see a little more contrast in the view which at a smaller exit pupil is more aesthetically pleasing (the view of the full disk (preferably <50% illuminated) through my masked 16" with a 32mm plossl is just breathtaking)
  
• cut through bad seeing a little better
  

I agree though that with full aperture you get a lot more sharp detail but do have to wait for this sometimes as the seeing comes and goes.

[Cath]

Does it not reduce resolution when the scope is looking through a little hole in the end cap? .. much like a smaller aperture scope has a lower resolution than a larger aperture scope?

[RichieP]

Does it not reduce resolution when the scope is looking through a little hole in the end cap? .. much like a smaller aperture scope has a lower resolution than a larger aperture scope?

Yes it does, which is why I would recommend a moon filter instead.

[planet15]

Interesting - I have a little 70mm refractor in the corner of my room I use for terrestrial viewing.

Just for a test, I dimed the objective lens with the cap but the little cover off in the middle of the lens cap, and looked out through the window. The view got darkend a little, but contrast has risen, and I could read small writings in the car park noitce board half a mile away. When I take the lens cap off totally, it gets brighter, the contrast goes down, and I cannot make out the writings although I can see the board.

Maybe aperture is NOT king after all.

[Cath]

.Maybe aperture is NOT king after all.

Lol, you've just written off all pro telescopes around the world.

I knew I should have got that 450x mag 1" tesco refractor scope instead the 6" SCT

[planet15]

Lol, you've just written off all pro telescopes around the world.

I knew I should have got that 450x mag 1" tesco refractor scope instead the 6" SCT

I said "Maybe" .... LOL

[emadmoussa]

Solar observing...

Definitely not for solar observing!

Always use a purpose-made solar filter when observing the Sun. If you do not you will suffer irreversible eye damage!

[gazza63]

Hi all

Just read Matt's idea of using the small hole on his dust cap for a solar filter, (seems a good idea). I'm looking to purchase a filter in the near future. Would using the small hole in the end cap be just as good as using the full aperature of the scope? (for viewing and imaging), also, would I lose any detail?

Unless i made my own....a pre-made 6" solar filter can be expensive. So i was thinking! if i could buy a pre-made filter for say a 50-60mm aperature, and fix it to the inside of the dust cover, It could save me a bit of money.

Any views on this?

Gary