Higher power eyepiece question

[richarddownunder]

Hi Folk

Right, this is likely to show my lack of understanding...

I have been enjoying the deep space sights with my new Orion Optic VX10 (f4.8) and a 22 mm Vixen eyepiece. I have collimated it the best I can without any gadgets (I have a laser collimator on the way but collimation may be less than optimal at present I guess) - and have been looking at nebulae and the few galaxies I have been able to find, star clusters etc. I recently added to my eyepiece collection (of one ) , buying a Vixen 9 mm and a 2X TV barlow. Having read a bit, I thought this combo would be rather good for planets. I thought the views with my 22 mm eyepiece and barlow were quite good... bright and sharp. However, the amount of light loss with the higher power eyepiece is huge...much more than I anticipated. Saturn, for example is fairly clear but dull, even bright stars are fairly dull. Certainly not vivid pinpoints of light.

So, the question...Is this what I should expect with this combo (essentially 4.5 mm) or is it more likely to be related to poor collimation? If the collimation is a little out and the focal point isn't in the middle of the eyepiece, (see attached) presumably a 9 mm eyepiece with a barlow will be more affected than a 22 mm eyepiece. , i.e. the sweet spot is smaller. Is that correct or have I missed the point?

Thanks in advance.

Richard

[Capricorn]

On magnification alone you are dropping the brightness to 16% so a fair step down. Think about it like this - you have stepped down from a 16" scope to a 6" scope.

Additionally the eye uses contrast as much, possibly more, then brightness and at the higher magnification you will lose contrast. So detail stands out less and a "poorer" image. Maybe the brain interprets a lesser contrast as dimmer ?

The scope will be critical of collimation at f/4.8 however I suspect improving this will give a better quality image more then a brighter one, unless of course the secondary is losing a large amount of the light from the primary. I would expect secondaries to be over size and so would need a fair misalignment for it to lie outside the light cone.

[crazyjedi]

Agree with Capricorn above. Another thought to consider is that the higher the magnification gets, the worse the affect of atmospheric artifacts. The image will start to look shimmery and fuzzy in anything but the best of atmospheric conditions. Also the light pollutions appears amplified by high power lenses. I have a Meade 5.5mm lens that rarely gets an outing as the conditions are almost never stable enough to get good views through it.

My most used lens is my 12mm TV Radian at the moment as it strikes a good balance between power and light grasp and as a bonus is tack sharp. I'm going to get a 9mm TV Nagler next but probably won't get anything much shorter than this.

[valleyman]

I would suggest not going below 9-10mm as you will be pushing the scope to its maximum magnification capabilities.

Imagine you are driving a car. nice high gear smooth drive good MPH

Stickm it 1st with your foot on floor and watch your petrol gauge drop

scope are much the same. The more power you use ie smaller the EP the worse the viewing gets. Once you overpower hyou get poor results

[faulksy]

I would suggest not going below 9-10mm as you will be pushing the scope to its maximum magnification capabilities.

Imagine you are driving a car. nice high gear smooth drive good MPH

Stickm it 1st with your foot on floor and watch your petrol gauge drop

scope are much the same. The more power you use ie smaller the EP the worse the viewing gets. Once you overpower hyou get poor results

quite a good analagie

[richarddownunder]

Thanks for the replies. Looks like at least part of what I'm seeing is just related to the magnification then. It did seem to me that I'd need a fair amount of mis-alignment to get a light drop and although the collimation is bound to be out by a bit, I thought it was reasonable. One other observation though, the 22 LVW mm eyepiece with the barlow is, IMO quite a bit better than the 9 mm NLV alone (so the 9 mm seems to be a bit of a waste of time) - the former is much brighter (it also is a 65 deg FOV compared with a 50 deg and all-round much nicer to use). I had thought that ~270X mag was within the range of the scope, but sounds like it is close to the limit.

One of the reasons I have become interested in astronomy is that light and general pollution in these parts (we have moved to a spot just out of town) is pretty minimal, so on a cloudless night the views are spectacular with or without a scope!

Cheers

Richard

[faulksy]

you could probably use 270 x mag on some nights here in the uk i have managed near 300x on good nights with great detail so out there it should be better

[umadog]

I would suggest not going below 9-10mm as you will be pushing the scope to its maximum magnification capabilities.

The OP has a 10" Newtonian with a 1200 mm focal length. A 10 mm eyepiece only gives him 120x. That's 12x per inch, which barely qualifies as medium power (http://www.cloudynig...php?item_id=547). If the scope isn't reaching at least 25x per inch then something's wrong. The 4.5 mm will produce about 25x per inch. Good optics will reach 50x and beyond if the seeing will allow. You can always stop it down to verify this if seeing doesn't allow.

Richard, if you're not used to what a higher power view will look like then it can be a surprise. At low power Saturn will look rather white but if you crank up the power it will start to turn a coffee colour as the image gets darker. That's normal, I'm afraid, unless you go for a larger mirror. This is why, even on planets, big aperture helps.

The worse view at higher power can be due to two things: both atmospheric turbulence and collimation errors become more of a problem at higher power. If at higher power you see the image "dancing" a lot then it's a turbulence issue. If it just looks blurry then it's collimation issue. It shouldn't look blurry at ~250 x with a 10" if everything is right. 250x isn't asking for all that much so you should be able to hit it on many nights.

The diagram you post isn't a good representation of what you're trying to achieve when you collimate a telescope. There is no single focal point; rather, there is a focal plane. What you're seeking to do when you axially align ("collimate") a telescope is get the objective focal plane coincident with the eyepiece focal plane. See diagrams here: http://www.cloudynig...&Number=3532750 (scroll down to the one with the tilted eyepiece). If the planes aren't coincident then you get blur. What I think you're getting at with that diagram isn't axial alignment, though, but field illumination. i.e. whether or not you're capturing all of the light for the primary. To make sure of that you need to center and round the secondary in the focuser. This isn't an axial alignment. It's covered here: http://www.cloudynig...&Number=3033065

[richarddownunder]

Thanks. Great links. Yes, spot on - am I capturing all the available light from the primary - maybe I am. The image is quite stable once the scope has cooled and reasonably sharp. I have centred the mirrors as well as I feel that I can just looking down the focusser, but haven't got the tools mentioned to do it better at the moment. I'm not sure how this aligns with the comment I made about the 22 mm and the barlow being so much brighter than the 9 mm - so a simialr magnification. Does that suggest I still have the secondary alignment out? On second thoughts, I'll go and have a fiddle with rounding the secondary - you never know it may be on a slight angle to the focuser.

Cheers

Richard

[NGC 1502]

Hi Richard, lots of good advice already given. I agree that 120x is medium power with your scope and it should be easily capable to go a lot higher. A lot depends on how steady the atmosphere is, but just as important is collimation.

If it were me, I'd make sure the collimation is good, then try your 9mm eyepiece again, and on a half decent night the views should be crisp. The Vixen NLV is 'only' a 50 degree apparent field, but should perform well with sharp views across the field, I'd sooner have that EP than a cheap wideangle that's only sharp in the centre of the field. So I really don't think the 9mm is a waste of time as you mention in post #6.

One collimation tip - sort the secondary first and get that right before sorting the primary - if you do it the other way round it will be dawn and you won't have got there

You can sort the secondary with a simple collimation cap in daylight, then do the primary on a defocused star at night, use the 9mm to get it close, then barlow it to get it spot on. Once you have sorted the secondary, don't touch it when star collimating at night, just use the adjustments on the primary. You will get there !!

Polaris is your best bet as it stays put on the sky. Chasing a star across the sky doesn't help when trying to collimate !

Once sorted, you will be thrilled with what your scope will do.

Regards, Ed.

[richarddownunder]

Hi Richard, lots of good advice already given. I agree that 120x is medium power with your scope and it should be easily capable to go a lot higher. A lot depends on how steady the atmosphere is, but just as important is collimation.

If it were me, I'd make sure the collimation is good, then try your 9mm eyepiece again, and on a half decent night the views should be crisp. The Vixen NLV is 'only' a 50 degree apparent field, but should perform well with sharp views across the field, I'd sooner have that EP than a cheap wideangle that's only sharp in the centre of the field. So I really don't think the 9mm is a waste of time as you mention in post #6.

One collimation tip - sort the secondary first and get that right before sorting the primary - if you do it the other way round it will be dawn and you won't have got there

You can sort the secondary with a simple collimation cap in daylight, then do the primary on a defocused star at night, use the 9mm to get it close, then barlow it to get it spot on. Once you have sorted the secondary, don't touch it when star collimating at night, just use the adjustments on the primary. You will get there !!

Polaris is your best bet as it stays put on the sky. Chasing a star across the sky doesn't help when trying to collimate !

Once sorted, you will be thrilled with what your scope will do.

Regards, Ed.

Hi Ed

Yes a collimation cap is what I need. I'll have to track one down - or is it easy to make?. My collimation proces is pretty slap-dash at the moment, it all looks aligned down the focuser and that is about it!. OK on the eyepiece - my comments may have been a bit hasty. I do really like the view through the LVW though.

Polaris will be a challenge!!

All the best

Richard

[umadog]

Thanks. Great links. Yes, spot on - am I capturing all the available light from the primary - maybe I am.

Follow the secondary guide I linked to. This is the easiest alignment. Then again, even if you're missing about 20% of the light you'd hardly notice it.

I have centred the mirrors as well as I feel that I can just looking down the focusser,

Then it's not collimated. There's no way you can achieve the required alignment accuracy for high power viewing in this way. You will get blurry views at higher powers because of this.

I'm not sure how this aligns with the comment I made about the 22 mm and the barlow being so much brighter than the 9 mm - so a simialr magnification.

The exit pupil is 50% greater in area with the barlowed 22mm so that means 50% brighter. That may be all there is to it.

Does that suggest I still have the secondary alignment out? On second thoughts, I'll go and have a fiddle with rounding the secondary - you never know it may be on a slight angle to the focuser.

Exactly. Judge the secondary positioning using the correct techniques. Don't second guess it based on what you think a high power view should look like. Besides, if your secondary position is leading to light loss then you'll get that at all powers not just high power.

[NGC 1502]

Hi Richard, you can make a collimation cap from an old 35mm film container (I got a bag of them free recently from a local photo processor)

Cut off the bottom, then drill a 2mm hole in the lid. Most of them will fit the focuser, some tape will add packing if it's loose.

With the CC in the focuser and the scope pointing at a blank lit wall, you can see if the secondary is centered under the focuser and if the reflection of the primary is centered with the secondary. At this point, ignore the reflection of the spider.

To check if the secondary is under the focuser, the edge of the secondary should be concentric with the outline of the bottom of the focuser. A longer sight tube is better for this, but with care the CC gets this bit done. Hopefully OO did this before sending the scope, but of course it has been posted a long way......the centre bolt and 3 tip/tilt screws on the spider are used to sort the secondary. You can use the reflection of the primary mirror clips to get the secondary pointing accurately at the primary, and also looking at the centre spot (or 'doughnut') on the primary and centering it with the hole in the CC.

Then, have a look at the reflection of the spider. If it looks central, you may get away with tweaking it into good collimation with an out of focus star at night. If it's a bit off with your daytime check, get it as close as you can using the primary adjustments, fine tune it with the out of focus star. An important point when star collimating is to centre the star in the field of view after each tweak. Make a mental note of which screw you last turned and whether you tightened or slackened. Did that last tweak help or make things worse ? If it helped continue with that screw, if it made it worse, reverse the screw. I'm sure you get the picture here, it's trial and error.

When I first had my OO, I found that when well collimated by star check, then the doughnut centre mark was a bit off when checking with the CC by day. I tried several times and came to the conclusion that OO had not precisely put the doughnut dead centre. The star check tells the truth, so I learned in which direction and by how much the doughnut was off. It was very close, but not precise. Since then I've had the mirrors recoated and added my own centre spot, and guess what, mines a tad off too, no prob as I've just described.

Now, there are lots of opinions on how to collimate, and I've sadly seen threads elsewhere get heated over what's best, so please guys, anyone reading this who has a method that works for them, I expect Richard would like to hear, but all I can say is my method works for me and I get great views through my scope

All this will seem daunting if you are new to it. Once the secondary is sorted, then most of the time all you will need is a tweak on the primary. Once you get the hang of it, it all falls into place, a bit like when you first drove a car and there seemed so much to get right at first.

I think the secondary is hardest one to get right, because it can be rotated on the centre bolt, go closer to, or further from the primary, or the 3 point tip/tilt. The primary is a piece of cake in comparison. But you can do the secondary by day, so that helps.

Please ask if you need clarification.

OOps Richard, sorry if you are too far south for Polaris ! Best if you use a bright star not too far from the south celestial pole then, it's apparent movement will be less than away from the pole.

Regards, Ed.

[richarddownunder]

Hi again Umadog, agreed it isn't properly collimated (although it isn't blury...or wasn't before I started fiddling), but at least, as far as I can see, the secondary is well aligned. I made a simple collimation cap, as Ed suggested, by drilling the centre out of a film canister - these are hard to find these days! Looking down it, the secondary seems perfectly centred. I'll see if I can borrow a laser collimator. However, it sounds like a cheshire collimator would also be a useful tool. No viewing for a while anyway, the weather is apalilng and no amount of alignment will fix that!

[LeeB]

Agree with most of the above I use basically the same scope and while most of the time I use 40-160x for deep sky, on planets and good nights 400x is possible , but collimation with this f4.8 is key , it needs to be spot on to hit 400x even with great seeing. The clear night we had a month ago for some reason was amazingly clear, so uranus and neptune became targets, they held up at 500x , couldnt believe my luck so went silly for 10 minutes to see how much mag I could pile on, (i know it doesnt help but hey its fun) , a 6mm and 2x and 3x barlow later i made that 1200x and uranus was as big as a button, no detail just a disc but amazing all the same. The scope you have is great but to repeat the above most of the time 40-160x is where you will be and its remarkable on a normal night how quickly extra mag will detract from the image. Its a test and see thing but do get collimating.

[richarddownunder]

Hi Ed, l followed you previous suggestion about the collmation cap and discovered the film canister idea on the net. Looks like great minds think alike

As mentioned, the secondary looks OK, so next time we have a clear night (whenever that will be) I'll try the star check. I guess a laser collimator is only as good as the centre spot on the mirror so I hope mine is fairly accurate - I ordered one of these gadgets a while ago but it hasn't turned up yet. Yes, Polaris is rather out of my field of view unless I drill a large hole . Everything moves down here (including the ground).

Cheers

Richard

[LeeB]

As an extra point this scope does reward the odd good eyepiece, the bst's are good as are the hyperians, for planets I still like an ortho in the bag, and for wide field I just tested one of those gold skywatcher 80 degree and it was wow.(its on the xmas list) The colours were the best I've seen. for normal hunting I use a cheap 2inch revelation 65 degree from telescope house.

[richarddownunder]

Not sure if this helps but it does show the primary centre spot is out a bit.

[LeeB]

the centre is just out (assuming your spider is central), the shadow of the secondary will show a slight offset, (3mm or so if my calcs are correct) due to fast f4.8, a cheshire would help to centre the centre mark. It may look small but this amount of decollimation will affect the image particulary as mag increases. Also a laser will help but make sure you check the laser itself for collimation or you just repeat its error on the scope.

[LeeB]

for a quick fix, given its hard to extract all info from photo,its the primary that needs the slightest oif tweaks , unlock the locking screws and give a knob a tiny tweak and try to centralise the donut. a second person helps.

[NGC 1502]

Hi again Richard, can't see from that pic if the secondary is exactly under the focuser, but OO probably got that right in the factory.

What the pic does show is that the secondary is pointed properly at the primary as the 3 primary mirror clips show.

If it were me, I'd not touch the secondary myself.

Looks like just the primary needs a tweak.

When star collimating to adjust the primary, it helps if you know which adjustment screw to go for first.

You can just try one and see what happens, as I said earlier, does what you just did make it better or worse, trial & error.

Or you can defocus by a large amount, put your finger over the front end of the scope so it's protruding into the light path, slide your finger around

so it's shadow can be seen where the error is worse, then trace that position along the tube to the primary end to find the screw nearest that point,

then adjust that one first. You can do that at medium power.

When you are close to correct collimation, go to high power to fine tune it. At this point don't defocus by much, and centre the star to see how things look.

This centering the star within the field of view is important.

Looking at the pic, you may find you only need a few tweaks ( I mean the scope, not you )

Regards, Ed.

[richarddownunder]

Hi All

thanks for the advice.

Ed, I have already fiddled with the secondary... quite a bit. Actually I took it out to have a look at it but I think I have it back in the right place - I have a bit of a compulsion to take things apart . Centering the camera on the focuser seems to work quite well as its easier to see the centre spot on the primary. I need more than a tweak. I have a rotten cold so am off work at the moment

LeeB, adjusting the primary worked OK. It sounds like laser collimators can be a bit of a gamble - I'm not sure how you'd precicely collimate a collimator without an accurate rig - perhaps just rotating it in the focuser will show whether it is projecting a parallel beam. Anyway, I need a clear night to have a go at star collimation. That may be a week away.

Cheers

Richard

[Naemeth]

Hi All

thanks for the advice.

Ed, I have already fiddled with the secondary... quite a bit. Actually I took it out to have a look at it but I think I have it back in the right place - I have a bit of a compulsion to take things apart . Centering the camera on the focuser seems to work quite well as its easier to see the centre spot on the primary. I need more than a tweak. I have a rotten cold so am off work at the moment

LeeB, adjusting the primary worked OK. It sounds like laser collimators can be a bit of a gamble - I'm not sure how you'd precicely collimate a collimator without an accurate rig - perhaps just rotating it in the focuser will show whether it is projecting a parallel beam. Anyway, I need a clear night to have a go at star collimation. That may be a week away.

Cheers

Richard

Do you have a Cheshire to check the collimation of the secondary. I'm sure the shadow of the secondary needs to be off centre, especially in a fast newtonian.

[faulksy]

Agree with most of the above I use basically the same scope and while most of the time I use 40-160x for deep sky, on planets and good nights 400x is possible , but collimation with this f4.8 is key , it needs to be spot on to hit 400x even with great seeing. The clear night we had a month ago for some reason was amazingly clear, so uranus and neptune became targets, they held up at 500x , couldnt believe my luck so went silly for 10 minutes to see how much mag I could pile on, (i know it doesnt help but hey its fun) , a 6mm and 2x and 3x barlow later i made that 1200x and uranus was as big as a button, no detail just a disc but amazing all the same. The scope you have is great but to repeat the above most of the time 40-160x is where you will be and its remarkable on a normal night how quickly extra mag will detract from the image. Its a test and see thing but do get collimating.

wow thats some power, must have some good skys down there

[umadog]

Richard, if anything your first attempt looked better than the second. Everything is concentric in the second image and it shouldn't be. Read this: http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=reflectors&Number=3033065 Pay attention to the images. The film cannister is better than nothing but it won't be good enough at sub f/5. You need a Cheshire. You need a sight-tube. You can get away with a combination tool comprising the two.

The shadow of the secondary isn't round in the second image, so something's wrong. I think you've tilted it too much whilst attempting to make everything concentric (see the link, don't do that).