Posts Made By: John Bambury

Hi all,

I am new to this group and I live about 1 hr north of Sydney Australia. I have a couple of questions regarding exotic reflector design which I am hoping someone can help me with.

I attach two web pages below and if you click on the thumbnail on each page it gives you a schematic of the design of each telescope. I have a couple of questions regarding these designs:-

http://www.kitgear.com/25%20inch%20plans.htm

http://www.kitgear.com/flex/Flex_Project.htm

1. I have on order from Willman Bell, two books, these are coming by surface mail and should arrive in the next 2 or 3 weeks, The Dobsonian Telescope by Kreige and Berry and How to Make a Telescope by Texereau. My question is do either of these books have any mention of these exotic design dobsonians. If not, is there a book that does
talk about these designs?

2. Obviously there is no free lunch so to speak, and these designs have some advantages / disadvantages over the standard newtonian.

With regard to the 25" folded reflector design I assume the disadvantages are a large obstruction in the light path, with an additional obstruction in the secondary light path and an additional reflective surface to suffer light loss and to keep collimated. Are my assumptions correct ? Are there additional disadvantages that I have missed? What are the disadvantages / difficulties of the flexed mirror design.

Ideally I would love to be able to build a 15 or 16 inch long focus dob and get the focuser close to the ground, I would like my wife to be able to use the telescope without a ladder. I dont think either design is the answer. Maybe there is another option, other than building the telescope at f5 or have the eyepiece 8ft in the air.

Clear Skies and thanks for any help in advance

John Bambury

I have a 10"/f5 Steel tubed Guan Sheng reflector, clone of the old Orion XT10. I wish to baffle the tube to eliminate stray light and reflections. The scope also suffers from tube currents. I have read several articles about baffling, including the S&T one by Gary Seronik. I have also read articles about ring baffles and used Newt 2.5 to calculate the positions of these. The problem I have is deciding whether to use ring baffles which I understand will worsen the tube current problem or just use flocked paper for general blackening of the tube and baffle the focuser etc.

I am also considering installing a fan to fix the tube current problem, can someone advise which is the best way to do this on this type of telescope.

ie. Do I suck the air down the tube or blow it accross the face of the mirror etc etc. I have a number of 12 volt computer type fans which I think would be suitable for the job.

Has anyone addressed these issues with this particular telescope(yes there are lots of them out there) and advise me the best way to improve its performance

Thanks in advance.

Clear Skies
John Bambury

I am planning to build a 12.5"/f6 Truss dob. I need to source the primary cell, secondary holder, secondary mirror, spider and the focuser. Novak are available in Australia and JMI focusers are also available, both are extremely expensive. Can someone give me the URL for these companies please as it may be cheaper for me to import direct into Australia. I have looked at the websites for Protostar and University Optics and they are under consideration. UO prices seem very reasonable, can someone please rate the quality of the components from UO, Novak and Protostar and advise of any advantages or disadvantages of these different suppliers' components. I will be using a premium grade primary so dont want to compromise the quality of the telescope but I am looking to save any money I can. I know that the JMI focusers are very good but they are very expensive in OZ, is there anything else thats good quality and a bit cheaper.

Thanks in advance.

Clear Skies
John Bambury

For about 32 years I have been bugged by not being able to see the crosshairs in my finder scope under dark skies. The finder scope that I have is the standard 8 x 50 straight through supplied with the Guan Sheng 10"/F5.

I believe it is not such a difficult job to illuminate it internally with a RED LED and I have a couple of questions in this regard.

I planned to mount the LED and a small battery in the OTA of the finder with a small switch on the side of the tube, obviously I need to keep these bits out of the light path.

My main question is will the LED provide enough internal illumination if it is mounted anywhere in the tube or does it need to be mounted very close to the crosshairs.

The 2nd question is what sized LED would do the job, I was thinking 3mm would be about right.

Are there any major foreseable problems with this that I should know about.

Thanks in Advance

Clear Skies
John Bambury

Hi Group:

I currently own a Guan Sheng (GS) manufactured 10"/F5 scope (247mm mirror face, 1255mm FL = F5.1). The standard secondary is 63mm which is a 26% CO, I am hoping to be able to drop the 1.85" (47mm) secondary supplied as standard on the 8"/F6 GS scope straight in without modification for a 19% CO. I have plugged all the numbers into NEWT but I am not 100% sure on what all the numbers mean. The scope will be used for visual work only. My widest FOV eyepiece is a 2" 32MM (63 deg AFOV) plossl for 40X and TFOV of 1.6 degrees. I don't use this eyepiece all that often and normally my Maximum FOV that I use is 1 deg in a 25mm ortho. I have listed all the necessary paramaters for NEWT below, could someone please check it out for me and tell me if it works ok. I think it will work fine.

Primary Diameter = 247mm (illuminised surface)
Focal Ratio = F5.1
Diagonal Minor Axis = 47mm or 1.85"
Tube Inside Diameter = 295mm
Tube Thickness = 1mm
Focuser to Front of Tube = 140mm
Mirror Face to Back of Tube = 70mm
Focuser Height = 57mm
Spare Focuser in Travel = 23mm
Additional Camera Height = 0
Focuser Inside Diameter = 50mm

Sorry to give you guys the measurements in metric but thats primarily what we use in Australia and I didn't have an imperial ruler or calipers handy.

Thanks in Advance
John Bambury

Hi All,

I have a colleague in the middle of building a 10"/F5 dobsonian and he has some Wilson Arts "Ebony Marble" laminate on hand. He has asked me if it is the same as "Ebony Star" and will it work as well. I am not familiar with this material and am hoping someone out there may be able to advise if it is a suitable alternative with a similar co-efficient of friction to ebony star.

TIA
John Bambury

Hi All,

I currently have 2 pairs of binoculars, having sold my
older pairs a couple of years ago. I currently have the
Pentax PCF WP 16 x 60 and the Pentax PCF WP 8x 40. I am
very happy with the PCF WP 16 x 60's they offer a very
nice view with a sharp flat field almost to the EOF
and IMO perform somewhat better than their selling price
of $AUS 300 would indicate. The 8 x 40 PCF WP I am not
as happy with, they cost $AUS 240 and have a little bit
of field curvature and astigmatism, not a bad binocular
but not to the same high standard as there bigger
brothers IMO.

I happened to stumble upon the Fujinon 8 x 42 BFL which
I can buy for $AUS 290 which seems a very good price to
me as they sell in USA for about $US 270. Remember also
that a pair of 8x40 Nikon Action cost about $AUS 250 and
a pair of 10 x 50 Vixen made Celestron Ultimas cost
almost $AUS 600. The Fuji 8x42 BFL are listed as a
birding binocular and are listed as Fujinon's entry
level birding binocular, but being a made in Japan product
with a Limited lifetime warranty I am assuming these would
still be a pretty reasonable binocular.

I am a glasses wearer so I need long eye-relief, which is
another reason I went with the Pentax's.

Has anyone used the Fujinon 8x42 BFL and can give me
some idea of how they perform and what would be even
nicer is if anyone can give a comparison between the
8x40 Pentax PCF WP and the Fuji's ?

Clear Skies and TIA
John Bambury

I dont know whether there are reasons why you need to go with a refractor. For the money you are planning on spending you can buy an 8" dob and you will see considerably more with an 8" reflector than you will with a 4" achromat, this relating to all forms of observing, Planets DSO's etc. The Guan Sheng and Discovery scopes are excellent value for money and an 8" is an excellent starting point. I suggest you go to a star party or local astronomy club and try to have a look through both types of telescope. In my opinion and I am often wrong, a 4" achromat doesnt even begin to compare with an 8"/f6 reflector on the planets or DSO's, yet the cost is very similar. You can push the reflector easily to 300x on nights of good seeing, at 300x the 4 " achromat will be crying out for mama.

Clear Skies
John Bambury

I purchased the Pentax PCF WP 16 x 60 Binos about 2 months ago and I am extremely happy with them. They have 20mm of eye relief for people who wear eyeglasses. They also have 4.0 diopters of adjustment in the right eyepiece if you wish to observe without your eyeglasses. For a binocular of this size they are extremely light and can be hand held for shorter periods with no trouble. They also have an extremely flat sharp field to the edge, optically they are extremely good for this price level, maybe they are a little more expensive than you wish to pay, but you do get fully multi coated lenses, BAK 4 prisms, they are also Nitrogen filled and waterproof. I rate them excellent value for money and especially suited to eyeglass wearers. The only drawback with them is that they have a narrower field of view than most binos this size at 2.8 degrees, I dont find this a problem at all. I paid $AUS350 and I think they are a little over $US200. If you want a bit more magnification consider the 20 x 60 model which also incorporates a special field flatening lens.

Clear Skies
John Bambury

The scopes from Hardin and Hands on Optics are identical you get a longer warranty and more accessories from Hands on Optics, Laser collimator, barlow etc. They are made by Guan Sheng in Taiwan, whereas the Orion scope is made by Synta in China. The Guan Sheng telescopes are in my opinion slightly better optically and mechanically than the Synta manufactured Orions. Guan Sheng are striving to attain a minimum standard of 1/12th wave P to V, Khanscope are guaranteeing them to be at least 1/10th wave. All their mirrors are factory tested on a laser interferometer. I have used and tested 6 examples of these GS telescopes from 6 inches to 10 inches. They have all had mirrors which were better than 1/8th wave. 1/8th wave is better than what 99.9% of amateurs will ever need. My current 10" has an excellent mirror at least 1/12th wave. Maybe I have been lucky with the examples I have used, but I havent heard of a bad one anywhere. Here is a test on the 10" done by Geoff Gaherty an observer with over 40 years experience.

http://www.khanscope.com/10_sky_mentor_review_by_g_gaherty.htm

This scope is the same as that sold by Hands On and Hardin, some earlier balance issues with these scopes have now been resolved. Personally I would buy the scope that had all the extras included, although you may wish to use a cheshire and not a laser and also buy a better quality barlow.

Hope this helps

Clear Skies
John Bambury