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what is the hole for?

oldnut

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John

John

Posted
Posted

You can put some special solar filter film across the inside of the hole (completely covering the hole) and view the Sun in white light. Some folks like viewing the Moon through the smaller hole to cut down on glare, chromatic aberration (in refractors) etc but this does also reduce the resolving power of the scope.

Which scope is it ?

Note: You MUST use purpose made solar filter material for the 1st use above and make sure that there are absolutely no gaps for the unfiltered Sun to shine through. If you have an optical finder that should be fully capped when viewing anywhere near the Sun.

Sorry to harp on about the safety stuff but you can't take any chances where the Sun is concerned.

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Uplooker

Uplooker

Posted
Posted

It is an aperture reducer / mask. I have no idea what your scope is, but the focal ration of your scope is calculated by dividing the focal length of the scope by the primary mirror diameter. So if you had a SW 150P, the focal length is 750mm and the diameter of the primary is 150mm the focal length of the scope is 750/150to give a f/ratio of F/5. By attaching the end cover to the scope and then removing the small cap you are effectively increasing the F/ratio of scope as the diameter of the small hole left by removing the cap that is AFAIK 2 inches or 50mm diameter. Therefore the calculation if as follows - 750/50= F/15.

By increasing the focal length of your scope you will by default reduce the resolution ( detail that you can see in an object)

You will probably have noticed that there is a "similar" circular protrusion next to the removable cap - this is so that you can fit the removable cap over it so that you do not lose it, when you have removed the removable cap :grin:

Ian

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astro mick

astro mick

Posted
Posted

It is an aperture reducer / mask. I have no idea what your scope is, but the focal ration of your scope is calculated by dividing the focal length of the scope by the primary mirror diameter. So if you had a SW 150P, the focal length is 750mm and the diameter of the primary is 150mm the focal length of the scope is 750/150to give a f/ratio of F/5. By attaching the end cover to the scope and then removing the small cap you are effectively increasing the F/ratio of scope as the diameter of the small hole left by removing the cap that is AFAIK 2 inches or 50mm diameter. Therefore the calculation if as follows - 750/50= F/15.

By increasing the focal length of your scope you will by default reduce the resolution ( detail that you can see in an object)

You will probably have noticed that there is a "similar" circular protrusion next to the removable cap - this is so that you can fit the removable cap over it so that you do not lose it, when you have removed the removable cap :grin:

Ian

Spot on.

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spaceboy

spaceboy

Posted
Posted

This is a question that does seem to be asked an awful lot and not surprising given there is not real instruction to what it actually is in any of the telescope manuals.

While I agree with the replies to what is the most probable use of the reducer it does also beg the question of why a 2" aperture stop down across the entire range of Synta scopes available from 90mm to 450mm ??? Surely a 3" or 4" mask would be better suited to owners of a 16" and why stop down a 3 1/2" scope to 2" ? I understand the advantages of stopping down the aperture and increasing the focal ratio in achromatic refractors in doing so so reducing CA. I also to some extent understand that a reflector can benefit to some degree by reducing turbulence in the OTA and removing the central obstruction of the secondary. What I don't understand is what is the practicality of reducing the resolution in the case of an 10" newt to only 2" ??

The most plausible idea and obvious advantage of stopping down the aperture is when viewing an almost full moon but given the choice I would rather retain the resolution of a scope over 2" by using a moon filter.

Yes it makes for a quick and easy solar filter when the appropriate safety film is taped to the rear but I'm guessing in the modern days of health and safety Synta would be setting themselves up for a law suit if it was suggested their intentions of putting this hole in the dust cover for this reason.

Whatever the manufacturers design reason for this 2" hole it must be of some significance as they clearly meant of it to be used on a somewhat regular basis as they included somewhere for you to put the cap when in use. Could it be that it allows air flow and is a means of cooling your scope with out running the risk of your optics being fully exposed to the elements ?

Whatever the reason we have all found a use in one form or another for this 2" hole and that really is all that matters but I would be interested to see if there is a documented manufacturers explanation why it is there. Might sound daft but does anyone know of some historic reasoning for it ? I mean some of these telescope designs have been around for quite a long time relatively unchanged.

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swamp thing

swamp thing

Posted
Posted

The 2" hole is there because this is the size of nozzle of the cloud injection unit at the manufacturer.

The end cap is fitted, then the optical tube can be filled with clouds prior to shipping :grin:

Once you've opened your new scope the clouds are released and hey presto! A nice overcast sky with your new scope. Obviously the bigger the OTA the more clouds there are that can be packed within it. 16" owners often find its cloudy for weeks after opening their scope for the first time. ;):grin:

Seriously, I always though it was for solar observing as John has already written. It's use for the moon is daft IMO.

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