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Long slow Newtonians

equipment mirror making observing planet reflector
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#176 SandyHouTex

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Posted 13 February 2019 - 10:07 AM

I finally had first light with a Celestron 114eq Power Seeker last night.  4.5 inch mirror and f/7.89.  The optics are wonderful, but the provided eyepieces and mount suck.  With a 18mm BGO though, the moon was razor sharp and M45 was awesome.


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#177 Philler

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Posted 24 February 2019 - 03:23 PM

When I was a beginner in the 1980's with my first scope I read that if you are using a Barlow like say a 2x or 3x, you are not only doubling or tripling your magnification but the Barlow negative lens is actually increasing the focal length of your scope and focal ratio, by changing the steepness of the angle to you eyepiece. (hope I explained this right).. So, if I used like a 2x Barlow, Tele Negative, or Power Mate with an f/4.5 reflector, would that end up making the f/4.5 Dob something like an f/6 or f/7 reflector?   But, on the other hand, the mirror itself is still an f/4.5, and that doesn't change. 

Something else: with more complicated, multi lens group eyepieces like Nagler's, etc. that use a kind of built in Barlow in their shorter focal lengths for higher powers, greater eye relief, and wide fields;  are they not also increasing the focal length and thus focal ratios also?


Edited by Philler, 24 February 2019 - 08:28 PM.


#178 Astrojensen

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Posted 25 February 2019 - 10:04 AM

 

Something else: with more complicated, multi lens group eyepieces like Nagler's, etc. that use a kind of built in Barlow in their shorter focal lengths for higher powers, greater eye relief, and wide fields;  are they not also increasing the focal length and thus focal ratios also?

They do indeed increase the focal length, as seen from the perspective of the rear elements in the eyepiece. 

 

But since the main mirror is still f/4.5, you still have the coma, very tight focus tolerances, etc., of the fast primary. 

 

 

Clear skies!
Thomas, Denmark


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#179 Deep13

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Posted 25 February 2019 - 01:45 PM

***  The optics are wonderful, but the provided eyepieces and mount suck. ***

That's a common lament.



#180 mic1970

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Posted 25 February 2019 - 02:06 PM

Oh man... I can't wait to see this when complete.

Had this little project sitting around for a few years...12 inch F/20.

 

Orion Optics UK ground this and another one for the BBC for a Herschel Telescope doco  they were making.

 

I'm going to start the mirror box soon as per image from the Smithsonian Museum with the same mirror holder and access panel.

 

Three 7foot sections....1/ .Mirror box...2/ .carbon fibre rods....3/.carbon fibre rods with secondary holder.

 



#181 Astrojensen

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Posted 25 February 2019 - 03:20 PM

That's a common lament.

Sadly, yes.

 

 

Clear skies!
Thomas, Denmark


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#182 SandyHouTex

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Posted 26 February 2019 - 10:42 PM

At least I finally have an anwere for someone who wants to buy their kids a scope for $100.  The Celestron 114eq Power Seeker fits the bill.  Just make sure you recommend a reasonably priced Symmetrical.  Maybe a 12 mm for 72X.

 

Whatever you do don’t get the 127mm Power Seeker.  It’s a Bird-Jones and hard to collimate.

 

I also finally got the 6 inch solid tube Skywatcher Dob f/8.  I tried it out Saturday night.  Excellent optics and a pretty nice mount.  I didn’t try the eyepieces.



#183 John Fitzgerald

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Posted 27 February 2019 - 12:11 AM

  This is the 8 inch f/8.6 I ground and made back about 1988.  It was a good scope, on an old Edmund mount.  I had a 6" Byers gear on it.  That fence is an L shaped light and wind block.  I had a pier in the yard, with the mount under a trash can and bags.  I would carry that big aluminum OTA out about 100 feet.  It had a redwood plate that attached the tube rings to the mount.  I had the tube and rings fabricated locally, bought the focuser, mirror cell, secondary, and spider mail order.  I cut / bored all the holes and did all the mounting myself.  It worked well with the simple eyepieces we had back then.   Sold it about 1999 or 2000 on "Starry Messenger" ads.

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Edited by John Fitzgerald, 27 February 2019 - 12:14 AM.

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#184 CHASLX200

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Posted 27 February 2019 - 06:13 AM

  This is the 8 inch f/8.6 I ground and made back about 1988.  It was a good scope, on an old Edmund mount.  I had a 6" Byers gear on it.  That fence is an L shaped light and wind block.  I had a pier in the yard, with the mount under a trash can and bags.  I would carry that big aluminum OTA out about 100 feet.  It had a redwood plate that attached the tube rings to the mount.  I had the tube and rings fabricated locally, bought the focuser, mirror cell, secondary, and spider mail order.  I cut / bored all the holes and did all the mounting myself.  It worked well with the simple eyepieces we had back then.   Sold it about 1999 or 2000 on "Starry Messenger" ads.

Just what i would love.



#185 Earthbound1

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Posted 27 February 2019 - 06:34 AM

It's a real shame how many telescope review sites out there recommend that Powerseeker 127, Space.com as well as SEVERAL others. I had my heart set on one for months!!! Got the chance to buy one used for $50 on CL but it was not to be... Then I discovered CN and all you guys set me straight! Thanks for saving me lots of frustration and headaches as well as buyer's remorse! I finally got the parts to make a REAL 4.75" f/8.3 back around October 2018 and have been happy ever since (except for the weather!!! Grrr!!!). Too bad those review sites don't recommend CN as a pre-buyers guide! Anyway, been in love with my medium-long moderately slow f/8.3 since first light!

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Edited by Earthbound1, 27 February 2019 - 06:38 AM.

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#186 Deep13

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Posted 27 February 2019 - 03:05 PM

  This is the 8 inch f/8.6 I ground and made back about 1988. 

Awesome!



#187 John Fitzgerald

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Posted 27 February 2019 - 04:18 PM

Awesome!

Thank you!  Gotta love that 6x30 finder and the yellow setup "bug" light hanging on the fence.smile.gif


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#188 rachnoman

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Posted 22 March 2020 - 09:57 PM

For me it's my 10" F/8 and  8" F/7.5. At these F/ ratios, the obstruction is about 18% with a low  profile focuser. At that low percentage, the final image is not compromised. 


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#189 SpaceConqueror3

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Posted 22 March 2020 - 10:45 PM

The only one I've ever owned was a Edmund Scientific Space Conqueror  3" f/10 



#190 larrytOMC200

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Posted 23 March 2020 - 04:43 AM

Hi,

    When I was unemployed in the early 1980's, I made a 6 inch F11 mirror for a Newt. It was mounted as a dobsonian and I used it for a few years until I couls afford to get a larger sized scope. It served me very well and gave very good images of Luna, the planets, double stars and Globs. I actually did not realise how good it was until a very experienced amature complimented me on the view of Saturn. He was convinced it was at least a 8 inch scope.  I gave it to the Scarborough astronomical Society and wonder what happened to it. Since moving form the UK to New Zealand I guess I will never find out.

What I will say is that the scope worked well with cheap eyepieces and was very forgiving with collimnation.


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#191 Spikey131

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Posted 23 March 2020 - 04:46 PM

ATM 12.5 f/7.  Great planetary scope.

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#192 Jeff Morgan

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Posted 24 March 2020 - 03:37 PM

What I will say is that the scope worked well with cheap eyepieces and was very forgiving with collimnation.

 

Forgiving?

 

At f/9, just putting the optics in the tube is almost good enough!


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#193 Galicapernistein

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Posted 24 March 2020 - 03:51 PM

The hardest part of collimating my 8”F9 dob is trying to get the laser collimator to land on the tiny secondary way up at the top of the tube.


Edited by Galicapernistein, 24 March 2020 - 03:53 PM.

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#194 25585

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Posted 24 March 2020 - 04:24 PM

The hardest part of collimating my 8”F9 dob is trying to get the laser collimator to land on the tiny secondary way up at the top of the tube.

The green, brightest Glatter laser may be good for long tubes.


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#195 Spikey131

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Posted 24 March 2020 - 05:16 PM

All you need for an f/9 is a combo Cheshire.


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#196 rachnoman

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Posted 24 March 2020 - 07:43 PM

For me it's my 10" F/8 and  8" F/7.5. At these F/ ratios, the obstruction is about 18% with a low  profile focuser. At that low percentage, the final image is not compromised. 

It's actually about a 15% obstruction, not 18%. as previously stated, my error.



#197 ed_turco

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Posted 25 March 2020 - 11:25 AM

There is something glorious about a well-made 6" f/10 Newtonian!


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#198 icomet

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Posted 25 March 2020 - 09:26 PM

Did someone mention a 6" f/10 newt?

 

A.K.A. "anti-refractor"

 

Moonlite - Astrosystems - Antares - Cave (mirror cell) - Beck mirror (6 inch f/10)

 

mount is an old, "made-to-work-again" CG5 - Adv. GT.

 

Clear Skies.

 

 

 

 

 

 

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#199 KI5CAW

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Posted 26 March 2020 - 10:56 AM

My first telescope was a 6" f/12. It gave great images and I regret ever selling it. I currently still have my second scope, a 6" f/8 as well as an 8" f/8 with premium optics that just celebrated its 20th anniversary. They both give superb images, especially the 8 inch. I've thought about larger mirrors but at f/8 they'd be unwieldy. My 12" is f/5.



#200 25585

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Posted 26 March 2020 - 04:11 PM

 Orion Optics UK can make a VX6XL a 6" F11. 




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