10" Skywatcher Dob, no better than my Orion 6" EQ

[brantuk]

A fan on the back is a good idea - if the scope was s/h then the old one may have been taken off before sale. It'll draw in cool air and the scope will come down to temperature quicker. Half an hour to an hour and it should start to show good views, maybe quicker with a fan.

This isn't really a can of worms - just standard operating procedure with a dob - there's all sorts of mods you can do to improve things e.g. dew heater on secondary, Wixey angle guage, AZ setting circle, levelling feet, right angle finder, counterweight for larger eye pieces, Telrad finder, low profile focuser, bobs knobs for collimation, etc, you can make it as technical as you like

[Brainstorm]

A fan on the back is a good idea - if the scope was s/h then the old one may have been taken off before sale. It'll draw in cool air and the scope will come down to temperature quicker. Half an hour to an hour and it should start to show good views, maybe quicker with a fan.

How would I attach the battery to the fan? I know I can solder a 9V battery clip to the wire from an old PC case fan, but the battery would just be hanging there, then there's the issue of finding the right screw threads to fit the bores in the scope. Or are there specific fans designed for telescopes?

This isn't really a can of worms - just standard operating procedure with a dob - there's all sorts of mods you can do to improve things e.g. dew heater on secondary, Wixey angle guage, AZ setting circle, levelling feet, right angle finder, counterweight for larger eye pieces, Telrad finder, low profile focuser, bobs knobs for collimation, etc, you can make it as technical as you like

Do you know of any good websites or books for beginners like myself who clearly need to learn more about dobs?

[Moonshane]

There are plenty of threads in the DIY section.

an 'advanced search' for fan in the DIY Astronomer section brought these up http://stargazerslounge.com/search.php?searchid=2277523

[brantuk]

If you click on the "search link" and choose "advanced search" for threads - then just put the words "Dob Mods" in the text box and you'll find at least seven pages full of info and pictures showing the things you can do with a dob.

As for the 9v battery - you can mount it on the front of the base and take the cable up to the tube in front of the pivot point. Then tape it along the tube to the fan. Be sure to leave some slack around the pivot point so it doesn't get stretched when elevating the tube

[Brainstorm]

There are plenty of threads in the DIY section.

an 'advanced search' for fan in the DIY Astronomer section brought these up http://stargazerslounge.com/search.php?searchid=2277523

"Sorry - no matches. Please try some different terms."

[swamp thing]

If the views are brighter through your 6" then the 10" has either got serious problems with the mirror, or more likely the secondary is covered in dew.

A dew covered secondary also explains the lack of detail on Jupes disc.

You need to make yourself a few shield.

Regards Steve

[kniclander]

if jupiter is a featureless white blob then it's nothing to do with cooldown or atmospheric disturbance etc etc. It's a fundamental problem with the scope or its way out of collimation. I wouldn't waste time trying to build/fit a fan until you've got to the bottom of whatever the obviously much deeper problem is. I've got a 10" dob - it doesn't have a fan and I've never thought of fitting one and I've had some lovely views through it. I'm not saying there are no benefits to a fan - I'm sure there are but it's not going to address the problems you seem to be having. Why are you so sure the collimation's spot on?

[RikM]

If the views are brighter through your 6" then the 10" has either got serious problems with the mirror, or more likely the secondary is covered in dew.

A dew covered secondary also explains the lack of detail on Jupes disc.

You need to make yourself a few shield.

Regards Steve

Good shout Steve. I always have a foam campmat dewshield on mine for exactly that reason.

[kniclander]

If the views are brighter through your 6" then the 10" has either got serious problems with the mirror, or more likely the secondary is covered in dew.

A dew covered secondary also explains the lack of detail on Jupes disc.

You need to make yourself a few shield.

Regards Steve

yup, could well be dew.

[cs1cjc]

Plossl's start to "fail" at around f/5 and being at f/4.7 perhaps what is being asked of them is too much.

This may be true of some, but it is not true of all Plossl eyepieces. My Vixen NPLs are fine and for those with more money, all Televue eyepieces are design to work well to F/4 or faster.

An f/4.7 scope will have a curved primary image, it will not be flat.

The radius curvature of the focal surface of a Newtonian is equal to the focal length away from the observer and is in practice negligible. Curvature is often much higher in eyepieces. Often the edge of the field is in focus slightly further away than the middle so it may be best to focus on a star near the edge of field and allow your eye to accommodate for stars at the centre.

Also for the curvature on the actual mirror you may be getting coma.

All Newtonians have coma - it is a feature of the design rather than a manufacturing fault. An F/4.7 does have noticeable coma, but many observers are happy to view without a corrector at this f-number. Coma is effectively proportional to the apparent field of view for a given f-number so a coma corrector is more important with wide angle eyepieces. A corrector is not necessary for viewing planets if you keep them in the centre of the view, because they are so small.

For more information online, see:

http://www.telescope-optics.net/

[kniclander]

A fan on the back is a good idea - if the scope was s/h then the old one may have been taken off before sale. It'll draw in cool air and the scope will come down to temperature quicker. Half an hour to an hour and it should start to show good views, maybe quicker with a fan.

This isn't really a can of worms - just standard operating procedure with a dob - there's all sorts of mods you can do to improve things e.g. dew heater on secondary, Wixey angle guage, AZ setting circle, levelling feet, right angle finder, counterweight for larger eye pieces, Telrad finder, low profile focuser, bobs knobs for collimation, etc, you can make it as technical as you like

...or you can just use it out of the box and have a wonderful time observing the universe

[stev74]

Hi Brainstorm,

Out of the box your scope should be able to give you excellent views, as long as it's collimated. 'Cool down' time only refers to the time it takes for your scope to cool down to the ambient outside temp. The bigger the scope the longer it will take to cool down unless you store in it outside in a shed (like me) in which case it will be near to ambient temp already.

When I head out to a dark site I always have the heating in the car off as my scope is in the back seat, that way the scope will take less time to cool down when I arrive.

As has already been said I suspect you had some dew on the secondary mirror and/or eyepiece's as you had the heating on in the car.

I'd recommend you try using it again to see if you have the same problem. Also do a quick star test which is the easiest way to check your optics - TELESCOPE SUPPLIERS - SKY-WATCHER TELESCOPE

Lastly as your in Edinburgh I'd be happy to have a look at your scope for you as I'm just outside Edinburgh myself. I'm also a member of Central Scotland Observers Group and we have observing sessions not far from Edinburgh which you would be more than welcome to come along to. We could check out your scope then if you prefer.

All the details are on the forum feel free to join - Central Scotland Observers Group Forum - Index

I'm very familiar with the Skywatcher 10" dob as my mate has one and I have the 12" version, I'm pretty sure we could get you up and running in no time.

Cheers

[stolenfeather]

I own a Sky-Watcher 10 inch Dobsonian and am quite happy with it, The fact that you are not seeing as much as you wanted since your upgrade,... could it be for other reasons? I definitely see details in Jupiter which make me think that collimation may be the problem.

Isabelle

[Beulah]

You have to remember that the images you see of the Horsehead make it look big and bold, whereas visually it's just a very small notch against the brighter background.

Practise........

Merlin,

What would be the ideal filter to view the Horsehead nebula in a dark sky?

[swamp thing]

Even if collimation was a country mile out, and the scope suffering terribly from sudden temperature change it doesn't explain the fact that you said that M45 was brighter through the 6"

The only explanation for this is dew. Either on the secondary mirror, or on the eyepieces. Or of coarse your primary is figured so badly it is like a ploughed field, and optically useless ( don't worry, this is extremely unlikely ).

I didn't say dew on the primary because although this maybe a cause too, it is rare, as the scopes tube acts as a giant dew shield.

Regards Steve

[John]

I think a Hydrogen-Beta filter is the one for the Horsehead Nebula although I've read of it being spotted with other filters. David Knisely (Cloudynights forum) reports having seen the Horsehead with a 100mm refractor using an H-Beta filter.

[haitch]

When I drive out to a dark site I wrap up and put the aircon in the car on full blast to get the scope ready. There are 2 issues to cooldown - the figure of the mirror changes and you get heat currents, like a heat haze, within the tube.

Dew is a likely issude at this time of year - I've seen a few posts recently where people are getting badly dewed up early on in an observing session. You shouldn't get dew until after your mirror has cooled down though.

[Moonshane]

at a recent star party my primary was dewed each morning after a night in the cooking tent where it was stored. no problems in use and the wind soon cleared the dew out in the morning.

[Ptarmigan]

I have a tea strainer, do I put this over the empty eyepiece focuser, or do I have the eyepiece attached, if so which size eyepiece is optimal?

I wonder if Merlin has missed this in all the replies,, I guess empty focusser without an eyepiece - the "tea strainer" idea is new to me but a Ronchi test is a common method of testing optics.

One such test is to use a "Ronchi eyepiece" (which is a little misleading in that it is a Ronchi grating in an eyepiece holder/body/eyepiece substitute, no lenses) on a bright star.

Try this link to get you started http://www.btinternet.com/~mike.hanlon/mirror_testing.htm

(scroll down to "2. The 'field' test.")

and then Google "Ronchi eyepiece" for more. However unless you have been very unlucky and have got a rogue scope or one that has suffered somthing really unfortunate in transit I would be surprised if you needed to go to the complexities of Ronchi testing !

I would first follow the many suggestions of dew and collimation, I think !

[spaceboy]

I have heard of these same problems several times on SGL. Either the secondary, primary or EP has misted up. The resolve should be the same for you as it was for them. Put the scope & EP's (boxed) outside under shelter for around 1 hr and then try the scope. If after this you still have the problem and you are sure the scope is correctly collimated you need to get back in touch with the seller and start asking some questions. I doubt there will be any need for it though as it will most likely be that either the scope mirrors are to warm or to cold compared to the outside temperature and a mist of condensation develops over one or all of the optical surfaces and the views you get are like looking through light cloud.