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14.7" f/3 Dob/RFT

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#476 geovermont

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Posted 10 September 2020 - 12:13 PM

With the equatorial platform, that's going to be a pleasure to use. I'm going to guess that you'd be better off sticking with 3/4 inch plywood with cutouts rather than going with the thinner stuff. The 3/4 is so much stiffer than the others (and the glue surfaces are larger too, in case that matters). Anyway, I'll be standing in the line to look through that scope when you finally get it to Stellafane!


 

#477 coinboy1

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Posted 10 September 2020 - 12:30 PM

Very cool! Let us know what you think of the platform!!
 

#478 Augustus

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Posted 10 September 2020 - 12:37 PM

Very cool! Let us know what you think of the platform!!

I will! I'm excited that I will be able to do some casual imaging with the scope; if nothing else I'd like to do a star test with a friend's planetary cam and maybe shoot Mars just as a proof of concept. 


 

#479 stargazer193857

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Posted 10 September 2020 - 08:43 PM

If you keep anything thick, top priority should be the bottom of the rocker box.

Before you bought the eq platform, I was about to say get a shorter chair.
 

#480 BradFran

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Posted 11 September 2020 - 10:00 AM

If your main goal is weight, you can consider trying to modify the platform to act as a ground board for the rocker box. Here is an EQ platform, and another that is designed to do just that. You don't loose anything, as you can still use the old rocker box bottom without the platform, you just don't need it!


Edited by BradFran, 11 September 2020 - 10:00 AM.

 

#481 mic1970

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Posted 11 September 2020 - 10:58 AM

If you can burrow a dedicated astro camera.... try live stacking in SharpCap.  You can take 10-20 second shots and stack them to simulate a 5 minute shot for example.  Plus, I just love watching the stacking process as the pic gets brighter and brighter.  There is a DSLR add on discussed here in cloudy nights, but I haven't tried it yet.  

 

Here is my advice.... focus on a bright star.  Then try to find longest you can go before stars lose there shape.  Then come back 20% from that. Set you ISO so that you get about a quarter of the way on the histogram.  Shot as many as you can, get your darks and flats, and stack it.  

 

Also, balance might be a big factor.  If you are tracking well, but your dob is slipping just so slightly it will affect your pics.  So keep an eye out on that.  

 

Good luck and let us know how it goes.

 

Thought: If I put this on an EQ platform, how long of an exposure could I get with my DSLR before I got trailing? The Moonlite could definitely handle the weight....


 

#482 mic1970

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Posted 11 September 2020 - 11:00 AM

Our club president has it also.  He thinks it makes him a better astronomer.  

I have it, it’s not a disability


 

#483 Augustus

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Posted 11 September 2020 - 11:15 AM

Tonight I will be presenting on this scope at WAA's annual Members' Night on Zoom if anyone is interested. Will probably upload the PowerPoint and post it here as well.

 

If your main goal is weight, you can consider trying to modify the platform to act as a ground board for the rocker box. Here is an EQ platform, and another that is designed to do just that. You don't loose anything, as you can still use the old rocker box bottom without the platform, you just don't need it!

Yup, that's the plan

 

If you can burrow a dedicated astro camera.... try live stacking in SharpCap.  You can take 10-20 second shots and stack them to simulate a 5 minute shot for example.  Plus, I just love watching the stacking process as the pic gets brighter and brighter.  There is a DSLR add on discussed here in cloudy nights, but I haven't tried it yet.  

 

Here is my advice.... focus on a bright star.  Then try to find longest you can go before stars lose there shape.  Then come back 20% from that. Set you ISO so that you get about a quarter of the way on the histogram.  Shot as many as you can, get your darks and flats, and stack it.  

 

Also, balance might be a big factor.  If you are tracking well, but your dob is slipping just so slightly it will affect your pics.  So keep an eye out on that.  

 

Good luck and let us know how it goes.

I have quite a lot of friction on my alt bearings; I can remove the P2 and eyepiece and the scope stays still, it doesn't drift with the P2 and 21mm Ethos either. 

 

Not really interested in EAA or live stacking; DSLR shots are more for curiosity's sake than anything else, only planetary has some level of appeal.

 

Our club president has it also.  He thinks it makes him a better astronomer.  

Undoubtedly


 

#484 Augustus

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Posted 16 September 2020 - 11:35 PM

I made a little magnetic retainer system so that the UTA could safely be stacked and transported atop the mirror box without risk of sliding/bouncing. Additionally, I made some Velcro keepers for the dew controller and straps so that they could be more easily accessed - the controller sits on the mirror box, while the eyepiece dew heater has a holder for when I'm swapping eyepieces or not using it. Much better than having the controller under the mirror box and the heater always falling to the ground.

 

magnets.jpg

 

This enables the scope to be stored more compactly than ever before, and keeping the heaters attached to the UTA at all times allowed me to eliminate an entire carrying case. Plus, now that I can carry the UTA attached to the mirror box, I save at least part of a trip to the car.

 

transportnew.jpg

 

Here's a picture of the dew controller in action at WAS Public Night tonight. I got my first view of Mars this evening with the scope, which was really exciting.

 

pn.jpg

 

Next time I'm out, those cables will run under the shroud and be attached with Velcro to one of the truss poles.


 

#485 Augustus

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Posted 29 September 2020 - 11:06 AM

Cable management issues have been solved:

 

IMG_6926s.jpg

 

I also learned something interesting about collimating the secondary with diffraction rings that I didn't know about, but it's hard to explain how I did it in a way that makes any sense. Suffice to say planetary views are very good now - a bit better than my 5" apo even with poor seeing and dramatically better than my Coulter 13.1".

 

The mirror seems to suffer from dramatic astigmatism during cool down - close to 1 wave of it - which gradually subsides to basically zero within about 20 minutes. I would estimate that there is about 1/10 wave of support-induced astigmatism after cooldown, which I am working to alleviate.

 

Collimation shift has been eliminated with some slight modifications to the truss couplings (glue and wood putty on screws/inserts for a nice and tight fit). I can go straight from the horizon to the zenith and back again with zero issues. This was great last night because I wanted to view Mars as soon as it cleared the trees at an altitude of only about five degrees. 

 

There is an unknown diffraction spike source that is really troubling me on stars. It is not really symmetrical in any direction, and I have ruled out my spider, cables dangling in the OTA, mirror clips, and miscollimation or scratches. It is only a problem on bright stuff but still puzzling. 

 

The mirror also seems to have a very slight TDE; I may try to make a TDE mask to see if that changes anything.


Edited by Augustus, 29 September 2020 - 11:20 AM.

 

#486 tommm

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Posted 29 September 2020 - 12:56 PM

Are you referring to something like Lockwood's article (scroll down to Collimating the Newtonian Telescope) on using diffraction rings for collimating?


 

#487 Darren Drake

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Posted 29 September 2020 - 01:15 PM

You can use a knife edge test on a star to see if there is a turned edge.   This would help to understand where its best to mask off exactly..


 

#488 Augustus

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Posted 29 September 2020 - 01:57 PM

Are you referring to something like Lockwood's article (scroll down to Collimating the Newtonian Telescope) on using diffraction rings for collimating?

Sort of.

 

The best way I can describe it is that when the secondary is out of alignment stars have a weird flaring effect to them off to one side.

 

You can use a knife edge test on a star to see if there is a turned edge.   This would help to understand where its best to mask off exactly..

How would one make a knife edge for an eyepiece or focuser?


 

#489 Darren Drake

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Posted 29 September 2020 - 02:17 PM

I used an old eyepiece and took out the lenses.  Then I taped a straight edge across the middle.  It needs to be very clean and straight.  This basically works like a ronchi grating but with just one edge.  Place the scope on a star with tracking or Polaris if no tracking.  Position it precisely to the focal point and you will see a 3D null image that will show a very accurate profile of your mirror.  A turned edge or spherical aberration will clearly show up as long as seeing isn't terrible.  You can also test a scope indoors if you have a scope with known superb optics as described in my article  here a while back...

https://www.cloudyni...null-test-r1586

Attached Thumbnails

  • 20200929_141117.jpg

Edited by Darren Drake, 29 September 2020 - 03:23 PM.

 

#490 Augustus

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Posted 01 October 2020 - 03:38 PM

EQ platform is on the way and I'm almost done designing the new, lightweight rocker box to go on top of it. 

I used an old eyepiece and took out the lenses.  Then I taped a straight edge across the middle.  It needs to be very clean and straight.  This basically works like a ronchi grating but with just one edge.  Place the scope on a star with tracking or Polaris if no tracking.  Position it precisely to the focal point and you will see a 3D null image that will show a very accurate profile of your mirror.  A turned edge or spherical aberration will clearly show up as long as seeing isn't terrible.  You can also test a scope indoors if you have a scope with known superb optics as described in my article  here a while back...

https://www.cloudyni...null-test-r1586

I'm gonna do this once I can find a cheapo eyepiece to cannibalize for the job.

 

Thank you!


 

#491 Darren Drake

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Posted 01 October 2020 - 04:34 PM

You could just place a knife edge across the focuser.  An eyepiece shell isnt required if your innovative.


 

#492 avarakin

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Posted 01 October 2020 - 05:14 PM

@Darren,

Would this method show astigmatism?


 

#493 Darren Drake

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Posted 01 October 2020 - 05:50 PM

Yes if you know how to look for it.  Any assymetry left to right is an indicator but its subtle.  The thermal gremlins may make it difficult to detect.  A star test would be much more revealing on astigmatism...


 

#494 Augustus

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Posted 01 October 2020 - 05:59 PM

Yes if you know how to look for it.  Any assymetry left to right is an indicator but its subtle.  The thermal gremlins may make it difficult to detect.  A star test would be much more revealing on astigmatism...

Star test shows 1/10 wave or so of astig, ~1/6-1/8 wave SA, and some minor amount of TDE that I can't determine.

 

Might just go read more Suiter....


 

#495 Darren Drake

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Posted 01 October 2020 - 10:28 PM

Definitely read Suiter but the null test can still easily tell you how much edge to mask off if any..


 

#496 Augustus

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Posted 02 October 2020 - 12:38 PM

New rocker is done. Shaved off about 7 pounds compared to the original, bringing total scope weight to 41 pounds.

 

I'm replacing the ground board with an EQ platform though, so in practice the weight will jump up again somewhat.

 

rockerprog1.jpg

 

rockerprog2.jpg


 

#497 Augustus

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Posted 02 October 2020 - 02:45 PM

The compacted scope, minus its EQ platform which will replace the ground board:

 

compacted.jpg

 

At just 37 pounds without poles I can carry the entire unit easily, and everything can make it to the car in two trips (scope, then poles + eyepiece case). 


 

#498 grzesznypl

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Posted 02 October 2020 - 02:55 PM

It packs very nicely waytogo.gif 
If .....and I mean if it had detachable bearings and short double ring upper cage it could be even smaller, not sure if lighter but more packable. Nevertheless this is really nice.


 

#499 Darren Drake

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Posted 02 October 2020 - 03:54 PM

Which eq platform?  I have and old  Osypowski and a Crossbow..


 

#500 Augustus

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Posted 02 October 2020 - 05:40 PM

I added some reinforcement to the lower truss brackets in the form of screws securing the Delrin inserts so they don't move, which has been causing collimation to be way off when I first set the scope up.

 

Also, here's just some proof that I can move the entire scope in one piece without difficulty:

 

IMG_6980s.jpg

 

It packs very nicely waytogo.gif 
If .....and I mean if it had detachable bearings and short double ring upper cage it could be even smaller, not sure if lighter but more packable. Nevertheless this is really nice.

UTA is as short as I could make it; I guess you could use a low-profile spider but the gain is marginal

 

Bearings come off with a screwdriver

 

Could make the baffle come off and slide under the mirror box, then nest the UTA upside down in the mirror box but idk

 

Which eq platform?  I have and old  Osypowski and a Crossbow..

Ed Jones, custom made for this scope.


 


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