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14" Hubble Optics Project

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#1 starman345

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Posted 28 April 2012 - 08:48 AM

The 14" Hubble Optics mirror has arrived and I'm at the point of making sawdust with the secondary cage construction but I'm having trouble understanding the questions NEWT is asking for input:

Tube thickness- Is this the actual width(not thickness) of the 1/2" baltic birch I'm using for the cage rings? I'm thinking of making them 1.5" to 2" wide, not sure on this yet.

Focuser to front of tube- Is this the distance from the center of the focuser tube to the top of the upper secondary cage ring?

Mirror face to back of tube- Is this the primary mirror in question?

Spare focuser in travel- Not sure what to enter here as I don't really know what it means. I'm thinking it probably has to do with adding a camera later?

I have a 13mm Ethos that I use almost exclusivly in my 10" Skywatcher and expect in the future to add a 6mm and 21mm Ethos as I really like the wide veiws. The Hubble Optics mirror is f 4.48 so I'm wondering about possibly needing a Paracorr and if so how I should tell NEWT about that? Or maybe I could get by without a Paracorr?

#2 polaraligned

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Posted 28 April 2012 - 10:14 AM

Tube thickness is the thickness of the panel that your focuser will be mounted on, NOT the ring thickness. You have the next 3 questions right. Not sure how much, if any, extra backfocus you need for the paracorr

#3 Mirzam

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Posted 28 April 2012 - 10:43 AM

A Paracorr needs about 1/2" of infocus. I usually put the focal plane about 3/4" above the fully racked in focuser height if it is a scope that will use a Paracorr.

JimC

#4 starman345

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Posted 28 April 2012 - 07:01 PM

Tube thickness is the thickness of the panel that your focuser will be mounted on, NOT the ring thickness. You have the next 3 questions right. Not sure how much, if any, extra backfocus you need for the paracorr


Thanks, I'm glad I asked that question, there is quite a difference in measurement between my cage rings width and focuser board thickness.

#5 starman345

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Posted 28 April 2012 - 07:21 PM

A Paracorr needs about 1/2" of infocus. I usually put the focal plane about 3/4" above the fully racked in focuser height if it is a scope that will use a Paracorr.

JimC


Thanks, is it safe to assume even if I find I don't need a Paracorr I will be alright with 3/4" spare focuser in travel ?
I have a Moonlite 2" focuser presently on my Skywatcher but will be using that on this project.
I've decided on making the cage 10" high with 1/2" thick rings 1 1/2" wide. Changing the various input numbers in NEWT has finally given me acceptable results. See below
NEWT

* 14. inch f 4.48

14. primary mirror diameter
4.48 f ratio
16. tube inside diameter
0.25 tube thickness
0.75 extra focus height
0. camera focus height
2.6 diagonal minor axis
2. focuser height
2. focuser diameter
8. focuser to front of tube
3.5 mirror to back of tube
1. fixed baffles (1 or 0)
inch unit of measure

If anyone notices anything jumping out at them as wrong I would appreciate a heads up...this is all new to me.
I've chosen a 2.6" secondary but could go with a 2.76", NEWT gives the ok for either keeping all the other numbers the same. Is there any advantage of one over the other?
Thanks for the answers so far, I appreciate it.

#6 sopticals

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Posted 28 April 2012 - 10:31 PM

Hi Brian,

I think I would go with a 3.1" secondary. When I set up my 14" f5.07 Hubble Optics mirror powered dob, I instaled a GSO 2.76" secondary. Since going the binoviewer way, with 1.5"-2.0" further infocus required,I moved secondary down tube to achieve this, so light cone size arriving at the secondary being larger, I needed the 3.1" to accommadate this. Also going slightly oversize, deals with edge problems that many secondary mirrors seem to have. In my case, the image quality was improved when I upsized. Could be that the image was better due to my new secondary, (from Hubble Optics), may have been optically superior to the smaller GSO. Any way all the best with the build.

Stephen.

#7 killdabuddha

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Posted 28 April 2012 - 11:59 PM


I wonder whether 2.6" is enuf too. Have you calculated the illumination values for a 6 or 7mm exit pupil EP? I like Mel Bartels' calculator

http://www.bbastrode...om/diagonal.htm

Also, have a look at Lockwood's chart

http://www.loptics.c.../diagonals.html

Collimation would be easier too. And minimalizing the CO (central obstruction) percentages are sumthin we've all needlessly obsessed over in an effort to tweak every bit of performance outa our scopes, but 3.1" is well within acceptable limits.

#8 starman345

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Posted 29 April 2012 - 08:06 AM

Stephen, thanks for the kind words. I'm really enjoying the process of putting this together. Some of the software programs are a bit humbling, Plop mostly, still trying to get my head around that.
Thanks for your experience with your 14" Hubble and secondary sizing, it looks like I should be considering something bigger than 2.6"

Killdabudda:
Thanks for the links, both Bartels and Lockwood's sites give acceptable results for a 3.1" secondary with my 14" primary using my 13mm Ethos with its 22.3mm field diameter.
The only problem I have now is NEWT does not like the 3.1" secondary choice with my setup, it says I now have "vignetting at focuser of 75% ray". I've tried changing the various values in NEWT but can't seem to make the 3.1" secondary an acceptable choice. Any suggestions with that? At this point no actual construction has started yet so I can change anything but the Primary diameter and focal ratio. Or maybe I should just ignore the warning?

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#9 Mirzam

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Posted 29 April 2012 - 09:16 AM

Ignore the warning. Also, you will be fine if you allow 3/4" infocus with or without using a Paracorr. At f/4.48 you will likely end up using one though.

JimC

#10 Dave O

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Posted 29 April 2012 - 10:19 AM

Yes, the 'warning' is simply letting you know that the light cone including everything that is 75% illuminated (or more); is larger than the 2" diameter of your focuser. This would normally not be considered a 'bad' thing.

#11 killdabuddha

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Posted 29 April 2012 - 11:23 AM

Yes, as the others say, the vignetting is inconsequential. NEWT gives you the size of the 75% illuminated field and yer field stop is still well within this range. The spare wiggle-room you have--the difference between yer EP field stop and the size of NEWT's 75% illuminated field--is what gives you yer greater ease of collimation, etc. (Moreover, some very knowledgeable people here say that you can even satisfactorily get away with an illumination drop-off down to 50%, but you don't have to worry about that.) I ran yer numbers, 1st thru a telescope calculator for exit pupil and field stop per the 21mm Ethos

http://www.stargazin...a/scopemath.htm

which gives an exit pupil of 4.68mm at a field stop of 36.6mm, then plugged these values into Mel's calculator. Yer fully illuminatin 20 mm with nary (.2) any magnitude drop-off. Let's say you decide later on to really push the exit pupil for good dark sky conditions--max it out to near 7mm exit pupil with a 31mm Nagler for a 1.6* TFOV...At a field stop of 44.4mm yer fully illuminated field hasn't changed, and the illumination drop-off is still less than .3 magnitude at the edge of field. I wish I had yer curves :p and yer room to maneuver.

#12 starman345

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Posted 29 April 2012 - 12:30 PM

Thanks for the replies everyone, it looks like the choice is a 3.1" secondary. I have materials on hand to construct the secondary cage so that will begin, I think after that is completed I will give the dimensions to Astrosystems for a spider, holder, and 3.1" secondary. They seem to have a good reputation here and elsewhere, maybe there are others just as good?

#13 Mirzam

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Posted 29 April 2012 - 01:58 PM

Astrosystems has excellent spiders and secondary holders, but for the secondary itself I like to have one that comes with test data. That pretty much limits the choice to Protostar or Antares Optics. Protostar also makes nice spiders/holders but their delivery times have been quite long. Maybe they are caught up? Anyone ask them at NEAF?

JimC

#14 killdabuddha

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Posted 29 April 2012 - 06:34 PM

Astrosystems has excellent spiders and secondary holders, but for the secondary itself I like to have one that comes with test data. That pretty much limits the choice to Protostar or Antares Optics. Protostar also makes nice spiders/holders but their delivery times have been quite long. Maybe they are caught up? Anyone ask them at NEAF?

JimC


Yeah, Protostar and Antares seem to be the consensus choice from everythin I've read here at CN. We woulda gone with one of these. But I would caution against goin overboard here too...the standard secondaries of these are likely to be as good to the eye as any of their others (an Antares 1/14th wave PV will be as good as their 1/30th as far as you can tell, and you'll save $120).

Also, for yer reference, here's a nice list of links/resources

http://stellafane.org/misc/links.html

#15 SteakFury

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Posted 30 April 2012 - 06:03 AM

Astrosystems has excellent spiders and secondary holders, but for the secondary itself I like to have one that comes with test data. That pretty much limits the choice to Protostar or Antares Optics. Protostar also makes nice spiders/holders but their delivery times have been quite long. Maybe they are caught up? Anyone ask them at NEAF?

JimC


I recently ordered a spider from Protostar. They list a two week lead time but mine shipped in just a couple of days.

#16 sopticals

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Posted 30 April 2012 - 05:35 PM

Hi Brian,

Got my spider/secondary holder for my 14" dob from "Meridian Telescopes", fits 16" tube size. Description: 4-vane Spider & Secondary Holder Assembly for 16"ID Tube w/1.8" hub, Unpainted. Price $37.95 plus shipping. Their address: MTG Products LLC, PO Box 571, Mt Clements,MI 48046. Not at all expensive and works well.

Best regards
Stephen.

#17 starman345

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Posted 08 May 2012 - 06:01 PM

I haven't forgotton about this project, I was side tracked with a barn door tracking system replacement. My neighbour enlisted me to help him change to a new system, he figured his 105 year old tube track had paid for itself. Neither of us had ever done anything like that before so it was an education.
Back to ATM:
I've spoken with Astrosystems and Protostar about spiders, holders and secondary mirrors and its just a matter of deciding which, they both seem to be very professional.
Stephen, I e-mailed Meridian Telescopes as they were out of stock on the 16" spider/holder on their wepage, sadly, they are no longer going to stock any more of them(in that size). You got a deal there!
I got a 5x5 sheet of 1/2" baltic birch and have been cutting tube rings. I didn't have a router circle cutting jig so made one but it took four tries to get the inside dimension for the rings just right, a lot of trial and error but its coming.

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#18 starman345

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Posted 08 May 2012 - 06:13 PM

Is there a way to attach more than one picture per post?

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#19 sopticals

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Posted 08 May 2012 - 11:32 PM

Hi Brian,

Great stuff. Watching your progress with interest.

Best regards
Stephen.

#20 starman345

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Posted 09 May 2012 - 02:08 PM

Spent a little more time with the tools today and produced this. I wanted the cage height to be 10" but I forgot about there being strut seats on both rings so I'm short of 10" by 1/16"...I"ve had bigger disasters

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#21 starman345

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Posted 09 May 2012 - 02:09 PM

My jig for cutting four struts all the same length

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#22 starman345

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Posted 09 May 2012 - 02:29 PM

What is the best way to install tube inserts? I ran a 1/4" bolt through a piece of 3/4" oak and fastened an insert up tight then backed it off about 1/4" and tapped it into the tube.A couple of them didn't go in straight, they seem to be tipped to one side. Any way to straighten them out?

#23 starman345

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Posted 13 May 2012 - 11:33 AM

I"ve been playing with Plop and am wondering what others are using as a thickness value for their Hubble Optics mirror. The selections in Plop are for solid mirrors not the sandwich type that Hubble makes. At least that is what I think, I see "fused silica" is that a sandwich type?

#24 davidpitre

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Posted 13 May 2012 - 02:32 PM

Astrosystems has excellent spiders and secondary holders, but for the secondary itself I like to have one that comes with test data. That pretty much limits the choice to Protostar or Antares Optics.

I believe Ostahowski's secondaries come with a test report:
http://www.ostahowskioptics.com/

#25 starman345

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Posted 13 May 2012 - 02:53 PM

Thanks David, I've already ordered a Protostar 3.1", good to know about Ostahowski, I"d never visited that site before.






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