Tonights first observing gone wrong?

[JamieH]

Hi guys. After assessing my actions last night & using the telescope. I find myself asking some more questions regarding the telescopes PAE feature & the focuser with the EP. Also I didn't need to previously set up red dot in daylight. It was all done on the Moon & fine tuned last night I hope? I guesse you can get the red dot as close to the centre as possible. So here goes....

I know I can use PAE which is supposed to make the telescope re calculate its position by pressing the ESC key to re centre on an object in the centre of FOV. I don't know if this helps tracking be more precise or just find objects more preciseley.

Without using the electronics last night I managed to setup the red dot completely in centre of the Moon without having to do it in daylight prior. I noticed when magnifiying the Moon it was getting badly out of focus with the 20mm EP. So when I'm magnying an object how do I get it in focus when I'm closer to it?

I can turn the Focuser Wheels forwards to get Focus. But in reverse turning Wheels backwards will magnify an object thus become out of focus. So how do I achieve Focus when magnifying?

Jamie

[Trevor-Austin]

Hi Jamie, you don't magnify using the focusser wheels, the eyepiece selection dies that.

I doubt there is anything wrong other than a lack of understanding over what you've got. Is there no chance the place you bought it from can help or was it mail order?

I honestly think 10 mins from someone who knows it will sort it for you.

You shouldn't move it manually at all when using synscan, including up and down, or at least not on my Meade equivalent. Just use the motors or it won't have a clue where it is. Also I think you should be doing the clutch up tighter, I don't think you should be able to move it with your hands.

[JamieH]

Hi Trevor-Austin. Don't worry, Everything is completely fine. I know you are trying to help & give me advice. If I tried moving the AZ GOTO SynScan mount in Azimuth manually I would break the stepper motors. But my mount is built to allow manual control of Altitude scale Up & DOWN so I can do that there is a certain level of free movement that is specifically made for manual operation. So that's not the issue I'm concerned about now

The issue I am concerned about is the Rack & Pinion type Focuser. Turning the knobs forwards a bit does indeed Focus on the Object. But what does it do as I turn the knobs backwards on the rack & pinion focuser. The refractor tube extends backwards?

[Trevor-Austin]

the reason for the large amount of movement in your type of scope is that it has a huge range, mine can focus from 30' to infinity, but it takes a lot of focusser movement.

And depending what is mounted to the "viewing port" and how far away that is from the actual exit can also massively change focus point.

[JamieH]

Trevor I have included with the telescope a 2x Barlow lens, a metal one, & 2 metal EP's. A 20mm super wide angle long eye relief. A 10mm one aswell.

I used the 25mm one for focusing on the moon & only had to move the knobs forwards a bit. What would I need to have to reverse the knobs backwards. Would using a different EP change this ?

[sixela]

Exactly. Not all eyepieces are "parfocal", i.e. have their focal plane at the same location with respect to the shoulder of the barrel that is inserted in the focuser.

[JamieH]

WoW that is a mouthful of words

[JamieH]

Now I have to find the Little Dipper & Big Dipper... but I heard "Now... if only bears HAD tails...!"

[JamieH]

p.s. I am REALLY grateful for the astronomical amount of advice I am getting in this space! really thanks guys...!

[sixela]

WoW that is a mouthful of words

In more plain language, there is no standard as to where the barrel of the eyepiece should end and the rest of the eyepiece should begin, so in general, changing eyepieces means refocusing.

You can actually buy rings to stop eyepieces that insert more deeply than others going all the way in the focuser, and they're called "parfocalisation rings".

[JamieH]

So each EP requires re focusing. Because different EP have varying magnifications & will be closer or further away if their barrel is shorter or longer. If the EP is closer & higher power it might require less focussing. If the EP is closer & lower power it might require more focussing. The same for the other way round if the EP is lower power & further away it require more focussing, but if the EP has more power & it's further away, then less focussing is needed. I nearly confused myself then.

[JamieH]

My failed attempt to try & explain things once they've been spoon fed to me is still a failed attempt! Still...

[sixela]

Ok. If I am getting your drift properly & thats a big IF... I'm guessing each different sized EP will have a shorter or longer magnification, a longer or shorter lens. Plus the barrels on the EP can be longer or shorter so thus meaning depending on how much closer or futher away the EP lens is to the focuser when inserted will directly impact the amount of light the focuser is going to bring

Not really. The eyepiece always uses *all* of the light that comes through something called the "field stop" (the thing that makes the hard edge around the field of view that you can see when looking through the eyepiece); it's not a matter of "getting more light in".

If you want a focused image with your eye focused at infinity, then the eyepiece's effetcive field stop *has* to be at a certain distance to the objective of the scope. If it's not, the light bundles coming out of the eyepiece for a given point in the field will either be diverging (meaning your eye will have to focus closer than infinity for you to see a sharp image, up to a point, and deliver a fuzzy view beyond that) or converging (which will make your eye unable to give you a sharp image).

And as no one has decreed "thou shalt end the 1.25" barrel of an eyepiece at the location of the field stop", different eyepieces will require a different position of the focuser draw tube to make the eyepiece field stop be at the proper position (called the "focal plane" of the telescope).

It's also physically impossible for some eyepieces to place the effective field stop in the 1.25" or 2" barrel. Some eyepieces --in particular wide field eyepieces-- have a lens group in the barrel and the field stop inside the eyepiece between lens groups, in a wider part of the eyepiece that simply wouldn't fit in the barrel.

[sixela]

Ok. If I am getting your drift properly & thats a big IF... I'm guessing each different sized EP will have a shorter or longer magnification, a longer or shorter lens. Plus the barrels on the EP can be longer or shorter so thus meaning depending on how much closer or further away the EP lens is to the focuser when inserted will directly impact the amount of light the focuser is going to bring

Since you seem interested in the theory, allow me to expand (at the risk of becoming a bit technical, but you can stop reading at any point).

The eyepiece always uses *all* of the light that comes through something called the "field stop" (the thing that makes the hard edge around the field of view that you can see when looking through the eyepiece; it's not a matter of "getting more light in".

If you want a focused image with your eye focused at infinity, then the eyepiece's effective field stop *has* to be at a certain distance to the objective of the scope, so that each point in the field will correspond to a parallel light bundle exiting the eyepiece. If it's not, the light bundles coming out of the eyepiece for a given point in the field will either be diverging (meaning your eye will have to focus closer than infinity for you to see a sharp image, up to a point, and deliver a fuzzy view beyond that) or converging (which will make your eye unable to give you a sharp image).

And as no one has decreed "thou shalt end the 1.25" barrel of an eyepiece at the location of the field stop", different eyepieces will require a different position of the focuser draw tube to make the eyepiece field stop be at the proper position (called the "focal plane" of the telescope).

It's also physically impossible for some eyepieces to place the effective field stop in the 1.25" or 2" barrel. Some eyepieces --in particular wide field eyepieces-- have a lens group in the barrel and the field stop inside the eyepiece between lens groups, in a wider part of the eyepiece that simply wouldn't fit in the barrel.

In the "one picture is worth more han a thousand words" series, here are some light bundles for different points in the field in an eyepiece:

[Left an right are the same thing, but the right image is someone with glasses using the moveable eyecup set low]

The field stop is at the left, where all the light for one object point focused by the scope converges to a single point on the focal plane. You see that for this eyepiece (a 10mm Pentax XW), the shoulder of the eyepiece is indeed roughly where the field stop is (they haven't drawn the 1.25" barrel).

[JamieH]

Sixella. I don't fully understand this. You'll have to try & explain it easier by marking points on the pictures or pointing things out when illustrating what you say in relevance to the pictures. I really want to understand this. I'm sure it's all pretty easy, really!

Thanks

Jamie

[Glider]

Hi Jamie, I'm starting to feel as bit desparete for you as I agree with a previous post that for the sake of spending 10 minutes with someone you would have a lot of things sorted and be happily enjoying some actual quality time observing with you scope.

Is there a local astronomy club nearby?

Without giving your address away, what is your nearest town?

[JamieH]

Nevermind. It's ok

[sixela]

Given that you've said you want to understand it (but if you don't, remember that you don't have to understand any of it to enjoy the views!):

-The telescope has a lot of light bundles coming into the scope, all hitting the objective at the front in your case (the "entry pupil"). For each point in the sky (star, point in a nebula, etc.) the light bundle for one point is a parallel bundle of light hitting the objective. There is an infinity of bundles, one parallel bundle for each point in the field you're observing.

The scope's job is to focus each of these bundles onto a single point at the back of the scope. The collection of all these points, one per point in the sky, is called the "focal plane". If you point your scope at the moon and put a piece of paper at the focal plane (or a camera sensor), you'll see an image of the moon perfectly in focus.

So the job of the objective is to transform each parallel bundle of light into a cone of light that hits the focal plane, with each different agle of incoming light corresponding to a different spot where that particular light bundle converges on the focal plane.

Once you've got that, the job of the eyepiece is to convert those cones of light (which continue merrily beyond the focal plane into a diverging cone of light for each bundle if you don't put a camera sensor there) back into another set of parallel bundles coming out of a circle hovering over the eyepiece. In effect, the eyepiece is a kind of "mini telescope in reverse", but it's a lot more complicated because the angles are larger.

The bundles at the exit of the eyepiece (through the "exit pupil", which is actually an image of the objective) are going to be

a) more narrow,

exit at a larger angle than the bundles coming into the telescope,

so that you have magnification (otherwise, the scope plus eyepiece wouldn't do anything). That's what the figure is showing.

In the middle, you have the bundle for what is called the "on-axis object". It starts as a diverging cone of light from the telescope's focal point and the eyepiece converts it into a parallel bundle that also goes "straight ahead", and that's what you're going to see in the middle of the eyepiece view.

At the top and the bottom, you have light bundles that enter the scope at an angle (from the objects at the edge of the field you're seeing with the scope and this eyepiece. These go through the eyepiece and are converted into light bundles that enter your eye at angles of (for this eyepiece) 35°.

What you then do is place your eye at the "exit pupil" of the eyepiece, and then those parallel bundles of light are focused by your eye's lens onto the retina.

For all of that to work, the eyepiece expects the light bundles from the scope to converge to a point at a specific location with respect to the eyepiece. The focuser lets you move the eyepiece closer and further away until that is the case. When it isn't the case, you get blurry views (unless your eye corrects for it by focusing differently, as your eye is perfectly capable of focusing on objects closer to you than stars, i.e. to make diverging light bundles converge to a point on your retina).

For terrestrial observation, the light bundles entering the scope aren't entirely parallel as the objects are closer than "infinity", the image plane (where you'd see a focused image if you inserted a piece of paper) actually moves a bit, and moving the eyepiece also lets you correct for that.

There's an excellent figure here:

Astromart Articles - Wild Card 001 - The Exit Pupil of a Telescope

[andy]

Hi JamieH,

I cant help you will all your issues, (and I'm nowhere as near intelligent as sixela!) but I have a Synscan AZ (with Skymax127 mounted), I'll try help a little on that. And a couple of other tips

OK, first of all I usually never bother with worrying what the RA/Dec or alt/az coordinates of an object are, and I don't usually bring my laptop outside while observing either.

What I do is take a look at Stellarium a while before I go outside to identify good alignment stars, and also observable targets.

I would put Stellarim into fast fwd to see how my view of the sky will 'pan out' over a few hours, and what constellations are visable to see what objects to go for (the handset will tell you what constellation an object is in, plus rise and set times).

The best alignment stars are usually the brightest, but to get the best results it is recommened that your 2 alignment stars are at roughly the same alt, and at least 60deg apart in Az.

Once you do your inital set up, and choose 'yes' to begin alignment, choose a star you've already identified, and slew the scope with the handset directional buttons, at an appropriate speed. As a tip, before leveling my mount, I usually put the tripod down so that the OTA roughly points to my first identified alignment star, just to save time, as the scope does not need to be polar aligned like an EQ mount.

When you slew at a high rate and start getting close to the star, sight along your tube until it's fairly close, then maybe reduce the rate and locate the star in red dot finder/finder scope. Press enter once, when centered in finder, then centre in the main scope and press enter again.

Then choose your second star from the list and the scope should slew itself fairly close to it. Then, again, locate the star in red dot finder/finder scope. Press enter once when centered in finder, then centre in the main scope and press enter again. The handset should (hopefully!) say 'alignment succesful'. You can test the accuracy be sending the scope to something else (perhaps Jupiter) if it doesn't land exactly on target (remembering to let it finish slewing completely... it will beep when done) you can then centre Jupiter with the handset and hold Esc to use the PAE function.

It is better to make sure your mount is perfectly level, and isn't moved once you begin the alignment process, instead of relying on the PAE.

As for your OTA moving in Alt axis, it may be a question of balance. As I have said I have a different scope witha shorter tube length, so I can't really comment about yours, but I still have to balance mine by sliding the dovetail to a balanced position before tightning it up... has your scope got a decent length dovetail bar?

As for the slew limit message, you can set slew limits in the setup menu, to avoid get the message and the scope stopping. Just be ready to stop the scope if it looks like there is going to be a collision. Again, your longer scope may have more of a problem in this area.

On to identifying stars once you go out to observe. The first thing you'll wll probably notice is the scale of asterisms and constellations. If you're looking at the plough on stellarium (depending on zoom level) or on a planisphere, it will be much bigger when you go looking for it yourself, as will the distance from the pointers to polaris. Maybe just use stellarium to take note of the direction and orientation of the plough before going out, to help you find it.

Also I don't think I have ever used polaris as one of my 2 alignment stars (it's not that bright) Generally you should choose brightest stars you can. Depending on your available clear viewing directions and then time you go out at, you might try Vega and Capella, or maybe one of the stars in Orion.

Anyway, keep at it, and enjoy it as much as possible.

[sixela]

then centre in the main scope and press enter again.

Always end the centring using the same two arrows too. That helps avoiding backlash confusing the alignment.

[JamieH]

Sixella thanks SO MUCH for taking your time & being patient with me. I am really grateful. When I am into something which interests me. I can't let it go, I must find a way to understand & take in the knowledge which is useful likewise with computer & technology. Likewise now with Lenses, mirrors, astronomy!

I understand that explanation much much easier than the earlier ones. Their just might be a few terminology words I need to be clear about & where they exist in an actual telescope or EP etc.. just so I can read through your excellently detailed explanation & easier findout where they are.

These are the "Objective", "Exit Pupil", "Infinity", "Focal Plane" & also where is the "Field Stop". I get the gist of it better than ever. It's actually very easy really it's just the complicated words that explain them. Once I understand them I will understand it much better.

Andy. Thankyou. After my first observing session Mon night, I went through with people on the forum the SynScan setup on Tues night. They & I realized I was cold, confused, miserable & that nothing was wrong with my scope. It was just me over the cold & confiusuion & I was doing things wrong & maybe a certain degree of panicking. We decided that I just have a hard time finding Stars. I need to identify stars better.

On tues night in bedroom I realized my scope was on the SLUG SLOW (default) rate of 4* so I change it to 9* just before someone mentioned this. It dawned on me. Stupid silly doh. I figured just before the forum member how to change it & get the thing moving as they picked up on the tracking rate & they also picked up on the time being wrong.

When I was out on Mon night I realize I didn't have the local time set properly it was probably still at the default 20:07. I kept wondering why the time wasn't changing everytime I tried the alignment in front & back garden. This might be why my scope was way out of direction anyway even though I hadn't successfully pointed it to the first or second alignment star, I pressed enter too quickly from first to second alignment star & wondered why the scop slewed not far & not the correct direction.

No wonder & I didn't realize that certain setup parameters needed to be put in at the start of every observing session. Also I kept wondering whether the timzone was correct. As my SynScan ver is an early version & not the latest one which has a problem unless -00:00 is entered or alignment gets screwed up. I still set mine to -00:00 because I in the west even if it doesn't do anything ir is useless. It's just the telescope manual says to set - for west coordinates. SO... I play safe.

So now my scope slews at 800x rate9* speed which is better than rate4*64x speed. I suffice to say that rat2* would win the slowest slug competition for guiness world records. Andy I really enjoyed & ook some very good advice & tips from you. I need to set up the slew limit. As I went through setup twice tues night & the first time the slew limit was 23 degrees wehn bringing scope on the DOWN AXIS. So this wasn't going to affect it hitting the mount as slew limit was wrong direction. Also when I went throught with them the tues night first setup. I had the wrong date set. Doh...

The second setup my scope went all the way past 0 degrees on the DOWN AXIS bringing it down. When I took it up, I managed to get it to nudge the mount & when it nudges the mount, it's the focussing wheels that hit the mount gently. So when it hits the mount, it's not even hit the 90 degrees marker, it's something like 8 somehing degrees. I don't think there will be many stars needing 90 degrees or even 80 something, if they do my telescope will hit the mount & never make. I guesse they'll be no go stars. I must remember to make I get all the settings right on the SynScan setup. It's imperative. Silly boy!

Thanks for explaining everything to me Sixella, it's wonderfully made. & thanks Andy for you equally brilliant advice & tips too.

Jamie

[sixela]

These are the "Objective",

Whatever forms the image (i.e. focuses the light bundles from the objects far away). In your scope it's the set of lenses at the front.

"Exit Pupil",

The little circle through which all the light bundles go when they exit the eyepiece. You place your eye's lens there to catch all these bundles.

"Infinity",

Objects so far away that all the light rays emanating from them are parallel. Kind of "far enough to focus at the same position as stars do".

"Focal Plane"

The collection of points where the different light bundles (one for each point in the field of view) converge (each point on that plane corresponds to one light bundle converging there).

also where is the "Field Stop".

At the focal plane of the eyepiece; each light cone that emanates from a point on that plane will actually exit the eyepiece as a parallel bundle.

The "field stop" is used to stop the light for all the objects that would look too ugly at the exit of the eyepiece. As in a scope it's placed where each light bundle collapses to a point, the field stop either lets all the light go through (for the objects that are in the field) or none (for the objects that are hidden). When you look through the eyepiece, the circle around which everything is black corresponds to the field stop, which in the eyepiece is simply an obstacle with a round circular hole. If you have a Plössl, you can physically see the field stop by looking in the barrel.

I guess they'll be no go stars.

Everything at the zenith eventually makes its way to smaller elevations with a bit of patience, unless you happen to observe too close to the North or South pole on earth . So a bit of patience will make these "go stars" again.

[JamieH]

Focal Plane & Field Stop still not sure about.

In the picture on the far right of the diagrams I see a big round circle, is this meant ot be the EP. Inside this I see there are 3 yellow parallel bundles of light which are being spread out to the exit pupil at far right of the picture?

The 3 yellow light bundles are being spread out from a smaller full circle which is partailly blocked with a line through it. There is a little space here on the left side of the smaller black circle. This space is right next to the telescope tube where the 3 yellow bundles of light come to a point & meet this little space on the left side of the little circle.

The top yelloe line of light comes across the tube & hits this little black circle but crosses it & comes out at the bottom. The middle line of yellow light comes & hits the centre of the black circle & pases through the middle of the other side of the little black circle. The bottom yellow line of light comes across the tube & crosses the little black circle & comes out at the tope on hte other side of the little black circle.

[sixela]

Focal Plane & Field Stop still not sure about.

In the picture on the far right of the diagrams I see a big round circle, is this meant ot be the EP.

If you mean the Pentax XW diagrams...from left to right: focal plane (where light bundles have just collapsed to a point then start to widen again before entering the eyepiece) - cross section of the eyepiece with all its lenses - exit pupil (where all bundles cross again) with eye lens of the observer - eye ball (in white) - retina (where the bundles once more collapse to a point). Everything pink is the observer's facial flesh.

The scope isn't depicted at all (it's on the left of the focal plane). The diagrams only depict what happens from the focal plane through the eyepiece into the observer's eye, until the bundles hit the retina and the light is detected by the cones and rods.

The diagram on the right is someone with glasses (where you'll see that between the eyepiece and the exit pupil there's an extra lens; that's a section of this observer's glasses.)

If you want to see what happens in the scope then the eyepiece, you have to follow my other link to this image for a conceptual sketch, but it's not at all to scale.

[Celeste]

Your first post sounds so much like my attempt at aligning our scope If it makes you feel any better, Polaris was 0.60° out I can safely say it was a disaster!

Oh and BTW...I wouldn't go posting your co-ordiantes on a public forum :S (if indeed they are your actual co-ordinates) You never know who's looking.