Some basic queries about my 8'' dob

[cybernautic]

Hi

I hope someone can answer a few queries I have about my 8'' dob. I've not had the chance to use it much because of the weather but several things have cropped up that I'm a bit puzzled about.

I) When I take it outside to acclimatise do I leave the dust cap on or off? Leaving it off seems like the natural thing to do but then I don't want dust getting into it while it's adjusting to the outside temperature. If I leave the dust cap on then won't it take much longer to acclimatise?

II) A similar question: when I bring the scope back inside do I need to leave the dust cap off so any moisture inside the OTA can escape? At the moment I wipe any dew from the outside of the OTA but leave the dust cap off with the tube pointing down towards the ground. I'm not sure this is necessary but I don't want moisture to get trapped in the OTA and make the mirror go moudly (or something). Is this nothing but paranoia?

III) I can't get my finderscope aligned. I can't align it during the day as there's no horizon far enough away so I've tried doing it at night. Tonight I used M31. I could get it in the centre of the main EP and close to the cross-hairs in the finderscope. The problem is that there doesn't seem to be enough tension provided by the little spring-loaded bolt to push the finderscope onto the thread of one of the grub screws. It's really hard to explain. I could get M31 in line top to bottom but when it came to moving it left to right the grub screw had to be unwound so much the thread lost contact with the finderscope. Any ideas? I think I should pack something out but I've no idea what and doing it in the dark is proving a bit of a mare.

IV) An old problem from what I've read online, but the azimuth control of the base isn't as smooth as I think it ought to be. It's possible to pan left and right relatively easily but it takes quite a shove to get it going. I'm not sure if this is normal or not though. I've tried sandwiching a single milk bottle washer between the two parts of the dob mount but even one was enough to make the mount wobble when I tightened the central bolt. I have a feeling a lazy susan mod might be the only thing to improve it.

Thanks in advance

[ian_j_kirk]

1 Leave the dust cap off as this allows the scope to cool quicker, eliminating air currents inside the tube that could affect your view.

2 When bringing the scope back inside, keep doing what you are doing. Leaving the scope pointing down allows the dew to evaporate naturally, with out running the risk of any drops forming on the mirror and staining it, and also pointing down prevents as much dust getting on your mirror.

3 Don't know how to explain about this really, just trial and error. Have you got the finderscope in the bracket straight? I have the 200p and the spring loaded screw has more than enough tension;

4 There is a modification that you can do to the washer/bearing assembly that is quite popular and not too hard either from what I hear. Try googling it. I will have a look as soon as I can too to see if I can find it for you.

Hope this helps

Ian

[ian_j_kirk]

Oh yeah the lazy susan mod, that was it. Sorry wasn't paying enough tension

Ian

[lestergibson]

Hi

I hope someone can answer a few queries I have about my 8'' dob. I've not had the chance to use it much because of the weather but several things have cropped up that I'm a bit puzzled about.

I) When I take it outside to acclimatise do I leave the dust cap on or off? Leaving it off seems like the natural thing to do but then I don't want dust getting into it while it's adjusting to the outside temperature. If I leave the dust cap on then won't it take much longer to acclimatise?

Yup, leave the dustcap off.

II) A similar question: when I bring the scope back inside do I need to leave the dust cap off so any moisture inside the OTA can escape? At the moment I wipe any dew from the outside of the OTA but leave the dust cap off with the tube pointing down towards the ground. I'm not sure this is necessary but I don't want moisture to get trapped in the OTA and make the mirror go moudly (or something). Is this nothing but paranoia?

I don't know what the 'form' is but I leave mine off till the following day.

III) I can't get my finderscope aligned. I can't align it during the day as there's no horizon far enough away so I've tried doing it at night. Tonight I used M31. I could get it in the centre of the main EP and close to the cross-hairs in the finderscope. The problem is that there doesn't seem to be enough tension provided by the little spring-loaded bolt to push the finderscope onto the thread of one of the grub screws. It's really hard to explain. I could get M31 in line top to bottom but when it came to moving it left to right the grub screw had to be unwound so much the thread lost contact with the finderscope. Any ideas? I think I should pack something out but I've no idea what and doing it in the dark is proving a bit of a mare.

No idea about this one but possibly the mount is a little 'off' or maybe, as you say yourself, the spring is a bit weak?

IV) An old problem from what I've read online, but the azimuth control of the base isn't as smooth as I think it ought to be. It's possible to pan left and right relatively easily but it takes quite a shove to get it going. I'm not sure if this is normal or not though. I've tried sandwiching a single milk bottle washer between the two parts of the dob mount but even one was enough to make the mount wobble when I tightened the central bolt. I have a feeling a lazy susan mod might be the only thing to improve it.

I did the lazy susan mod and it was well worth it.  Tracking is a good bit smoother now.

[Andy_E]

As for the dust cap exactly as Ian said (by the way, getting dust on your mirror isn't a problem so don't worry ).

Where alignment of the finder is concerned, I wouldn't bother doing it in the day. My memory of the SW finders is they are pretty flimsy and get knocked out of alignment very easily. Also, I wouldn't use anything diffuse like M31. Much better to use bright star. Centre the star in the EP then re-align the finder to match.

[Avocette]

When you assembled your scope did you use the O ring properly on the finderscope body? The O ring may have been wrapped around the finderscope arm and seems to be easily overlooked (this was the case with my 200p bought second hand). If the O ring is seated propoerly in its slot, then you may need to find some further adjustment range by slackening off the two bolts which fix the finderscope base to the tube. Moving the base within the tolerance of the fixing holes in the direction which would give you more left-right adjustment may do the job.

I do my finderscope alignment using Polaris during the cool-down time.

[Avocette]

The azimuth axis friction adjustment is a bit tricky! I've had hands-on experience of three SW Dobs - the 200p to which I fitted a Lazy Susan bearing and three PTFE furniture gliders carefully shimmed, a SW 250px which I modded with six PTFE furniture sliders half way out from the central bolt, and the 300p flextube Dob which came fitted with its own factory Lazy Susan bearing. I'm hesitating whilst contemplating the inevitable mod to the 300p to add some friction (it just spins too easily at present) when I add a setting circle (which I did to the other two scopes). I'll post a full explanation one day but in the meantime I don't think a LS would solve your stiction problem whereas replacing the three stapled SW PTFE rectangles with three or better six 25mm furniture sliders might well sort you out. In any case the azimuth bearing friction is much less of a problem in my experience than the alt bearing friction on SW Dobs. My conclusion on the latter is that it's all down to tube balance so the next mods will be a set of adjustable counterweights - one per eyepiece!

[cybernautic]

Thanks for the replies. I'm not in a rush to do the lazy susan mod at the moment so I'll try the furniture sliders and see how I get on. I've not used a Dob mount before so it's hard to know what's normal and what isn't. I just know that it seems stiffer/heavier to move than I think it ought to be.

The finderscope issue is weird. When I first assembled the scope I fitted the rubber O ring into the correct slot, pulled back the spring-loaded bolt and slide the finderscope down into mount until it wouldn't go any further and then released the bolt. I'm not sure what else I can do with it. The bolt and the two grub screws hold it pretty securely in place, it's just in the wrong place! I'll try something other than M31.

It's a lovely sunny day here in France so I'm hoping for clear skies this evening. I'll try to fiddle with the mount and see if I can get anything out of adjusting the screws. I'll report back later!

[Andy_E]

Just a thought, would it be possible to post a photo of your finder set up? It will just help confirm that everything is in the right place.

[cybernautic]

Just a thought, would it be possible to post a photo of your finder set up? It will just help confirm that everything is in the right place.

Oops. I think something is on back-to-front. I had a look at this photo I just took and noticed that the mount doesn't fit into the foot properly (never noticed until now!)

The foot hasn't been moved since I unpacked the scope. I just assumed it slide into the foot from the top but clearly something is wrong.

[Reeny]

The finderscope is sitting in the bracket back to front (upside down).

The screws should be at the back, and seat into the reces shown at the bottom of your photo.

The bracket will slide in from the bottom once it is fitted correctly.

Check the instructions which came with the scope.

page 5 - sections 11, 12, and 13

[cybernautic]

The finderscope is sitting in the bracket back to front (upside down).

The screws should be at the back, and seat into the reces shown at the bottom of your photo.

The bracket will slide in from the bottom once it is fitted correctly.

Check the instructions which came with the scope.

page 5 - sections 11, 12, and 13

Ah yes. It's ok now, lol. It seemed counter-intuitive to slip the mount in from the back rather than drop it in from the top.

*embarrassed*

Thanks to all

[Charic]

cybernautic.......EVERYONE SUFFERS THE WEATHER WITH A NEW PURCHASE?
When I leave my scope outside, I remove just the small`lens cap` from the dust cap (the lens cap then mounts on the raised section of the dust cap for storage) and I leave the scope pointing towards the house at about 45 degrees, allowing any warm air currents to rise. This also reduces the amount of  any dust,rain to enter the scope should I fall asleep during the cool down period, which for me is about 60 mins. My main worry here, are the `eyes` watching me put the scope out! ( I`ll maybe  consider a security system, chain or motion alarm?)

On return to the kitchen, same again with the dust cap configuration.
Glad you got the finderscope corrected. (I always use Polaris for alignments, because it doesnt move in the Sky! ) Do you actually use both finders now, as I see you have the Rigel fitted?
As for the rotation of the base, how tight is the centre bolt, it just needs to be nipped to take out any slack.

[cybernautic]

cybernautic.......EVERYONE SUFFERS THE WEATHER WITH A NEW PURCHASE?

When I leave my scope outside, I remove just the small`lens cap` from the dust cap (the lens cap then mounts on the raised section of the dust cap for storage) and I leave the scope pointing towards the house at about 45 degrees, allowing any warm air currents to rise. This also reduces the amount of  any dust,rain to enter the scope should I fall asleep during the cool down period, which for me is about 60 mins. My main worry here, are the `eyes` watching me put the scope out! ( I`ll maybe  consider a security system, chain or motion alarm?)

On return to the kitchen, same again with the dust cap configuration.

Glad you got the finderscope corrected. (I always use Polaris for alignments, because it doesnt move in the Sky! ) Do you actually use both finders now, as I see you have the Rigel fitted?

As for the rotation of the base, how tight is the centre bolt, it just needs to be nipped to take out any slack.

Hi -

I've not really had chance to use the Rigel in conjunction with the finderscope yet but I suspect I'll be using both of them together. Even with the off-centre alignment of the finderscope I was finding it hard to place the scope in the right part of the sky as the finderscope's magnification is quite high but I found M31 easily enough in it. Needless to say it's cloudy here this evening (and blowing a gale!). I like the idea of removing the small aperture-reducing cap within the dust cap and letting any warm air vent just through the hole so I might try that out. I'll continue to fiddle with the base but I think I'll probably go down the furniture slider route (I'll check the local DIY store and see if they stock them).

Thanks to everyone who has helped answer my queries. Since signing up I've found that the forum has been incredibly helpful, friendly and informative and it really is appreciated. It's a fantastic site. I dread to think how long I would've struggled with that finderscope unless I'd asked about it on here.

Clear skies

[cybernautic]

Did the dob mount mod earlier using Teflon furniture movers! I used four, about 3cm across, placed around the central bolt. I then packed a couple of milk bottle washers under the little circular washer and reassembled the mount. It's made a big difference to the smoothness of the azimuth movement and most of the stiffness has now gone. I don't know why Skywatcher don't just ship the mount with the better Teflon pads.

Thanks for the suggestion.