Daft question for the night ?

[Labman]

I've got a Celestron Astromaster 130EQ. I have had it for 10 days and am still getting to grips with it but am loving it so far. I can't believe I'm having so much fun at freezing temperatures, looking at the moon !

Daft question for the night:

The large front lens cap (huge fun trying to get it back on!) has what looks like two rear lens covers from a camera lens on the front of it. They look very impressive but what on earth are they for? They are not mentioned in the instruction manual. They are too big to be spare covers for eyepieces. Anyone know what they are there for?

As an aside, I bought a Telrad star finder and after aligning it to the scope immediately found getting stars into the eyepiece was very easy. Great piece of equipment. It stopped working second time out so sent a mail to Telescope House to say it was malfunctioning and they replied inside 5 minutes and after a couple of mails to make sure I was using it properly they immediately said send it back for a replacement and they would refund my postage. Brilliant service! Is this normal in the telescope world?

Does the Telrad unit have a reputation for failing or am I just unlucky to have got a bad one?

Graham.

[Uplooker]

One of the caps, the removeable one acts as an aperture mask. The focal ratio of your scope is worked out by dividing the clear aperture of your scope into the focal length. So  if your scope has a focal length of 650mm the focal ratio is 650 / 130 to give a focal ratio of f/5. By putting the end cap on the scope and removing the "small" cap you have effectively changed the clear aperture to 30mm. Therefore 650/30= f/21. The other "cap" that does not come off is so that you can store the cap that you can take off, once you have taken it off Its not a daft question, you would be amazed at the number of people that ask that one.

Ian

[Labman]

Hi Ian.

I have just tried it and the removable cap does indeed park nicely on the smaller boss. From playing with my cameras, f21 is not going to let very much light through. Why would I need to do this? I thought my next mortgage was going to be for a much bigger diameter telescope to get more light gathering for deep space astronomy.

I've got so many questions, this is a fascinating hobby. I think I am going to join the Peterborough Astronomical Society to find other people as knowledgeable as you to pick their brains. I wish I had started this when I was younger!

Thanks for you reply.

Graham.

[Uplooker]

Hi Graham,

I only used some figures off the top of my head to illustrate a point, not sure it will actually work out at f/21, but it will significantly change the f/ ratio.

Ian

[RichM63]

Try it out when either observing or taking video of the Moon. Reduces glare, and increases contrast, although not generally employed.

[Ruud]

If you take both caps off, you get a focus assist tool.

When much out of focus, each star will give two blurred images. If you get closer to focus the images get nearer to each other as they get sharper. The images merge when the telescope is in focus. If you turn the focus knob too far the images will separate again.

Here is a video:

http://youtu.be/UPoh76f60kA?list=PL8E8362735A736A30

at 9 minutes into the video the focus mask is discussed

[ronin]

Easy answer is forget them and take off the full cover only. The moon might be a bit bright but that is about it.

Some people will put a solar filter over the small aperture but again that is mainly not adviced - safer to cover the full 130mm aperture at the front.

Before the remortgage a big aperture is not required for astrophotography, so do not go buying a 200P or bigger for AP. A small 80mm APO/ED or 130PDS will be better. As a scope goes bigger then the physical size and parameters counts against it and causes problems.

[jason.p]

Hi Graham. From my own experience the Telrad is pretty reliable. I've got two of them and the only trouble I 've had is when I left it out and the sun came round and fried the reticle. Fairly easily sorted fortunately!

I'm sure I'm not the only one who's put it away still turned on and found the battery still ok after a several days! As you say, a brilliant bit of kit. I couldn't do without it.

[hazegood]

If you take both caps off, you get a focus assist tool.

When much out of focus, each star will give two blurred images. If you get closer to focus the images get nearer to each other as they get sharper. The images merge when the telescope is in focus. If you turn the focus knob too far the images will separate again.

Here is a video:

http://youtu.be/UPoh76f60kA?list=PL8E8362735A736A30

at 9 minutes into the video the focus mask is discussed

The OP is talking about the standard lens cap that comes with the scope, which yes does have what he has described used to reduce aperture for moon or solar, not a focus mask,

OP be wary about what cameras you use on this scope,  a webcam will work great fro planetary, a DSLR will not achieve focus with the standard focuser, there is not enough inward travel on it

[rowan46]

as has been said its used to enhance contrast on the moon and solar. Although it stops an amount of light getting in. Because its off axis it takes the central obstruction out of the equation  thus cleaning up the image. I am not sure how effective they are on small scopes but on big newtonions people claim it gives a view like a larger apparture refractor excellent for splitting doubles and lunar work

[jaydeeuk1]

I had ots little brother the 114eq as my first scope and i was too afraid to ask hehe. I couldn't work out the point of it so glad to read the above!

[rwilkey]

Hi Graham, sounds like you are having a good time.  I have had experience with TH and having to return something, nothing is too much trouble, excellent customer service!

[Labman]

There is a lot more to this than first meets the eye!

I have made the calculation that Ian explained. Focal length is 650mm and the aperture when the small lens cap is taken off is 42mm. This gives a focal ratio of f/15.5 which is still a substantial reduction in the amount of light arriving at the eyepiece (no big surprise considering the small aperture) but this links in with my knowledge of lenses I use on my cameras. Fast lenses have large apertures.

Thanks everyone for the good advice and knowledge :-)

Graham.

[Carina Lass]

Also diminishes chromatic aberration for day use.

[rowan46]

Also diminishes chromatic aberration for day use.

might be so on an achromat but reflectors don't suffer from ca 

[Alien 13]

Although reflectors dont suffer from CA eyepieces do and the increase in F ratio can help get the best out of cheaper ones.

Alan

[rowan46]

Although reflectors dont suffer from CA eyepieces do and the increase in F ratio can help get the best out of cheaper ones.

Alan

Wouldn't know about that

[cotterless45]

This is a great help with a Telrad,

http://www.atmob.org/library/member/skymaps_jsmall.html

I have two and have never had any trouble with them. You can easily rig up a dew shield with black foam sheets or buy one off of Astroboot,

Nick.

[Alma]

Telrads are fairly functional pieces of equipment but designed in the stone age and i.m.h.o. over-hyped and cheaply made for the price. Most people seem to have no problem with them but others of us have had less luck - I had two fail in quick succession and wouldn't buy another. Good luck with yours!

[Charic]

I sold my first Telrad due to an eyesight issue (needed to wear spectacles to see the rings clearly defined ?) but I  felt its one of those gadgets, that when its gone, its as if somethings missing.? I bought the second one very cheap, but still go for the 9x50 in the first place. From the darkest skies, the 9x50 reticule would probably become invisible, where my Telrad will take over. Stellarium has a Telrad reticule, which is of some help when a Telrad is fitted to the telescope.