Nudging a Dobsonian

[Hairy Gazer]

I currently have a Celestron Powerseeker 127 and am obviously finding it hard to see DSO's (Andromeda and M13 looks like a grey smudge).

I was looking at buying a 10inch Dob, but was wondering if Andromeda and M13 would still look like a grey smudge, but just a bigger one.

 

Also, I was worried about nudging a dob at 400x magnification, would the object just shoot out of view?

Any advise on this would be great.

[John]

400x is generally not useful in any telescope, whether on a driven mount or being manually tracked.

With a little practice, tracking a dobsonian by nudging becomes almost 2nd nature at useful high magnifications of 180x - 250x.

The Andremeda Galaxy is M31 (Andromeda is the constellation) and it will still look like a smudge but it will be a brighter smudge and more extensive. Close to it the galaxies M32 and M110 should be visible as well with a low power eyepiece - low power is the key to this target.

M13 will be resolved into stars in it's outer parts and quite far towards it's core if the seeing conditions are good. This target can be usefully observed at around 80x -100x with a 10" scope.

Obviously what you will actually see at the eyepiece will depend on your local seeing conditions and to some extent your experience as an observer so that those factors into account when considering the above advice.

The 10" dobsonian is a really worthwhile upgrade in terms of observing potential over the Celestron 127 Powerseeker.

 

[Paul73]

10" Dob is a cracking scope. Most things are still grey smudges.

Nudging becomes 2nd nature with practice. I spent my first six months of dob ownership trying to work out a cheap way of getting it to track. Now I really can't see much point. Although, a quick change of eyepiece is needed my larger dob at high magnification. No probs at 10" f4.7.

However. Sorting out a Telrad / Red dot finder and/or a Raci finder is a priority.

Paul

[Louis D]

Tracking is really only an issue with Dobs when sharing the view at star parties.  That's where having a equatorial platform or goto system really helps.

[Paz]

Low contrast things like m31 will still look like smudges, maybe more extended. To see more details in those sorts of things needs darker skies.

However globular clusters and fainter open clusters will look fundamentally better, much more in the way of resolved stars.

I have a tracking platform for the dobsonian and will use it when I'm expecting to use higher magnifications on objects nearer the equator where things move quicker. It is no bother to set up... Put it down, put dob on top and off you go.

[Hairy Gazer]

Thanks everyone, you've been a great help.

I did wonder whether the size of the scope or light pollution would be the cause of the grey smudge. I have quite a dark back garden, but it's in the middle of a village with lots of houses around and street lights around the front.  I have taken my scope out a few times, but everytime I find a good dark spot, it either clouds over or a mass of car headlights appears.

[billyharris72]

It will probably be both the scope and the light pollution, but also depends on the object - M31 is not especially forgiving in that regard (true of galaxies in general). I recently had the pleasure of observing it (though only through a 130mm reflector) from the edge of the caldera of Mount Teide on Tenerife - it was a grey smudge, albeit a clearer one than usual with very faint hints of detail. From a somewhat lighter site I also had a go with a 12 inch Dob - the view was not as good due to the light pollution.

To see real detail in the Andromeda galaxy is a big ask. But globular clusters such as M13 should resolve well with more aperture - I see a some barely resolved stars in a 130mm Newt, a lot of resolved stars and some structure in a 200mm; a 300mm gives awesome views, with stars resolved almost to the core, lots of structure and obvious colour. Other objects, such as M27 and M57, should also be really good in a 10 inch, even from moderately light polluted locations.

On the LP front, it might be worth seeing if you can rig something up with sheeting or tarps to block some of the more local light. An observing hood (or a t shirt worn upside down around your neck and pulled forward to do the same job) can also be a big help. As for the weather - wish I could help with that one, but no joy...

 

[swamp thing]

17 minutes ago, billyharris72 said:

 

To see real detail in the Andromeda galaxy is a big ask.

 

Not really. It's only a big ask if you choose to observe it from a LP back yard.  

The dust lanes are seen in a 4" scope from dark skies, a 10" makes them obvious as they spiral around the core. The detail is in fact quite striking if your sky is dark enough.

In larger aperture scopes one can start to observe extraordinary amounts of detail within the dust lanes. Some of M31's globs and open clusters can also be seen. It does require some patience (detailed maps) and scrutiny but there is much to see in our larger neighbour  

Have fun out there  

[ronin]

17 hours ago, Hairy Gazer said:

I currently have a Celestron Powerseeker 127 and am obviously finding it hard to see DSO's (Andromeda and M13 looks like a grey smudge).

I was looking at buying a 10inch Dob, but was wondering if Andromeda and M13 would still look like a grey smudge, but just a bigger one.

 

Also, I was worried about nudging a dob at 400x magnification, would the object just shoot out of view?

Any advise on this would be great.

I think you are going to have to change your thought process and swap from "magnification" to "field of view".

M31 is 3 degrees across, so to see it in one go you need at least 3 degrees which in a BST is 20x magnification: Field = EP Field/Mag = 60/20

On a 10" that means a eyepiece of 1200/20 or a 60mm eyepiece. Basically you are not going to get it all in one go.

M13 is given as 20 arcmin, lets say 30 as that frames it and gives 0.5 degree. For 0.5 degree in a BST that is 120x magnification and 120x magnification in a 10" is around 10mm, so the 12mm BST would be the one. Really for M13 I suggest a 1 degree field and that means 60x and a 20mm eyepiece.

You are not going to nudge a dobsonian at 400x, things would just shoot across and out of view too fast. Even if on an Eq mount I suggest you are looking at one costing around £5K for that accuracy, and I even doubt any would do it.

Start thinking in object size terms and then work back to the magnification which that implies. I think you will find that 80x is the most appropriate for the majority and often less.

[Timebandit]

 

Hello 

I have compared my 120mm frac with my 350 mm reflector (dob) on numerous occasions. You will get a larger more defined image in general the larger the aperture. There is a noticeable and very worthwhile experience difference in the 120mm to the 350mm on DSO. 

And I think you will see a worthwhile difference if you upgrade from a 127 to a 10" dob. So much more light gathering ability. Let's not forget the moon is around 250,000 miles from earth as such so great to look at in most telescopes ,  M13  is I understand 25,000 Light years away from us . This is a vast distance,but you can still get a good view in a decent aperture telescope and start to resolve the stars of the outer edge.

As for nudging the dob. I don't even think about it now, it just comes as second nature if I need to track an object, and I don't even consider it a draw back. A simple and effective way to track an object, no star alignment to do ,no battery packs, no electric gremlins to go wrong? obviously the wider the angle of the eyepiece used at high magnification this makes a considerable difference in tracking an object and how much nudging is required.

The most effective way are making sure you get the best out of any telescope of any give aperture is seeing conditions. On DSO the darker the site the better to get the most contrast, atmospheric conditions , pollution and weather conditions all have an effect on a telescope  

I hope the above helps 

 

[Moonshane]

16 hours ago, Louis D said:

Tracking is really only an issue with Dobs when sharing the view at star parties.  That's where having a equatorial platform or goto system really helps.

Very true Louis. I always place the object in right of field so they have time to see it.

[Louis D]

2 hours ago, ronin said:

You are not going to nudge a dobsonian at 400x, things would just shoot across and out of view too fast.

Oh ye of little faith.  I regularly track objects in my Dob at very high powers.  Mostly, you need a smooth, stable design and a light touch so you can track while observing.  Having your fingers on the tube tends to dampen the low frequency settling time vibrations.  I've tracked satellites and airplanes for fun without any problems.  Don't knock it 'til you try it.

[niallk]

A 10" dob will give great views - multiple dust lanes in Andromeda, and M13 will be a little brighter/more resolved than the 8" view in this:

http://www.obsessiontelescopes.com/m13/index.php

Globs respond wonderfully to aperture!

Regarding nudging - I've a Skywatcher 250px solid tube dob, and it gives great views, but it can be a little challenging at 250x in that you can overshoot the fov and there is also a very small flexure present, tbh.  Note I haven't applied soap or any lubricant to the bearing which might help make it that bit smoother in use.  However, I'd wholeheartedly recommend this scope, and love it.

I do have a 15" dob which moves exquisitely - large diameter bearings on teflon, and it is very rigid.  I've manually guided at 560x on Jupiter one night of very good seeing - making adjustments of a fraction of the 100° afov EP without issue.  This night was an exception for supporting such high mag though!  I do have a tracking platform that will take my 15", but tbh, I don't use it (set up, adjustment fuss; resetting).

The 250px is a great buy, and people have made very nice DIY mounts with large bearings (like Moonshane above for several fine looking examples ) which will help with smooth movements and balance.  Bresser have also brought out 8" & 10" dob with nice looking large diameter bearings:

[Hairy Gazer]

Thanks again everyone, the response I've got from you all on here has been amazing.

As some of you have realised, I'm still quite new to astronomy (6 months), but the pointers you have given me have been great.

That being said, I do know that 400x is a high level of magnification, and I am, concentrating more on aperture over magnification (that's why I'm looking at the 10inch dob over the 8 inch one that I first thought about). I only used that example because of the advert I'd just read, referring to the 10 inch dobs highest realistic magnification.  

 

I think I'm going to go for the 10" solid tube Dob and start practicing my nudging. I was thinking about one with a goto if the nudging was going to be a pain, but I prefer finding things for myself and I enjoy being out there under the stars all night. Also, the only goto Dobs I have seen are the flextube ones and I didn't fancy one of those.

Now I just need to find somewhere to put it.

[Charic]

I had a Celestron Powerseeker 127EQ...........I now have the 8" Skywatcher Skyliner, and only took one night to make the decision?

The 10" Dobsonian 'should' offer a tad more than the 8" but location is the issue.

M31 from my back yard, is nothing much to look at, just a smudge, but a short way from home, its impressive, requiring the use of a 70° eyepiece to try and get it all in? 

As for the power of the scope, assume the aperture of the scope matches the power, so on my 8" 200mm scope, 200x is fully achievable under uk skies. Much over 200x, most images actually deteriorate, depending on what your looking at, though I have observed at 375x looking at the Moon, but to be honest, the longer the focal length of the eyepiece, the better the image, but the image scale is reduced, the further you go with  longer  EP's. I use ALL my eyepieces, one of them will produce the correct image and detail, depending on the conditions.

Don't think that nudging will be an issue, yes at high power, you need to move the scope quickly, but its more of a glide than a nudge, especially when the scope is setup correctly. Nudging sounds as if your jerking the scope! 

A jump from your 5" to an 8" or 10" is massive, but not only that, the 127 is no good for a beginner or visual use, the scope does just not do any justice, and believe me, I tweaked and cleaned, and even used better eyepieces to no effect, the 127 had to go, and all that knob twiddling and having the eyepiece end up in some very strange places?..............yeah, go ahead, get a 10", but don't discount the 8", its still a  popular scope, and with a couple of  BST Starguiders, if your conditions allow, you could have a scope that will last a lifetime.

Now Winters setting in, with the longer nights, if you could locate a club, and go and look through the 10" and or 8", you'll sell yourself the right scope for your needs. Weight and portability are something to consider, with the 10" heavier, and a bit larger.

[Merlin]

Regarding using high magnifications. They have their uses for splitting close doubles. It's on extended objects that high magnifications are prone to the law of diminishing returns.

[Louis D]

20 hours ago, Merlin said:

Regarding using high magnifications. They have their uses for splitting close doubles. It's on extended objects that high magnifications are prone to the law of diminishing returns.

Also useful for resolving globular clusters if the optics are well figured and the skies are steady.  It's really neat to see a faint fuzzy break up into tiny sparkling lights.

[Astro Imp]

Earlier in the year I went from a 6" Newtonian on an equatorial mount to an 8" Dob and after a couple of nights was surprised the difference this small increase in aperture made. I view from my back yard at the edge of a large village so not the best but the increased detail in globs. and clusters was surprising. The increase in aperture you are considering would make a considerable difference.

Good luck.

[Timebandit]

1 hour ago, Astro Imp said:

Earlier in the year I went from a 6" Newtonian on an equatorial mount to an 8" Dob and after a couple of nights was surprised the difference this small increase in aperture made. I view from my back yard at the edge of a large village so not the best but the increased detail in globs. and clusters was surprising. The increase in aperture you are considering would make a considerable difference.

Good luck.

 

 

In smaller aperture scopes it is surprising that an inch or two increase in aperture what the extra percentage in light gathering ability it results in.

A jump in aperture from a 6" reflector to a 8" reflector may be only a 2" jump in aperture. But in extra light gathering percentage terms, a jump from a 6" to an 8" means a massive 77 percentage  more light gathering ability?

[alanjgreen]

On 11/8/2017 at 16:29, ronin said:

I think you are going to have to change your thought process and swap from "magnification" to "field of view".

M31 is 3 degrees across, so to see it in one go you need at least 3 degrees which in a BST is 20x magnification: Field = EP Field/Mag = 60/20

On a 10" that means a eyepiece of 1200/20 or a 60mm eyepiece. Basically you are not going to get it all in one go.

M13 is given as 20 arcmin, lets say 30 as that frames it and gives 0.5 degree. For 0.5 degree in a BST that is 120x magnification and 120x magnification in a 10" is around 10mm, so the 12mm BST would be the one. Really for M13 I suggest a 1 degree field and that means 60x and a 20mm eyepiece.

You are not going to nudge a dobsonian at 400x, things would just shoot across and out of view too fast. Even if on an Eq mount I suggest you are looking at one costing around £5K for that accuracy, and I even doubt any would do it.

Start thinking in object size terms and then work back to the magnification which that implies. I think you will find that 80x is the most appropriate for the majority and often less.

I agree with this.

Wider Field of view FOV eyepieces really reduce the burden of nudging when using a manual dob. I am now using 100 degree FOV eyepieces and the amount of nudging has reduced 10 fold.

Using increased magnification only increases the amount of nudging needed

So, for BIG targets like M31 you need a nice low magnification, wide FOV eyepiece. Unfortunately, wide FOV eyepieces are expensive too

Plan to buy a "quality" low power two inch eyepiece to go with your new dob with as wide a FOV as you can afford. This will make a great finder eyepiece for all your targets and be great for all the "big" DSO.

Also worth noting that UK skies are only really good for up to x235 magnification. Anything above this will lose image quality as the atmosphere will get in the way.  A x400 eyepiece will get little or no use in any scope.

Alan

[alanjgreen]

13 minutes ago, Timebandit said:

In smaller aperture scopes it is surprising that an inch or two increase in aperture what the extra percentage in light gathering ability it results in.

A jump in aperture from a 6" reflector to a 8" reflector may be only a 2" jump in aperture. But in extra light gathering percentage terms, a jump from a 6" to an 8" means a massive 77 percentage  more light gathering ability?

Light gathering calculation is ( larger diameter * larger diameter ) / ( smaller diameter * smaller diameter).

So an 10" scope would gather pi * (10 * 10) / pi * ( 5 * 5 ) = 314.1/78.5 =4 times as much light as your 5" scope (ignoring the size of any obstruction such as a secondary mirror)

If you use the same magnification on these two scopes you will have a big increase in image brightness at the eyepiece. With this should also come more detail. As you increase magnification then brightness will decrease. Which means that you can get the same brightness image (of the 5") at higher magnification (up to the atmospheric useful magnification!) in the 10".

Ideally, you want the magnification set to give the object a reasonable size and of reasonable brightness. You can use too much magnifcation in any scope resulting in a dull mushy image that you cannot bring to focus at the eyepiece.

[sockgoblin]

On 11/8/2017 at 15:07, Hairy Gazer said:

Thanks everyone, you've been a great help.

I did wonder whether the size of the scope or light pollution would be the cause of the grey smudge. I have quite a dark back garden, but it's in the middle of a village with lots of houses around and street lights around the front.  I have taken my scope out a few times, but everytime I find a good dark spot, it either clouds over or a mass of car headlights appears.

i have an 8 inch dobs and have the lights of greater manchester as a background 10 miles away. Galaxies are always very faint smudges and are great to be able to say that you have seen a galaxy and then a bit meh..

So choice of target is key , open and closed clusters , some of the brighter nebula such as the ring nebula and some good double stars makes being cold in the early hour worthwhile. Good star charts and a route plan and then some searching work , some failures, finally the aah Gotcha! is the game for me. And on a bad night I damn the LED light into hell.

[Hairy Gazer]

On ‎10‎/‎11‎/‎2017 at 17:36, alanjgreen said:

Also worth noting that UK skies are only really good for up to x235 magnification. Anything above this will lose image quality as the atmosphere will get in the way.  A x400 eyepiece will get little or no use in any scope.

Alan

 

This is a really interesting point, does anyone else (from UK) agree with this ? (not that i'm saying your wrong alanjgreen, I would just like to see everyones opinion on this, as i think this is a really good point).

[AdeKing]

I'd agree with @alanjgreen on this, the shortest fl eyepiece that I regularly use in my 8", 1200mm f/l  Dob is a 5mm eyepiece which gives 240x.  The shortest eyepiece that I own is a 3.2mm that gives 375x in the Dob, but seeing never allows me to use it in the Dob.  The 3.2mm does however get used in my 6" 750mm f/l Newt and my short tube ZS66SD frac (388mm f/l) where it gives me 234x and 121x respectively.

Most of my observation is done between about 50x and 150x and occasionally (though rarely) up to about 240x.

 

[Charic]

On 10/11/2017 at 17:36, alanjgreen said:

So, for BIG targets like M31 you need a nice low magnification, wide FOV eyepiece. Unfortunately, wide FOV eyepieces are expensive too

This is the reason I use the Skywatcher Panaview 32mm 70°  eyepiece, the only 2"/50mm fitting eyepiece in my collection.

At £79 from First Light Optics, their not overly priced,  even less for the 32mm whereas something like a Delos new, would set you back over £300