I have recently been thinking about getting a Barlow lens for my dob, and found out earlier this week that we were going to make a trip to Brisbane on Saturday. I have requested to stop at York Optical on the way down so I can pick one up. (http://www.yorkoptical.com.au/ their site for all interested).
I have had a quick read of some of the other Barlow lens related posts, but they didn’t seem to help me too much.
Anyway, as there seem to be a wide range, I was hoping I would be able to get some help on the subject.
Question One:
As most of you probably know, I use my video camera held up to my 32mm eyepiece to take movies which I stack in Registax. Would it therefore be better for me to get a 2" Barlow, as it would also double the magnification of the camera? Or does camera zoom + telescope focus trick completly cover that anyway? I do eventually plan to buy a ToUcam (is that the right spelling?), so I’m not sure weather the extra cost of a 2” compared to a 1.25” would be justifiable.
Question Two:
As with most things, I am guessing that choosing a Barlow should be based on what I wish to do with it? Although I have been chasing after so Nebula and DSO's, viewing and imaging planets is definitely my main interest (along with Lunar viewing and imaging).
Anyway, I hope you guys can help me with my questions. I’m really looking forward to getting closer views of my favorite night objects.
BTW- My telescope stats are probably important, so here they are:
10 Inch Dob (GSO) F5 (1250mm)
My eyepieces are the standards with Bintel’s BT-252 ‘scope, 32mm, 15mm and 9mm.
I have a budget of about $100 to throw around, though I will go to $150 if I really have too.
For under $100 (around $70), get the GSO 2x (2"). For over $100 (around $140-150), get the Orion Shorty Plus 2x (1.25").
And a barlow in front of your eyepiece with your video camera attached will make a significant difference to your image scale when you do your imaging, so that's a good thing.. and you will still be able to use it when you get a ToUcam.
Plus, you'll use it for visual too, and you'll have effectively doubled your eyepiece set.
Just another word of advice shop around ---like down south .
You will find it cheaper to buy from Sydney and pay postage than use our very un-competitive astronomy shops up in SE Queensland.
The GSO 2" 2x is not a bad barlow and very well priced
and it's with in your budget.
I just this minute got a quote via email from Andrews for a 2" 2x Barlow and a Cheshire EP. I have copy/pasted it for you:
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Hi Ken,
Yes, a black aluminium Cheshire collimation eyepiece is $29. A 2" 2x Barlow is $79. Adding $10 for express mailing costs gives a total of $118. However, the shop is closed from now until Thursday next week as Mandy and I are going to China for a week's holiday.
Regards,
Lee Andrews.
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Hope that helps.
p.s. Is that the Cheshire EP you have all been telling me to get?
Are there good and bad Cheshires? It sounds too cheap at only $29.
Well its a cheshire and probably as good as any other cheshire except the longer ones which double as a sight tube for aligning the secondary under the focuser.
As for being too cheap, hmm it probably costs about $3 to make at the factory.
See Geoff's post also. The sight tube really needs to be matched reasonably closely to the F-Ratio of the scope. It doesn't have to be exact ie An F5 sight tube on an F6 scope will be fine, an F7 sight tube on an F4.5 scope won't help much. My combination cheshire/sight tube is optimised for an F5 scope and the sight tube is about 5 inches long. The F-Ratio that Andrews device is optimised for is anyone's guess with the thing only being 3" long. I have attached a picture of mine, they are made by Synta.
Thanks John. The ID of the crosshair end of the Andrews Cheshire is around 2.5cm. Is finding the f-ratio of the Cheshire the same as a scope (making it F3)? If so I might need to reinvest in a more appropriate Cheshire.
Thats correct its about an F3 device. It would still do a "fair" job of collimating the scope but not as good a job as the correct tool. If your scope was F6 or slower I would say go ahead and use that tool as collimation accuracy is less critical in slower scopes. Owning an F5 scope I would really be inclined to get the correct cheshire/sight tube particularly if you want to get the best performance possible from your scope for high power viewing of planets/lunar and double stars.
I dare say that as a cheshire , the Andrews job should be as good as any other for collimating the primary, and you just need a sight tube for aligning the secondary.
If only 1.25 inch pipe was a common size, life would be simple.
Thats very true, the primary collimation with the cheshire will work fine, its only the sight tube thats too short. Whats got me done is why GS would produce this device as an F3 device when 80% of newts sold today are between F4 and F6 and those that aren't are slower anyway. Unless its designed purely as a cheshire and not a combination collimation device ? I have not seen 1 so I wouldn't know.
My cheshire/sight-tube combo doesn't fit snuggly in my 1.25" adapter, and can be wobbled back and forth unless I tighten the compression ring.
I'm pretty sure this is a bad thing, and have read that I really should be using a 2" sight-tube/cheshire in the 2" focuser (without the 1.25" adapter).
John or Geoff (or anyone else), what are your thoughts on this?
Originally posted by iceman My cheshire/sight-tube combo doesn't fit snuggly in my 1.25" adapter, and can be wobbled back and forth unless I tighten the compression ring.
I'm pretty sure this is a bad thing, and have read that I really should be using a 2" sight-tube/cheshire in the 2" focuser (without the 1.25" adapter).
John or Geoff (or anyone else), what are your thoughts on this?
Mike,
You should only be using a 2" collimation device if that is the normal size of your accessories including your eyepieces.
The objective in collimating your scope is too properly align the optical axis from the primary objective with the central axis of the eyepiece or at 90% radially to a photographic plane. If you collimate the scope without your 1.25"/2" adaptor in place any misalignment in the adaptor will affect collimation when the scope is used with 1.25" accessories. You need to decide what accessories you are likely to use the most keeping in mind that you are far better to stick with 1.25" collimation devices if you predominantly use 1.25" eyepieces, my guess is this will be your situation. Something else to consider is that I "ALWAYS" insert my 1.25"/2" adapter with the same orientation in the focuser using the positioning of the lock screws as the means of alignment.
In regard to the slop with your cheshire/sight tube in the adaptor barrel, you need to fix it and the best way to do this is to wrap some 2" wide clear cellotape (good quality stuff, not Reject Shop crap) around the barrel of the focuser to achieve a snug fit where the cheshire will slide into the adaptor without grabbing but has minimal freeplay. I have found that generally about 1.5 turns around the shank of the sight tube is what is necessary to achieve the desired fit. Mine has it and you didn't even notice
Collimation , what's that ??
Now If herd that term some where before .
I do remember what it is i think.
O that's right that what I did a long time ago when i first made my scope with the comical mirror that does loose collimation
Seriously I used an old Synta Cheshire eyepiece that came with my old synta refractor and it's only 2" long.
My newt is an f6 though and was easy to collimate.
I did a friends 12"GSO f5 and found it doable but hard with this short unit.
I also use a laser colimater as well.To check center alignments .
Interesting to note on the friends GSO the focuser was out of alignment and cocked to one side.
We needed to shim underneath it to get the laser dot in the center of the diagonal. It was out not because of diagonal off set but from side to side and by about 15mm.
Check those focusers.
I would say the tube was slighly out of round near the end and this played a part in the focuser mounting.
Start at the focuser and work back to the primary mirror.
I use my 11/4" cheshire in a 2"focuser .
It will need to be a snug fit or the cheshire will cock over to one side when tighned.
If it's a slopy fit as mine was , shim around the eyepiece with paper , but if you have to go around more than once with the paper strip make sure you stop where you start with the paper so the thickness is the same all round.
Originally posted by mhodson Seriously I used an old Synta Cheshire eyepiece that came with my old synta refractor and it's only 2" long.
My newt is an f6 though and was easy to collimate.
Mark
Mark,
You need to consider the fact that you have been playing with scopes a long time and collimating them becomes 2nd nature, consequently you can do an excellent job with not really the correct tool.
The original poster is obviously new to Newtonian collimation and to a beginner it can appear to be a somewhat daunting task, although its not. Personally I think a beginner is best off trying to use the correct tools to collimate a scope as fast as F5 notwithstanding an experienced person can do an excellent job with poor or no tools. I can collimate my F5 scope with no tools other than my eyes and a bright star but that doesn't mean its the best way for a beginner to go about the task. Notwithstanding the fact that it can be done without tools they certainly make the job easier and quicker. I particularly like the EZ-collimator but when using it, or any laser collimator for that matter, you need to be aware of the limitations of laser collimators and the errors they can mask in regards to secondary and focuser positioning. In terms of quick adjustments to primary and secondary tilt only, they are an excellent aid in collimation. The advantage of the EZ collimator, particularly with solid tubed scopes is you can watch the laser strike back on its own exit hole in the device from the bottom of the scope as you adjust the screws.
BTW I would love to get a look through your scope with that Royce mirror one day if I am up your way, as I am contemplating purchasing a mirror from him or Mark Suchting in the next 18 months or so to build a 12.5"/F6 truss dob (time permitting). I like the idea of the CCS mirror from Royce for center bolt mounting to eliminate diffraction noise off the clips.
