Looking through the EPs

callingrohit (Vivek) · #1 22-11-2007, 03:02 PM

Hi all,I just have a question which may sound really silly but I really would like to get it cleared. With my GS 8" dob i received 9mm and 25mm 1.25" plossl EPs. Now, yesterday in daytime (no i did not point towards the sun and blind myself...ha ha)I pointed my scope to a far away electric pole to align the scope and the finderscope. I was able to do that successfully but my question is, when I went really close to the EP, i lost the focus of the object and I couldn't see anything. Is this correct ?Since this was in daytime it was easy for me to navigate the scope EP to the target which I wanted to view by looking in the EP from far but in the night time, would I have to always use the finderscope to locate the object first and then see it from the scope EPs; even for moon ??? thanksvivek

iceman (Mike) · #2 22-11-2007, 03:08 PM

Hi Vivek

Yes, the finderscope is almost always used at night to line the scope up to what you want to look at first. That's why it's important to ensure your finderscope is aligned with the EP view.

As for getting too close to the eyepiece and losing focus? Was the image focused? The typical viewing distance of your eye is about 3-8mm from the eyepiece lens, depending on the eye relief of the eyepiece. How close was your eye?

Did you turn the focuser knobs to focus it?

callingrohit (Vivek) · #3 22-11-2007, 03:22 PM

Hi mike,

thanks for your reply. yeah the object was very clear and I think I was more than 3-9mm away from the EP. I started off by locating the target and the focusser completely extended out. Then I started to move it towards the secondary mirror and stopped when it was crystal clear.

Next time I'll try to be within 3-9mm and see how it goes. I think I had a misconception of telescopes in regards to looking through the EPs, because I have used microscopes and binoculars and in those you really have to stick your eye really close to the EP to see the object.

thanks

----I also noticed something right now that my eyepiece always shows the image of the cross-hair and the secondary mirror, so on the EP it looks like the crosshair with the centre portion being black. Is this correct or something wrong here ? I wish the dob came with a good manual.

iceman (Mike) · #4 22-11-2007, 03:43 PM

Hi Vivek.
That black spot in the middle is normal when viewing it during the day. At night, you won't notice it - your pupil will be dilated.

callingrohit (Vivek) · #5 22-11-2007, 03:58 PM

hmm...that gives a bit of relief and I also did an initial test of collimation by removing the EP and just looking into the focusser and I can see my eye and the small dot on the primary mirror, all in the center so I think my scope is well collimated....but will wait to receive my instruction manual from the vendor as he forgot to put them when he sent me my package.

Last night, at 0400hrs the cloudy weather in adelaid had cleared off, i got excited and got stuff ready , took the scope out....man its heavy !!! and what do i see, clouds back all over the place....and this happened within 5-10mins.....i waited patiently outside, it was 0500hrs and the clouds were in place....argggghhh.... came back inside and slept...hopefully tonight will be a clear night in Adelaide....atleast wanna see the moon....

thanks

ngcles · #6 22-11-2007, 04:30 PM

Hi Vivek,

I think the "issue" you are describing here is related to the exit pupil the telescope is providing with a particular eyepiece -vs- the lack if dilation in your own pupil in daytime.

The "exit pupil" of a telescope is the diameter of the beam of light emanating from the eyepiece and varies from 'scope to 'scope, eyepiece to eyepiece in combination. The exit pupil is approximated by dividing the aperture in mm by the magnification in use with a particular eyepiece.

Assuming your 'scope is an f/6 this gives 1200mm focal length.

1200 / 25 produces a magnification of x48.

Magnification of x48 on a 200mm aperture 'scope provides an exit pupil about 4.2mm diameter.

In the middle of the day the iris in your eye contracts down because there is a lot of light available -- typically not much more than 1mm on an ordinary sunny day, but the 'scope is producing a beam about 4.2mm diameter.

When the exit pupil is much larger than the iris, unless you have the optical axis of your eye _exactly_ aligned with the optical axis of the 'scope, you will get "blackout". And when you move your eye out a bit from the eyepiece you can see an image of the secondary mirror (the black dot you saw) in the centre of the eyepiece.

I'll bet it wasn't nearly so noticeable with the 9mm eyepiece which provides x133 magnification and a much smaller 1.5mm exit pupil right?

Of course at night when your iris is fully dilated to near 6mm this ceases to be a problem. That is why it is important that your lowest power eyepiece _not_ provide an exit pupil greater than 6mm or the problem will persist at night _and additionally_ your own iris will be effectively vignette the light-gathering power of the 'scope.

I wrote a short info box in one of my Deep Sky Delights articles in AS&T on the issues relating to exit pupil and field contrast about 4 issues ago (can't remember the date off-hand and you can read more info there.

Best,

Les D
Contributing Editor
AS&T

callingrohit (Vivek) · #7 22-11-2007, 04:45 PM

wow...thanks Les...that explains quiet a lot to me and yes you are right with the 9mm EP, it wasn't noticeable at all.

Unfortunately, I stay just next to a mainroad and the streetlights cause a lot of light pollution so i'm really not sure if I will be able to get pitch darkness for proper dilation of the pupil unless I view from my bedroom which provides a lot of darkness but then I will have to see through the scope behind the glass windows which is another problem causer.....damn.........

callingrohit (Vivek) · #8 22-11-2007, 11:18 PM

hello friends,

Today was my first viewing with my new 8" dob. I knew the moon is amazing and I have photographed quiet often with my 400D but man it got even more awesome when you look through a telescope.... just wow..... me and my wife were amazed by the moon view through the telescope.....we saw it today evening at 8.40pm, adelaide from our backyard, and yes with the EP, i had my eye very close to it and saw the moon....saw each and every detail - the craters and all which were not so visible with the photographs that I take since the subject is very bright.....later sometime I'll post the moon images i have taken using my 400D and today later in the night I'll try and capture a photo through the dob.

The 25mm EP has a good field of view as compared to the 9mm which is good for details as it magnifies more...I'm waiting for my barlow to arrive which will magnify it further....

Just quickly I did see Rigel and Canopus through the scope and even after fine focussing they looked nearer and magnified, but they still looked the same....is that correct ? Should they look any different ?

thanks

astroron (Ron) · #9 22-11-2007, 11:26 PM

Hi Vivek,they will look the same except Bright! but Rigel is a Double star, Have a look with your 9mm eyepiece and you will notice the companion
Regards Ron

ngcles · #10 23-11-2007, 12:10 AM

Hi Vivek,

Sounds like you had a great "first light" for your new toy.

The way you describe the images of Rigel and Canopus is the way they should look -- like tiny intense, brilliant points of light. Except the Sun, all the stars are way, way too far away to be resolved as a disc like the Sun, Moon or the planets.

Assuming your 'scope has a 4-vane spider, bright stars should also have 4 diffraction spikes 90 degrees apart. These spikes are caused by the vanes holding your secondary mirror and the wave nature of light. They are generally only seen on fairly bright stars. If it is a 3-vane spider, it will show 6 spikes 60 degrees apart.

The very largest and most sophisticated telescopes can produce "synthesized" images of the surface of a handful of other stars via interferometry -- even so, the images are pretty poor and not well sampled. For example see here:

http://en.wikipedia.org/wiki/Betelgeuse

Stars become much, much more interesting when they are in double and multiple systems, and when many are gathered together in a cluster. As noted above by Ron, Rigel is a double star, but the companion star is very faint by comparison to Rigel itself and pretty close in. Now you know that I'll bet you look for it and find it with the 9mm ep.

Hope you've got a basic star-atlas to find some more of these things.

Make sure you join an astronomy club where there will be lots of people to help you along the way and help you to find lots of interesting things to look at.

Best,

Les D
Contributing Editor
AS&T

callingrohit (Vivek) · #11 23-11-2007, 04:00 AM

Hello once again,

sorry to keep posting in the same thread but I just finished my second round of my first viewing night and it was just amazing....being a beginner , the 8" dob rocks the world and shows a hell lot of amazing stuff which the naked eyes can't see especially in the light-polluted urban areas. Thanks Les & Ron for explaining me the reason of the 4 diffraction spikes seen around the stars.

I don't have a star-atlas book, I do have the Astronomy yearbook but thats for 2008, so for now, I use the following site http://skychart.skytonight.com/obser...t/skychart.asp and print the pdf it generates and thats as easy as it can get. I have a handy compass with a LED on it, sticked a red tape on it, and bang..i had a compass with RED light. It works for me...got the sense of direction and thats it then it was easy to find the usual constellations. The stuff that I saw was - the complete Orion constellation, M42, Mars, Procyon, Cirius, Canopus, Achernar & Alpha Centauri.

The clouds/interstellar gas at M42 looked beautiful although it was a bit faint as in I wasn't able to see any colour but very crystal clear along with the stars around it. I wasn't able to really differentiate M43 from M42 but then maybe the 2x barlow + 9mm Plossl EP will help....

Mars looked like a small red ball, took me a while to focus properly on it.

After reading the replies regarding how the stars look, i was relieved that my scope works..... I did have a look at Rigel again, i could make out the 2 seperate stars but not quiet well...will try again later sometime.....

I plan to stay awake for another hour, watch my favourite planet - Saturn and then sleep...ofcourse dreaming about me sliding around the saturn rings....i just hope my EP's are able to show me the rings.....

Thank you all for helping me out in understanding how to choose a scope and helping me out with my weird questions...you really made it easy for me to enter the world of Astronomy. Even though there is a hell lot more to learn and see, for a beginner and a person like me who since childhood always wanted to see constellations and planets upclose, its really a dream come true. Just awesomely amazing.....Ice In Space & Astronomy rock....

Goodnight....well actually goodmorning....its 3.30am here....

Vivek

erick (Eric) · #12 23-11-2007, 09:14 AM

Quote:

Originally Posted by callingrohit

Just awesomely amazing.....Ice In Space & Astronomy rock....

You said it! I'm with you on that!

Glad you are having fun, Vivek. Makes me desparate to get out there again. But have to be patient - this rain is needed in Melbourne!

cahullian · #13 23-11-2007, 10:47 AM

Hi Vivek
I have had the same scope as you for a couple of years and it still rocks my world. They are suce a good scope (some will say starter scope) I will use mine for years and years to come. There are many, many more sites that will thrill you no end in your nor too distant future.

Gazz

tileys · #14 23-11-2007, 11:28 AM

Vivek,

Thanks for your posts...you enthusiasm is infectious

- I too can't wait to get out again soon (lots of cloud here at the mo

I'm right behind you...I bought my 8" dob a few weeks ago with the 9mm and 25mm EPs. I took a long lunch...ahem...and went over to Andrews Communications yesterday and bought a 2.5x barlow (looks very nicely put together and cost me all of $39 !) that has just arrived with their recent shipment direct from GSO - can't wait to 'conquer' Mars with it when the cloud disperses...

I've gone a bit crazy and also recently bought a red light finder - one with an adjustable brightness. I've yet to pluck up the courage to bolt it onto my scope though (want to keep the 8x50 finder as well).

Haven't been up for Saturn yet but got some great views of Jupiter the other night - I used the online java applet to check which moons were which - superb...

I've been using Stellarium on my laptop - I drag it out with me for viewing sessions to help me find things - pretty lost without it for the time being in fact.

Cheers,

Steve

tileys · #15 23-11-2007, 11:31 AM

Vivek...oh yes, I've got a Canon 350D as well that I'm keen to use with the scope for the moon at some point - I'd be keen to see your snaps and also get an idea of how you're using your DSLR with your dob.

PM me if you can...

Cheers,

Steve

callingrohit (Vivek) · #16 23-11-2007, 01:05 PM

The night was very beautiful yesterday. I just woke up as i slept at 5 in the morning. I should have done this on the weekend but you see a beginner is very curious to see - How well is my scope ??? What can I see ??? How much detail can the telescope resolve ? All my questions were answered yesterday....

Thanks for your comments Eric.

Gazz, you are right, I don't plan to ever ...i mean ever give that dob away..its awesome...I just may buy a EQ6 in future for proper astrophotography.

Steve, have a look at this page http://www.iceinspace.com.au/index.p...63,201,0,0,1,0. Its a guide on how to do astrophotography using a dob by Iceman (Mike).

I tried to do the afocal method as I didn't have the t-ring or the t-adaptor to connect the camera directly onto the focusser. Ofcourse since it was my first time connection, I didn't try much as I wanted to see more with my own eyes and then record it later but it was too late and i was very sleepy. So the next time I go out....I'll be more interested in photography now, so will post or let you know how it went.

But I think even though there is no tracking available on the dob, if the finderscope's crosshair is perfectly aligned with the dob's, then you can find the star, nebulae or planets and center them on the finderscope.. Have the camera attached to the focusser via the T-ring & t-adaptor method and you are ready to shoot. From what I saw yesterday, like the M42 and Mars, they drift very easily from the field of view especially if you are using the 9mm EP as its field of view is small but high-magnification. So what I did was, I printed the current sky chart from http://skychart.skytonight.com/obser...t/skychart.asp, its apparently valid for the next hour, so for example, you get the skychart for 1.30am, that particular map is valid till 2.30am, then the positions change. So I also got sky chart for the next hour that is for 2.30am which would be valid till 3.30am. Then I sat with both the maps and saw the direction in which these stars move. I marked the direction on both maps for the stuff which I planned to see through the scope.

Then for example, for the M42, I located it with the finderscope, and I knew approximately what direction it drifts into, so when I used to see it very close to the end of my field of view on the 9mm EP, i would move my telescope slowly with the other hand, still with my eye on the EP so that the M42 always stays in view. As i see it, for us, who have dob mounts, this is the only way for tracking. If anyone else has even more better solution other than the DIY tracking platforms, I'll be more than happy to take on board....

I have also made a list of simple and easy to locate DSOs which I have planned to see on my next viewing. I too used stellarium and the sky charts as I know the time and date when I would be viewing next (TONITE....hee hee...hopefully the weather will permit to do so...) to know the exact location of the DSO. Always make one or two stars or constellations as your astro-marks (thats what I call them) as they can easily guide you where to look. I made some astro-marks yesterday as they were easy to spot - Spot orion, if you know its orientation, then its easy to locate Betelgeuse, just below it and little to the right is Procyon, diagonally straight up to Procyon and next to κ Ori (Saiph - Orion's right knee) is Cirius and so on. This helps a lot in locating and planning for spotting the next DSOs on the list.

so there you go thats my way of tracking the objects. Tada.......

Thanks