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Dobsonian questions (mainly)


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I've had 130 P reflector on an Alt-Az GoTo mount for a few years now and am thinking about a new scope. As money is tight but aperture desired I'm considering a second hand 10 inch dobsonian mounted scope. Seems to make more sense to spend money on aperture rather than having to also get a new mount to take a new scope...? Plus it will be fun (hopefully!) to locate everything manually. Anyways, I'm wondering:

1) On something like the Skywatcher dobs just how easy it to achieve manual tracking of objects at high power? I'm worried that when, for example, looking at Mars at highest possible magnification (considering conditions, eyepiece limitations etc) it might be quite difficult to keep the target in view. At high magnification isn't it hard to keep things centred? I envisage getting something into view, giving the scope a nudge to try to keep it centred but moving way, way away from it, never to be found again. ;-) What were the early experiences of existing dob owners like?

2) Do any dob owners leave their scopes outside for any extended periods of time, covered for protection? Ideally I'd like to be able to leave it set up outside most of the time, ready for action, but covered when not in use. I've seen various scope covers advertised, some proclaiming to be all-year round weather protection (some costing a real packet). Are these a viable option? I suspect the advice will be to always bring the scope back indors after every session but just wondered what existing dob owners do.

3) I'd be looking to get a RACI finder or a Telrad. My skies are pretty light polluted and sometimes even spotting "easy" targets can be a pain. I think I need some aperture and magnification on the finder to even see big, bright objects as starting points for galaxy hunting etc. But folk seem to love Telrads. I'm worried that if I used a Telrad, which I understand has no magnification, I wouldn't actually be able to see very much at all under these skies to use as a finder target? Is this likely? If a Telrad is the better option, why?

Ok, thanks in advance for any replies. I'm sure I'll have more questions soon!

Tot

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hi, a 10" scope will be great and you will love it.

no)1 nudging at high power is easy and nothing like you imagine, dont worry about it

no)2 when we go to our dobfest 3 of the4 of us leave there scopes out no matter what the weather, and there big dobs and no problems

no)3 some use a telrad and a raci finder. some one will be along soon to help more on the telrad but i no you can down load charts for easy reference

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To have a go @ Q3 - My skies are pretty bright. I use a Rigel to point to the 'general' area (or go to a particular bright star if that's what I'm after) and then the 9×50 RACI to have a look around and narrow it all down before going to the eyepiece. Works well, for me...

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The Telrad is just like a gun sight that appears to be projected onto the sky? You can dim the reticules  to allow you to see the Stars. I need Glasses to use mine,  to better see those red reticules.  These reticules are calibrated to 4° 2° and 1/2 ° increments which makes finding easier in relation to some books/charts or with the option in Stellarium.

I sold my first Telrad, due to the glasses situation. I just didn't want to or need to wear the glasses for the telescope or the finder alone so sold it on to a fellow SGL member. Not only that, they customised it with a hack-saw ( made it much  shorter ). My newer Telrad was purchased as a result of the first one being sold,  which honestly just left me with the feeling that something was missing from the set-up. I really don't use it much from my normal site, but when I go to darker sites, and not being able to see the 9x50 finder-scope reticule against the dark sky, I find the Telrad comes into play. Placement is another issue for reach and height?

If I ever upgrade my Skyliner ( without testing - Hands on )  I would opt for the 12" solid tube if one became available at a good price. I no doubt would still like a 10" Skyliner, as this would still give just over 50% more light grasp over the 8".

As the Aperture increases, especially for just observing, then the fainter the Stars you should be able to capture, under the right conditions. That said, to me, one Star is no different to the next! I'm not interested in individual Stars, i`ll have a quick look and move on. I just want to see the Galaxies and    any other DSO  up there. I have seen Jupiter only once under what for me was perfect seeing conditions ( they don't come around too often ) but have viewed Jupiter every opportunity that I can. 

As for tracking, it  all comes with practice. Often a satellite will streak through the field of view. I can lock on and follow on-axis with ease. A comfortable seat is a good option, I've been using a king side leather kitchen chair with the perfect head rest for leaning back when/if scanning wide views with the 8x40 binoculars. The Mrs was not too happy with my use of the dining chairs, so have invested in a new drum stool, which I have still yet to test, and I often  use my 'Dob- tent'  to keep stray light at bay, I  can leave the telescope inside to cool if required, and out of view from prying eyes. Trust No-One!

I don't think you will have any problems using a Dobsonian telescope,  having considerations for buying the biggest you can afford,  and your ability to carry  the 10" which will be slightly heavier than the 8"  over short spaces. My 8" takes up  some 52cm² of floor space assembled. 

Edited by Charic
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Hi. I own an 8" dob. I find tracking absolutely fine, even using my most powerful ep (5mm) you don't need to nudge every second, probably every 45-60 seconds roughly. Keeping jupiter in view is no problem, and if slips out of view its very easy to get it back again, the dob is very easy to manoeuvre in small increments. I keep my dob in the garage under a pizza oven cover. I've had mine in there since May 2012 and haven't reported any problems, I'm not sure I would keep mine outdoors though, even under a cover, i would be worried about damp, but I haven't looked into that as an option so don't know much about it. Re the RACI I can definitely vouch for that, I have one and it is a god send, saves your neck and makes it so easy to fine tune the search before moving to the EP. I also use a rigel quikfinder which I use to point the scope in the right direction before moving to the RACI, I have found it invaluable and could not do it with out it. I considered a telrad but wanted to keep something on my side of the optical finder rather than the other side and having to lean over to look through it and a telrad wouldn't fit there. I positioned the rigel at the top of the tube between the focussed and the RACI, works very well there. Hope this helps and good luck with whatever you decide.

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I love my SW 259P, in response to your questions;

1. It's not much of a problem, there is a little bit of a knack to knowing how much to nudge and in what direction. It's not as easy as either a GoTo mount or an EQ with or without motor but don't let that put you off.

2. I would never leave any scope outside covered or otherwise. Personally I think the British climate is too variable/harsh to consider this. However, I know, if anything, I'm over-cautious with my gear. I wouldn't even feel that happy about leaving it in a wooden (single skin) shed. I guess I'm lucky not having an other half to moan at me leaving my scopes all cased up in the kitchen!

3. I find my Dob much easier to use by mounting both a red dot finder and traditional finder scope. I use the bracket

http://www.firstlightoptics.com/finders/flo-multi-finder-adapter-for-skywatcher.html

It makes it a lot easier using the red dot to get the scope pointing roughly right then zeroing in with the traditional finder. My finder has a 45deg prism which saves cracking the nest a lot (as well as making the red dot almost essential).

Enjoy :-)

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Before I used a dobsonian I worried about tracking objects at high powers. I think that was because my mind was thinking about keeping the target centered in the field of view, in the same way a driven mount will. With a dobsonian though a different technique is used which I call "nudge and drift". At high powers you allow the target object to drift across the field of view, then apply a gentle nudge to move it back across the field again, then observe as it drifts across, and so on. As bendiddly says above, even at high powers you get a decent period of observing between nudges, more of course if your eyepieces have a wider than average field of view and the image remains sharp most or all of the way across it :wink:

I keep my dob indoors although it's survived a couple of star parties where it was outside, though covered, for longish periods.

For finders, I feel the ideal combination is to have a good zero magnification illuminated reticule finder such as a Telrad or Rigel Quickfinder as well as a right angle, correct image 50mm optical finder. This combination seems to cover all the possibilities although I have had sessions where the illuminted reticule finder ( a Rigel in my case ) has been all I needed.

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Hi Tot,

Good questions, hope I can give you my advice.

Regarding tracking at high powers (>x250): it will depend on your focal ratio, the size of the target, the magnification and FOV of the eyepiece. Using an extreme example I used to observe Jupiter with an f/5 using a high powered monocentric (30 degrees) and this gave me an ete (edge to edge) time of around 25 secs; useful time is much lower than this as you want to keep the target somewhat centrally located for best views. Put in a 31mm t5 and you are talking a relative ice age of viewing time ;) The quality and smoothness of your bearing and surfaces, and how well your upper and lower tube areas are balanced determines the accuracy of your tracking. Stiction is often the term used, and you need to have just the right amount of it or have the ability to alter it. I bought into DSC's and tracking for a large dob later and it was very convenient but unnecessary so my new scope will have an Equatorial platform so I have the choice. When I would overshoot the target during an over exuberant nudge or when I was doing outreach and I needed to keep the object fairly nearby whilst I waxed lyrically, I used a Telrad with the dew cover and right angled mirror so I could keep the object fairly well centred and tracked whilst someone else viewed; this also worked well with planetary imaging.

Your second question about leaving the scope outside: I am ocd about equipment so could not do so. The closest I ever get is under canvas at star parties and even that makes me anxious :)

Your last point about the Telrad: I would say that a zero mag finder is the only finder to use in high lp skies. You need to use the Telrad to star hop from your bright stars rather than finding the target itself and then narrowing it down with a low fl eyepiece. I do have to wonder why I see all these wonderful servocat go to scope pics showing Telrad, large finder scope and a 31 mm t5 sticking out of (usually a feathertouch :)) focuser, which is a bit overkill in most situations. I am guilty of the above crime and will use a Rigel and low power eyepiece for simplicity and to keep the weight down of the upper tube assembly.

Hope this helps

Regards

Dannae

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Manual tracking was something that concerned me when I first started to use dobsonians, yet as John has mentioned 'nudge and drift' is a natural instinctive movement, quickly becomes habit forming and works just fine.

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Wow! Thanks for all the replies, folks. A great help indeed.

My worries about tracking are now much reduced. My fears about finding faint fuzzies by star-hopping, however, remain sky high (pun intended). :-)

Think I have my heart set on a Skywatcher 10 inch dob, second hand. Tight budget. But looks like it won't be outside permanently, maybe just left out, covered, for a few days now and again. Best to bring it back inside after every session, really.

I'm used to using the straight-through small finderscope on the 130P, but it is a bit of a pain where positioned. I can easily see the appeal and use of a RACI and am tempted to make that my first addition. I still don't really understand the Telrads and their being especially useful in areas with lp. Surely a 9 x 50 finderscope would be able to pick out a star to use as a start for hopping better than a finder with no magnification/aperture? I'm sure I'm completely wrong about that, though, otherwise why so much love for the Telrad?

Ok, thanks again for all the replies. No doubt more questions to follow - I already have one about focusers!

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Using a Telrad is a doddle - I wouldn't worry about finding or tracking - both will come naturally. I also have an 9x50 raci on mine so I can see more stars if I want to do detailed star hopping. The scope sits outside for 10 days or so at star camps (under cover when not in use) with no probs. You'll also need a good collimation tool and may need a counterbalance system and cooling fan. Dob modding is an irresistible hobby all of it's own. Hth :)

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Using  the 9x50 keep both eyes open,  focusing is  a lot easier using both eyes! . You`ll be surprised how much you can see through a 9x50 refractor? And that's sometimes the problem,  the greater amount of Stars now  visible plus the image  will be different to the Newtonian image?

Check this link on Telrad use http://www.backyard-astro.com/equipment/accessories/telrad/telrad.html there are many other links on the web.

Edited by Charic
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My Telrad gets minimal use from the present site. But I could change that, by just removing the Finder-scope and bringing the Telrad closer. But you don't see the Stars like you do in the Finder scope?  The Telrad just 'projects' the 'Bulls-eye' against the sky, so what you see with a naked eye is the same through a Telrad, only surrounded by the three  'Bulls-Eye' rings, and dimming their brightness to suit your viewing conditions. It works well when used in conjunction with  the Telrad feature in Stellarium  or any other program /  device that incorporates the degree rings, then use the degree rings to locate the target. 

Its a Marmite accessory.? once I rid my first one, I thought I needed another, and so it was.

Edited by Charic
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I couldn't find very much when I started using a rdf or a finder scope.

However all changed with the Telrad, especially with these,

http://www.atmob.org/library/member/skymaps_jsmall.html

Just keep both eyes open when searching and refer to any bright stars that you can hop from using the Telrad. In addition you'll soon to get to know where some of these targets hide out just by star hopping.

The best thing to get with a Dob is something to sign on. I made a wide 18" high wooden seat/table. An old cushion is comfy as well as kneeling down at the primary for Barlowed laser collimation.

Tracking is no problem at all. Various products will aid , such as a light application and polishing of Turtle Wax on the base and plastic milk bottle circles on the alt. There's plenty threads about upgrading Dobs.

Any temperature changes in storing conditions will lead to humidity on the mirrors. At star parties, I've always noticed considerable dew, that a morning of hot sun will remove.

Super choice, if you can't handle the weight of a 10", then a 8" will find most targets in light polluted sites,

Nick.

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The Telrad is a good replacement for supplied Finder Scope, as for nudging a Dob to keep DSO/Planets in view i went the GoTo route, the main reason i run a imagine Newt and new Messier images i find manually and the GoTo system will track them if i have to attend to the Newt or go make a cuppa tea it keeps a DSO in the EP for a very long time, The GoTo by the way is extremely accurate, takes a couple of minutes to set-up and will find DSO's in opposite parts of the sky very well..... 

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I have every confidence that your fears of not finding DSO's and such will vanish in due course. But one thing does cause me concern: Leaving your Dob outside - even covered. I am thinking that you are wondering about doing this as a result of difficulties you percieve about moving the beast. If this be the case, you can get, or make, a wheeled platform-contraption to allow for easy mobility. There's "billions and billions" of them:

https://www.google.com/search?q=Dobsonian+Telescope+wheels&ie=utf-8&oe=utf-8

Do have a look? A scope will survive just fine for a few nights/days in the wild, but it really should should have a roof over it's head when possible.

Clear & Dry Skies,

Dave

Edited by Dave In Vermont
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I thought the general consensus would be to not leave the scope outside and so it proves.  Oh well, just have to lug it around.

I'm sure a RACI would be very useful for me.  I wear glasses but don't like to have to use them at the eyepiece or finderscope if possible.  That's why I think a RACI rather than Telrad might be for me.  But no doubt I'll find what is best for me through using the scope.   And I still need to buy the dob set-up yet!!

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