Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Optimizing an 8" f4 and 8" f5 for Visual?

Beginner DIY ATM Reflector Visual Dob
  • Please log in to reply
28 replies to this topic

#1 Echolight

Echolight

    Voyager 1

  • *****
  • topic starter
  • Posts: 12,103
  • Joined: 01 May 2020
  • Loc: Texas

Posted 03 October 2023 - 12:47 PM

I'd like to optimize my newtonians for visual use, and am seeking advice and experienced perspectives.

 

My telescopes are:

An Orion (Synta) 203mm f4.9 with 58mm secondary and an aftermarket Orion low profile Crayford 2-speed focuser.

And an Orion (GSO) 200mm f4 with 70mm secondary and stock GSO linear bearing 2-speed crayford focuser.

IMG_20230930_111634784~5.jpg

 

I'd like both to fairly well illuminate a Meade 28 PWA, with roughly estimated 40.7mm field stop. But also gain some contrast along the way.

 

My initial thought was to chop a few inches off of the f4.9 tube (I think I need just a bit over 2 inches, maybe 2.5", above the tube with low pro focuser for the focal point) and use the 58mm secondary (at least to start with), and transplant the f4 mirror into it.

I do like using the f4 on my Unistar alt/az mount for richfield. Where I can easily scan across and around zenith. 

 

And then, maybe, cut the f4 tube in half or about 9 inches off the top. Flip the top upside down to have a bit of extra light shield. And then join them together with struts, à la Skywatcher flextube, to give it a longer space between the primary and secondary.

And make a dob base for it.

Of course I'd like a rotating UTA? Even if it could only rotate 30 degrees or so. Although I don't think I've ever seen this done.

And I'm not so  sure any of this is the best way to go with the f4.9.

I'm still not yet sure the cells are interchangeable in the different tubes. So I might have to use the 203 f4.9 cell for the 200 f4. And go deeper into the ATM/DIY pool to make the 203 4.9 swim.

Eventually, I'd probably like to stretch it out a bit more, and use a lower profile focuser. So secondary size is TBD.

 

What do you guys think? Is this a reasonable path to optimize these scopes for visual?

Or should I just start from scratch with the 203 f4.9, as a strut or truss travel dob?


  • brentelgeuse likes this

#2 Dale Eason

Dale Eason

    Soyuz

  • *****
  • Posts: 3,755
  • Joined: 24 Nov 2009
  • Loc: Roseville,Mn.

Posted 03 October 2023 - 01:08 PM

The focal length of the main mirror and diameter of the tube is the driving factor as to the size of the diagonal. This is a guess but I think you will not get a discernable increase in contrast by doing anything to those scopes.  Why do you think the things you suggest will help.  

 

You can use Mel Bartels telescope design calculator to help you decide.  Especially the diagonal size calculator section.  https://www.bbastrod...wtDesigner.html



#3 Echolight

Echolight

    Voyager 1

  • *****
  • topic starter
  • Posts: 12,103
  • Joined: 01 May 2020
  • Loc: Texas

Posted 03 October 2023 - 01:34 PM

The focal length of the main mirror and diameter of the tube is the driving factor as to the size of the diagonal. This is a guess but I think you will not get a discernable increase in contrast by doing anything to those scopes.  Why do you think the things you suggest will help.  

 

You can use Mel Bartels telescope design calculator to help you decide.  Especially the diagonal size calculator section.  https://www.bbastrod...wtDesigner.html

Well, by lengthening the distance between the primary and secondary, I can use a smaller secondary to achieve the same level of illumination.

 

No?

 

Right now, the focal point on the f4 is about 7 inches outside the tube. Or so it seems. If I can move around 4 inches from outside the tube, to in between the primary and secondary, then the size of the secondary should be able to be correspondingly smaller. Therefore increasing contrast without a loss of illumination.

 

Are you saying this is not correct?


Edited by Echolight, 03 October 2023 - 01:42 PM.


#4 Dale Eason

Dale Eason

    Soyuz

  • *****
  • Posts: 3,755
  • Joined: 24 Nov 2009
  • Loc: Roseville,Mn.

Posted 03 October 2023 - 03:00 PM

Ok, I did not know the FL was so far outside the tube.  Perhaps but what does Mel's calculator say about the change.  What percentage difference is the change?  Will it make a big enough difference to make it worth all that work.  



#5 Steve Dodds

Steve Dodds

    Surveyor 1

  • *****
  • Vendors
  • Posts: 1,848
  • Joined: 02 Apr 2007
  • Loc: Utah

Posted 03 October 2023 - 03:25 PM

Well, by lengthening the distance between the primary and secondary, I can use a smaller secondary to achieve the same level of illumination.

 

No?

 

Right now, the focal point on the f4 is about 7 inches outside the tube. Or so it seems. If I can move around 4 inches from outside the tube, to in between the primary and secondary, then the size of the secondary should be able to be correspondingly smaller. Therefore increasing contrast without a loss of illumination.

 

Are you saying this is not correct?

Yes!

 

 Using a low profile focuser you should be able to reduce that 7" to about 1.5" and use a lot smaller secondary.


  • Echolight likes this

#6 rgk901

rgk901

    Vanguard

  • *****
  • Posts: 2,410
  • Joined: 28 Apr 2021
  • Loc: Beautiful Bortle 10 Midwest Skies

Posted 03 October 2023 - 03:29 PM

I had the very same idea but with my 6.. but don't feel like doing it :)

Will be eagerly awaiting outcome of your project!

I'd bet a nice anteres secondary will improve contrast all by itself!
  • Echolight likes this

#7 Steve Dodds

Steve Dodds

    Surveyor 1

  • *****
  • Vendors
  • Posts: 1,848
  • Joined: 02 Apr 2007
  • Loc: Utah

Posted 03 October 2023 - 03:30 PM

Looking at your tube, it may be too short to do what you want.  Really needs to be about 5" longer.



#8 Echolight

Echolight

    Voyager 1

  • *****
  • topic starter
  • Posts: 12,103
  • Joined: 01 May 2020
  • Loc: Texas

Posted 03 October 2023 - 03:36 PM

Ok, I did not know the FL was so far outside the tube.  Perhaps but what does Mel's calculator say about the change.  What percentage difference is the change?  Will it make a big enough difference to make it worth all that work.  

Yes. Particularly the f4 is designed as an astrograph.

 

I didn't check the calculator yet. But just rough in my head numbers tells me that putting 4 inches from outside the scope to in between the primary and secondary would let me go from the 70mm secondary (35% obstruction) down to a 61mm secondary (30.5% obstruction) without a loss in illumination.

 

I'm hoping to use the 58mm secondary that I already have. To get down to 29% obstruction without light loss.

I'm not exactly sure how contrast is measured. But I figure 18% increase roughly.

 

I'l have to do some more measuring. 

 

I was really wandering if my how was a reasonable idea. Robbing Peter to pay Paul. But I guess that is quite common in telescoping.



#9 Echolight

Echolight

    Voyager 1

  • *****
  • topic starter
  • Posts: 12,103
  • Joined: 01 May 2020
  • Loc: Texas

Posted 03 October 2023 - 03:40 PM

Looking at your tube, it may be too short to do what you want.  Really needs to be about 5" longer.

Since neither are optimized for visual...

 

My plan is to put the f4 mirror in the f4.9 tube and use the low pro focuser.. And chop off the excess.

It's roughly 10 inches longer. Although an inch or so of that is at the top. Above the diagonal. And an inch or more is below the primary. The f4 primary (GSO) is right at the back of the tube currently.

 

So I will chop somewhere between three and five inches off the bottom of the f4.9 tube to hold the f4 mirror... depending on which cell I use 


Edited by Echolight, 03 October 2023 - 03:45 PM.


#10 Echolight

Echolight

    Voyager 1

  • *****
  • topic starter
  • Posts: 12,103
  • Joined: 01 May 2020
  • Loc: Texas

Posted 03 October 2023 - 03:50 PM

I had the very same idea but with my 6.. but don't feel like doing it smile.gif

Will be eagerly awaiting outcome of your project!

I'd bet a nice anteres secondary will improve contrast all by itself!

I might get an Antares for the f4.9. I'm hoping I can stretch the f4 enough to use the 58mm I already have. 

We shall see.


  • rgk901 likes this

#11 duck

duck

    Apollo

  • -----
  • Posts: 1,411
  • Joined: 11 Jun 2020
  • Loc: madera ca

Posted 03 October 2023 - 04:41 PM

prevent one bounce light from getting to the focal plane.  This means an adequate "dew" shield.  Really, these are stray light shields.


  • starcanoe and Echolight like this

#12 walt r

walt r

    Soyuz

  • -----
  • Posts: 3,933
  • Joined: 13 Feb 2007
  • Loc: Doylestown, PA

Posted 03 October 2023 - 06:38 PM

How is the quality of the optics, Primary and Secondary???

 

If the figures of these mirrors are Very good then what you plan would optimize the visual use.

If the figures are Poor than it is all a waste of time unless you get better optics.

 

Have you tested the optics?

Have you upped the magnification on a night of good seeing to see how well they do?

 

Plus other's suggestions.


  • Echolight likes this

#13 Echolight

Echolight

    Voyager 1

  • *****
  • topic starter
  • Posts: 12,103
  • Joined: 01 May 2020
  • Loc: Texas

Posted 03 October 2023 - 08:02 PM

How is the quality of the optics, Primary and Secondary???

 

If the figures of these mirrors are Very good then what you plan would optimize the visual use.

If the figures are Poor than it is all a waste of time unless you get better optics.

 

Have you tested the optics?

Have you upped the magnification on a night of good seeing to see how well they do?

 

Plus other's suggestions.

Well that's no help at all! Killjoy lol.gif

 

I'm not sure I'd ever really know how the f4 would do at very high power unless I optimize it for visual and make it have less than a 35% obstruction.

 

Should I just chunk it in the dumpster and wish I had an Obsession with a $2000 mirror?

 

I viewed Saturn at around 200x. But I thought it could have been improved upon with a smaller obstruction.

I will try a star test before I dispose of it flowerred.gif


Edited by Echolight, 03 October 2023 - 08:16 PM.


#14 dan chaffee

dan chaffee

    Viking 1

  • -----
  • Posts: 736
  • Joined: 22 Mar 2006
  • Loc: North Carolina

Posted 03 October 2023 - 11:12 PM


I'm not sure I'd ever really know how the f4 would do at very high power unless I optimize it for visual and make it have less than a 35% obstruction.

 

I would take Walt's comment seriously. There really is no point

in optimizing it of the optics are poor to mediocre.     Star testing

is  readily available and should give you some idea of the quality

even if you are not experienced in using it. The larger secondary

will make no difference for the test. BTW, the only visible improvement

in using a smaller obstruction of equal quality will be better contrast

on bright extended objects, not sharpness.  Obstructions affect

the modulation transfer function slightly differently than spherical

aberration.


Edited by dan chaffee, 03 October 2023 - 11:16 PM.


#15 Echolight

Echolight

    Voyager 1

  • *****
  • topic starter
  • Posts: 12,103
  • Joined: 01 May 2020
  • Loc: Texas

Posted 03 October 2023 - 11:21 PM

I would take Walt's comment seriously. There really is no point

in optimizing it of the optics are poor to mediocre.     Star testing

is  readily available and should give you some idea of the quality

even if you are not experienced in using it. The larger secondary

will make no difference for the test. BTW, the only visible improvement

in using a smaller obstruction of equal quality will be better contrast

on bright extended objects, not sharpness.  Obstructions affect

the modulation transfer function slightly differently than spherical

aberration.

When you quoted me, why do you cut out the part where I said I would do a star test?



#16 durangodoug

durangodoug

    Vostok 1

  • *****
  • Posts: 188
  • Joined: 08 Jan 2015

Posted 04 October 2023 - 01:35 AM

Sounds like one scope too many, as they're so close in capability / I'd pick my favorite and give the other to a family member or sell.

 

Personally, I'd probably keep the shorter FL for the spectacular wide-field views and then look for a light bucket that could do justice to DSO's.


Edited by durangodoug, 04 October 2023 - 01:38 AM.


#17 Dale Eason

Dale Eason

    Soyuz

  • *****
  • Posts: 3,755
  • Joined: 24 Nov 2009
  • Loc: Roseville,Mn.

Posted 04 October 2023 - 02:01 AM

When you quoted me, why do you cut out the part where I said I would do a star test?

Probably like me he did not make it past your statement stating "I'm not sure I'd ever really know how the f4 would do at very high power unless..."

 

Indicating that you did not know about how to do star testing.

 

But good to know that instead you do plan to star test it which is indeed the way to find out.  



#18 dan chaffee

dan chaffee

    Viking 1

  • -----
  • Posts: 736
  • Joined: 22 Mar 2006
  • Loc: North Carolina

Posted 04 October 2023 - 04:47 AM

When you quoted me, why do you cut out the part where I said I would do a star test?

Sorry, I just quoted enough to show what post I was responding to. Yes,

you did mention the star test; sorry about that.



#19 bunyon

bunyon

    Fly Me to the Moon

  • *****
  • Posts: 6,412
  • Joined: 23 Oct 2010
  • Loc: NC

Posted 04 October 2023 - 05:50 AM

I disagree. If you want premium optics, yes, sell what you have and save up.

But you have two telescopes and you want to use them visually. Makes sense to me. I’m currently working on a similar project. Not every scope has to have perfect optics. Yes, do the Star test and see how they are but for a casual stroll through the sky, they don’t have to be perfect.

#20 Echolight

Echolight

    Voyager 1

  • *****
  • topic starter
  • Posts: 12,103
  • Joined: 01 May 2020
  • Loc: Texas

Posted 04 October 2023 - 06:02 AM

I disagree. If you want premium optics, yes, sell what you have and save up.

But you have two telescopes and you want to use them visually. Makes sense to me. I’m currently working on a similar project. Not every scope has to have perfect optics. Yes, do the Star test and see how they are but for a casual stroll through the sky, they don’t have to be perfect.

I've heard that some people even make their own mirrors!



#21 brentelgeuse

brentelgeuse

    Ranger 4

  • -----
  • Posts: 361
  • Joined: 18 Nov 2021
  • Loc: Northern New Brunswick

Posted 04 October 2023 - 07:20 AM

I would so totally play around with these! Chop the focuser end off the f4.9 and make it rotate on the F4 tube. I'd put duct-tape over the old focuser hole LOL (probably I'd put a lovely informational plate of some kind) 


  • Echolight likes this

#22 Echolight

Echolight

    Voyager 1

  • *****
  • topic starter
  • Posts: 12,103
  • Joined: 01 May 2020
  • Loc: Texas

Posted 04 October 2023 - 08:16 AM

I would so totally play around with these! Chop the focuser end off the f4.9 and make it rotate on the F4 tube. I'd put duct-tape over the old focuser hole LOL (probably I'd put a lovely informational plate of some kind) 

Awesome! Shedding some light on the insta-rotating focuser idea!

Which would be super ideal for richfield viewing.

 

The f4.9 tube is slightly larger than the f4 tube. So just maybe.

If it wasn't for  the seam, it just might work.

Maybe some judicious grinding and polishing to smooth it out on the inside.

And possibly a generous application of teflon tape.

If I had to, I could cut a relief notch or two in the f4.9 tube, chamfer the inside edges, and use a ring to supply enough tension so it wouldn't rotate on it's own.

With a ring on it, I could possibly just cut out the seam as the relief notch. Up to where the f4 tube ends.

Maybe attach a round from a piece of bucket on the inside at the top of the f4 tube and at the mating point on the inside of the f4.9 tube.

 

Then build a box around the leftover bottom half of the f4.9 tube to attach dob alt bearings to, with enough room to slip a strut in each corner. And build a lightweight UTA for the top.


Edited by Echolight, 04 October 2023 - 08:37 AM.


#23 walt r

walt r

    Soyuz

  • -----
  • Posts: 3,933
  • Joined: 13 Feb 2007
  • Loc: Doylestown, PA

Posted 04 October 2023 - 10:23 AM

I disagree. If you want premium optics, yes, sell what you have and save up.

But you have two telescopes and you want to use them visually. Makes sense to me. I’m currently working on a similar project. Not every scope has to have perfect optics. Yes, do the Star test and see how they are but for a casual stroll through the sky, they don’t have to be perfect.

Exactly, Star test to ensure the optics are acceptable.

Since there can be a huge variation of optic quality even from the same maker it is worth testing.

 

Also, the lower the f/ the more likely the optics are not good enough since fast f/ is harder to get the mirror figure acceptable.

Also this tests the diagonal and it is as important as the primary.

If you replace the secondary then ensure you get one that is good enough not to degrade the primary.



#24 Don H

Don H

    Surveyor 1

  • *****
  • Posts: 1,924
  • Joined: 28 Dec 2006
  • Loc: Near Tucson, AZ

Posted 05 October 2023 - 05:04 PM

The f/4.9 tube seems like it is already set up pretty good for visual. Does it need an extension for you eps to focus? If not, maybe just flock opposite the focuser and make a Dob mount for it. Then you won't need a rotating UTA.

 

Getting that 7" down on the f/4 sounds worthwhile. However, you might want to try a simple experiment with a piece of black construction paper first. Cut out a circle of 76mm or so and put a hole in the center that can fit over your diagonal holder stem. Put the scope on Jupiter at high power. Then look at the view. Place the circle on the stem to increase your obstruction 9% and see if you notice any difference. The theory of improved contrast is not always apparent visually in changes of less than 10%-15%. If you do see a difference, then it might make changing the 70mm flat down to 61 or whatever worth it for you. Cutting the tube, turning it around for extra light baffle, and joining it with trusses to cut down on that 7" extension all seem like good ideas. Then making a Dob mount for the f/4 would again eliminate the need for rotating UTA.

 

I would think you might already have an idea about the quality of your mirrors and decided they are keepers. If you are on the fence, try a star test or viewing Jupiter to refresh your opinions.


  • Echolight likes this

#25 Echolight

Echolight

    Voyager 1

  • *****
  • topic starter
  • Posts: 12,103
  • Joined: 01 May 2020
  • Loc: Texas

Posted 05 October 2023 - 06:49 PM

The f/4.9 tube seems like it is already set up pretty good for visual. Does it need an extension for you eps to focus? If not, maybe just flock opposite the focuser and make a Dob mount for it. Then you won't need a rotating UTA.

 

Getting that 7" down on the f/4 sounds worthwhile. However, you might want to try a simple experiment with a piece of black construction paper first. Cut out a circle of 76mm or so and put a hole in the center that can fit over your diagonal holder stem. Put the scope on Jupiter at high power. Then look at the view. Place the circle on the stem to increase your obstruction 9% and see if you notice any difference. The theory of improved contrast is not always apparent visually in changes of less than 10%-15%. If you do see a difference, then it might make changing the 70mm flat down to 61 or whatever worth it for you. Cutting the tube, turning it around for extra light baffle, and joining it with trusses to cut down on that 7" extension all seem like good ideas. Then making a Dob mount for the f/4 would again eliminate the need for rotating UTA.

 

I would think you might already have an idea about the quality of your mirrors and decided they are keepers. If you are on the fence, try a star test or viewing Jupiter to refresh your opinions.

Thanks for the thoughts and ideas.

 

The f4.9 is a little better about not being totally designed as an astrograph. But still not quite optimum for visual. The focal plane is around 5 inches outside the tube I believe.

 

The idea of slipping one tube over the other for a rotating top end is still in the calculations stage. 

I'd have to cut 2 or 3 inches off of the f4 tube to start. To get the primary around 4.5 inches farther away from the secondary.

I'd probably have to flip the f4 tube upside down. To keep from having to cut across the top of the focuser hole and weakening the tube.

But with a four or five inch overlap, it could be too top heavy. And I would of course be more limited as to how high I could put the rings for balance. Although I think there'd be enough room for balancing.

 

But the idea is still super intriguing to me.

I found some 3 inch wide, one side sticky, teflon tape that would make a nice buffer between the two tubes.

 

I do have a bit of hesitation. Mainly because of the added weight and the balance issue.

It'd be nice if they were both a little lighter.

 

Definitely a dob mount for each is a strong possibility. As high power with the Unistar mount isn't really ideal with either one. And I don't plan on buying  DM6 and pier extension anytime soon.

Probably just use whatever materials I can scrounge up out of the shed... in honor of Dobson himself. grin.gif

Although I did see an article here on Cloudy Nights about a 6" f4.5 dob build I believe, from 2005 or 6, that had a swing through base. Which would be a cool dob base for richfield viewing with the f4 on a short tripod.

And I have just the tripod with a wide stance and wide top. An old Nexstar heavy duty model. Product have to chop it down a few inches for seated viewing.


Edited by Echolight, 05 October 2023 - 07:09 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: Beginner, DIY, ATM, Reflector, Visual, Dob



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics