Which barlow and does secondary need to be offset?

[pianorealm]

Hi everyone. Posted for the first time a few weeks ago. The general advice was to keep using my binos for now and buy a telescope shortly.

OK.....so I took this advice...well for a week or so.! THEN..........

I saw a Skywatcher 130pm advertised.... couldn't resist having a look and managed to get it for 55 pounds....it's in great nick and all the packaging..manuals etc with it!

Since then it's been mostly cloudy........more than normal methinks....but maybe I've not noticed it before?

Anyway......looked at Jupiter and Saturn whilst on hols in Cornwall...WOW.....BRILLIANT! REALLY hooked now!

Right...here are my two questions..I hope someone might be able to help/

1. Using the stock barlow the image is not great. After reading various posts this seems to be the norm. So I'd like to upgrade to another 2x barlow

I've seen a few advertisements that state 'this barlow is ideal for fast scopes'. My Skywatcher 130p is an F5 scope, so from what I understand...this is bordering on a fast scope?

I've been looking at Tal...I've found a site in the US that stocks them...I've also looked at Orion shorty plus and Celestron.

Is it the case that one format of barlow is more suitable for fast scopes than another or is it just a case of the better quality the better suited? Hope this makes sense? Does anyone have any recommendations please? My budget is somewhat limited....I'd convinced my wife and myself to some degree that once I'd bought this telescope I wouldn't need to spend any more money...ha....how wrong was I?

2. Upon collimating the scope I found that the screws for the secondary mirror adjustment were locked tight. So much so I thought I was in danger of snapping/bending the support vanes. So I removed the mirror from the scope complete with vanes and then managed to free the screws.

I have used Astrobaby's guide to collimation and all seems reasonably OK now. However I understand that fast scopes have the secondary mirror offset.

Does this apply to my scope and if so....Is the offset adjusted by the vane screws on the outside of the tube or is it done in manufacture and not adjustable? Ie: Glued to the support in an offset position?

Any help or comments would be greatly appreciated!

Regards, Howard

[Moonshane]

you seem to have a good understanding already of what's often required with a newtonian - the confidence to strip it all down and have a proper go! this is a reason I love them.

you have it all pretty right - the offset is set when the secondary is glued. get the vanes/secondary stalk central in the tube (same length each vane).

re Barlows, like all optics, you get what you pay for but better quality will give better results in your fast scope (f5 is fast). I don't think there's a specific type of barlow for newts but I think the shorty ones are more designed for refractors.

[umadog]

You don't need to worry about the secondary offset: it's either already done or it happens automatically. In case you're interested, there are two ways of offsetting the secondary: http://www.cloudynights.com/item.php?item_id=2677

[Stargazer_00]

On the barlow. You may have found the image poor because you were over magnifying the scope. Essentially as the magnification rises the exit pupil (the shaft of light which leaves the telescope) shrinks.

The rule is:

clear aperture in mm / magnification = exit pupil

The accepted rules are that 0.5mm is the highest practical magnification but anything below 1mm exit pupil will start to reveal floaters (the small defects in the tissue of your eyeball). Also, as you would perhaps expect, the view gets dimmer and dimmer as the exit pupil shrinks. The result is an increasinly grainy image with floaters and lots of nasties you dont really want to waste your time looking at.

It is often better, in fact, to reduce the magnification. This will boost brightness and actually makes picking out details far easier. Different objects have different 'sweet spots' like this and you'll find these sweet spots for yourself as they are a personal preference.

My long winded point is you should first ensure that you were not pushing the scope too hard with your barlow and not assume the barlow is the fault. If you are driving the scope too hard then any quality of barlow is going to show the same exit pupil and same degraded, grainy, floater infested views.

For a 1mm exit pupil you would be using a 130x magnification (as your scope is 130mm, the rule is 130mm / 130x = 1mm) . I would expect the scope can deliver 130x all day long, 150x occasionally and 160x-170x on nights of very good seeing. I do not go along with the 'highest practical magnification' nonsense that manufacturers publish as 0.5mm although possible is barely usable in reality so practical should be replaced with possible!

[Stargazer_00]

By the way welcome to the forum and forgot to say that is a cracking price for the scope which should show you quite alot and is a good introduction to the hobby

[pianorealm]

On the barlow. You may have found the image poor because you were over magnifying the scope. Essentially as the magnification rises the exit pupil (the shaft of light which leaves the telescope) shrinks.

The rule is:

clear aperture in mm / magnification = exit pupil

The accepted rules are that 0.5mm is the highest practical magnification but anything below 1mm exit pupil will start to reveal floaters (the small defects in the tissue of your eyeball). Also, as you would perhaps expect, the view gets dimmer and dimmer as the exit pupil shrinks. The result is an increasinly grainy image with floaters and lots of nasties you dont really want to waste your time looking at.

It is often better, in fact, to reduce the magnification. This will boost brightness and actually makes picking out details far easier. Different objects have different 'sweet spots' like this and you'll find these sweet spots for yourself as they are a personal preference.

My long winded point is you should first ensure that you were not pushing the scope too hard with your barlow and not assume the barlow is the fault. If you are driving the scope too hard then any quality of barlow is going to show the same exit pupil and same degraded, grainy, floater infested views.

For a 1mm exit pupil you would be using a 130x magnification (as your scope is 130mm, the rule is 130mm / 130x = 1mm) . I would expect the scope can deliver 130x all day long, 150x occasionally and 160x-170x on nights of very good seeing. I do not go along with the 'highest practical magnification' nonsense that manufacturers publish as 0.5mm although possible is barely usable in reality so practical should be replaced with possible!

Thanks for your replies guys. Now I'm just a little more confused than before though, I'm obviously not seeing..well the obvious?

My scope is a Skywatcher 130 which has a focal length of 650mm and is F5 which is fast I believe?

I had a look at Warthogs sticky 'Eyepieces..the very least you need'. It seemed to be well liked on the forums. However he recommends 'For an f/5 scope, 2x Barlow, 8, 18, and 25mm.

Stargazer_00...thanks for your advice about pushing the scope too far...that also makes sense. But.....as I understand it, if I use the 2x barlow with the 25mm ep that would give me a magnification of 52x with an exit pupil of 2.5mm [i've worked this out using 650 divide by 25 = 26 x 2 = 52 mag. 130 divide by 52 = 2.5mm.....hope this is right]

Even so, the view is not great as I struggle to focus. If I use the 10mm ep the view is much better...even though the exit pupil, using the same formula is only 2mm.

This is the reason I was assuming the barlow wasn't that good....though I'm probably missing the point here entirely.

I hope I've explained this OK but I've probably confused everyone now.

Also, I read somewhere that the highest practical, as opposed to theoretical magnification can be worked out as 25 to 30 multiplied by clear inches of aperture. On my 5.1'' scope that works out at 127 to 153x magnification. This is bang on what Stargazer_00 recommended. Is this just a different method of working out the same?

Sorry If I sound like I'm obsessed with this and yes, I really am getting out there and just enjoying using the scope and having fun. It's just I want to maximize my viewing experience but haven't got a great deal of cash to fork out. I'd like to have a good idea before I buy anything that I'm on the right lines. Ha...but then once I know WHAT I want....which make...but that can wait.

Regards, Howard