What's the smaller hole in the main dust cap for?

[Hanuman]

My Skywatcher 130M has a smaller hole with removable cap in the big main dust cap.

What's that for?

Guessing it's a way to reduce the aperture but I'm not sure why I'd want to do that.

I have bought some Baader film to make a solar filter ready for the transit of Venus but I'm unsure whether I want to make a big filter to go across the whole end of the scope or just put some film across this smaller hole, which would then be protected by the removable cap.

Hope that makes sense.

[ronin]

Sometimes when viewing the moon a full aperture is too bright, people either use a filter or put the main cover in place with the small aperture open.

The aperture is off to one side to miss the secondary so no direct obstruction and less differction and scatter from the secondary.

Also you can fit a solar filter over the small aperture and view the sun with the main one in place.

Basically just a means of reducing aperture for when the viewed object does not need the full aperture.

[Cornelius Varley]

The small hole is used when you want to reduce the amount of light entering the telescope, suxh as when you are observing the moon when it is full.

Peter

[Fandango]

I think it's to reduce the amount of light coming into the scope. Perhaps for really bright objects? I've only just discovered you can take the whole cover off! Spent months thinking my scope was rubbish.

[zainraza]

I think it also helps with contrast with some of the brighter objects such as Jupiter too but someone might have to correct me on that

[Hanuman]

Thanks guys. So I was right. Hadn't thought about reducing the brightness of the moon that way. Will give it a try next time I see it.

[Glen]

by using the cap it'll give the scope a longer F ratio sometimes needed for the brighter planets giving better contrast and pulling detail when brightness can be afforded (moon, Venus, Jupiter)

[x6gas]

Apparently it is recommended to put your Baader solar film over the smaller hole. Apparently this will increase contrast...

[sologuitarist61]

by using the cap it'll give the scope a longer F ratio sometimes needed for the brighter planets giving better contrast and pulling detail when brightness can be afforded (moon, Venus, Jupiter)

Not too sure about this one . The f ratio is determined by the the length of the light travel from objective to EP, not the aperture.

By using the small hole the aperture is reduced and, has already been said, this reduces the amount of light entering the scope and also can increase the contrast (mainly by reducing glare).

[Crunchard]

If it helps I use the small hole on my solar setup (ST80) - just make sure you secure the film correctly (normal warning apply). This works well for me.

[L-space]

useful information for a relative newcomer like myself, thanks.

[Glen]

Not too sure about this one . The f ratio is determined by the the length of the light travel from objective to EP, not the aperture.

By using the small hole the aperture is reduced and, has already been said, this reduces the amount of light entering the scope and also can increase the contrast (mainly by reducing glare).

Oops, sorry Richard one and all, my apologies. I always thought it was the mirror diameter to focal point length ratio

*Hides*

[x6gas]

Not too sure about this one . The f ratio is determined by the the length of the light travel from objective to EP, not the aperture.

By using the small hole the aperture is reduced and, has already been said, this reduces the amount of light entering the scope and also can increase the contrast (mainly by reducing glare).

F ratio is determined by BOTH the focal length AND the aperture, so doing this does actually increase the focal ratio.

The focal ratio is focal length divided by aperture so if you decrease the aperture by using the small hole in the cover you are increasing focal ratio (basically capturing fewer photons) but the focal length remains unchanged (so magnification for any given eyepiece also remains unchanged...)

[Glen]

[John]

This trick of reducing the effective aperture is all well and good but be aware that it will reduce the overall performance of the scope to that of the reduced aperture size. So your 130mm aperture scope becomes, say, a 55mm one with a large consequential reduction in light grasp, resolving power and contrast.

[sologuitarist61]

Oops, sorry Richard one and all, my apologies. I always thought it was the mirror diameter to focal point length ratio

*Hides*

Now I'm not sure, but I think the mirror diameter (ie the aperture) divided in the focal length equal maximum usable magnication doesn't it?

Feel free to correct me if I'm wrong

[JimFR]

My Skywatcher 130M has a smaller hole with removable cap in the big main dust cap.

What's that for?

Guessing it's a way to reduce the aperture but I'm not sure why I'd want to do that.

I have bought some Baader film to make a solar filter ready for the transit of Venus but I'm unsure whether I want to make a big filter to go across the whole end of the scope or just put some film across this smaller hole, which would then be protected by the removable cap.

Hope that makes sense.

I have the next one down from yours (see me sig ). the hole is to prevent the melting of the fieldstop when doing solar projection practice in preparation for 6/6...

still, could've been my eyes, I guess

[x6gas]

Powerful thing ain't it...

But then it is burning more than 500 million tonnes of hydrogen... a second

[Glen]

Now I'm not sure, but I think the mirror diameter (ie the aperture) divided in the focal length equal maximum usable magnication doesn't it?

Feel free to correct me if I'm wrong

No correction needed bud you are quite correct in saying that. but when you are viewing something bright it can be afford to loose a little aperture to gain some contrast. So i'm told:icon_scratch: you may not be able to use the scopes full 'power' but having a longer f ratio is good for definition on the brighter views and a slow f ratio on the dimmer ones. if that makes sense

[JimFR]

Now I'm not sure, but I think the mirror diameter (ie the aperture) divided in the focal length equal maximum usable magnication doesn't it?

Feel free to correct me if I'm wrong

I was about to correct you into something that was wrong, cos I didnt bother to read what you'd written, and went off on one based on what I thought you'd written !

The rule-of-thumb theoretical maximum useful magnification is something like 50x per inch of aperture, but YMMV most will go for something between 30x and 40x (in good seeing) per inch. or 2x per mm if you prefer metric

[JimFR]

Powerful thing ain't it...

But then it is burning more than 500 million tonnes of hydrogen... a second

missus made me show the kids, as a demo of why we don't look into the eyepiece when the scope is sunward !

[Hanuman]

Well I gave it a go yesterday and taped Baader Planitarium film over the hole. Taped it over the outside as that felt safer. Less chance of it coming unstuck inside the tube while observing. The smaller dust cap over the hole also protects it in normal use.

Taped over the finder scope to be safe. Whilst it's only a RDF it seemed like good practice.

Targetted the filtered scope at the sun by moving the scope until it's shadow was just a circle.

And then very cautiously took a look with the 25mm. Got a filtered white disk. That'll be the sun then.

Stepped up the 17mm plossl which gave an image that almost but not quite quite filled the FOV.

Looked like a white disks and I could make out a couple of smallish sunspots. Not the most exciting image but a bit of a thrill to be thinking I was looking at the sun and how much energy is actually being unleashed there.