TheSkySearchers

I picked up one of these “notorious” short tube reflector telescopes some years ago. The specification is a 114mm diameter mirror, Focal length is f/8.7, or 1,000mm. It has a Barlow lens in the focuser's drawtube. Without the Barlow lens, the primary mirror’s actual focal length is an f/4. People call them “Jones Bird” reflectors but to be fair, Jones Bird is of a different design, with a corrector and not a Barlow in front of the primary mirror or in this case at the bottom of the focuser drawtube. A Barlow does not correct spherical aberration from the spherical mirror, it only increases the focal length.

I had a peek through the eyepiece, it was suffering internal reflection, i.e. diamond-shaped glares on the image when in focus.

As such, I left it alone at the time to contemplate what I want to do with it. I have the option to flex the primary mirror with a screw or push forward the main mirror and do away with the Barlow.

Fast forward to last week, I removed the doublet Barlow lens, darken the lens edge and test the scope again, the internal reflection glares have disappeared, but the outer 40% field of view was fuzzy. I swapped the doublet Barlow lens with either a glass or plastic singlet Barlows, the view improved to around the outer 25% field of view being fuzzy. I then decided to remove the focuser tube and the Barlow, and dipped the eyepiece into the drawtube bracket to achieve focus, and could see sharpness almost to the edge of the field.

Eureka! The decision is simple, the primary is not bad for a spherical mirror, I am going to move the primary mirror forward closer to the secondary mirror to achieve focus. Yes, the central obstruction will increase, but doing away with the Barlow will remove chromatic and its induced spherical aberration and I will have a short-tube rich field telescope equivalent to a 3-inch frac, which I think I will enjoy.

To push the mirror forward I added the spiral paper tube of a masking tape of around 9cm. Cut and glue two circles of hard transparent plastic to the spiral and cover it on both ends, and then glued one end of the paper tube to the mirror and the other end to the black plastic backplate. I put black tape around the primary mirror edge. After testing the now Barlow-less scope with 6mm, 10mm, 20mm, 25mm, and 30mm eyepieces during the day, I am very happy with its performance. Sharpness to the edge for the lower power eyepieces. Slight blurriness around the edge for the 6mm but insignificant for me. I am looking forward to testing my rich field spherical reflector at night.

As such, these 114mm short-tube scope are not a lost cause if you pick up or end up with one.

Clear skies! @@

Bill

Nicely done Bill! Your mods remind me of what I did to an old Tasco 60mm 55VTE; take the click-stop focuser garbage out, and turn it into a nice f/7. Well done on your part

Look forward to hearing your experiences with it and all the best,

Very good modification!

A correctly made Jones-Bird is probably quite decent. These with barlows instead of a true Jones-Bird corrector can hope for no better fate than what you've done.

Great way to rescue a hopeless scope Bill! Curious how it will perform in the field.

Some good modifications Bill, I hope it does well for you.

Thank you for all your interest and positive comments, much appreciated.

There was a gap in the clouds tonight, and I use a 25mm "Plossl" eyepiece (a Dialsight eyepiece to be correct) with a 50-degree field of view to take a sneak preview, as tomorrow is a cloudy day.

Before the cloud covers it, I trained the scope on Orion and saw the Orion nebula, it is always a target of real delight and I like what I saw!

The three stars of Orion's belt managed to fit edge to edge in the field of view. Again the stars are not blurry near the edge, confirming the sharpness of what I saw during the day on a distant building.

I am very happy with its current performance as a rich field scope.

I will keep the scope in its current configuration and secure the mirror properly further now that I am keeping as is and not reverting it back to the original Barlow lens configuration.

I will also not be going down the alternative route in gluing a screw and plate to flex the spherical primary mirror to a parabolic to max out its performance further. It is not necessary for me with this f/4 mirror.

Wishing everyone at SkySearchers a wonderful, peaceful and restful Festive Season and a Happy New Year!

Stay safe and clear skies! @@

Bill

Barnacle wrote: ↑Tue Dec 21, 2021 1:17 amPeople call them “Jones Bird” reflectors but to be fair, Jones Bird is of a different design, with a corrector and not a Barlow in front of the primary mirror.

When I was a young lad on the other astronomy forum, we were discussing ornithology and the infamous Jones Bird, when another gentleman from Australia was kind enough to yell at me for not addressing it properly as a "Barlowed Newtonian." After overcoming my initial shock from being singled out for this egregious offense when everyone else and their mother referred to the thing in question as a Jones Bird, or Bird Jones (Tom's brother) I wondered how one might perform with the Barlow removed from the optical chain. Thanks for exploring this. There are a lot of Celestron 127EQ's out there that I wonder might be improved as deep space scopes buy making this same modification.

Thanks Don,

Celestron Powerseeker 127EQ's are not cheap and plentiful in Australia (i.e. second hand).

There are short-tube Newtonians around from 4.5 - 6 inches from no brand name like mine to well-known brands.

I had a brief experience with a common mass-produced 6 inch (150mm) f/9.3 or 1400mm Barlowed Newtonian (yes, I forgot what these are called, thanks). I must say the image is very soft at the outer 50% field of view.

Thinking back, I do not know if my tactic will work well for this 6 inch unit, as the spherical mirror may be so badly figured that this tactic of pushing the primary forward may still not compensate the bad figuring, or I could be wrong in that perhaps the Barlow lens is the culprit in image degradation all along....

I am still rather pleasantly surprised at how sharp the image turned out in the end with the removal of the Barlow, as it is a fast mirror at f/4. This morning I tested it with an SR4 eyepiece during the day, and the field stop of that eyepiece is the size of a gnat's ar*e, so the image was sharp in that small field of view.

The only way to know how things will turn out with Barlow removal is to test it out with a 5 inch Powerseeker or the 6 inch f=1400mm. This method of pushing the mirror forward is very simple and does not take long to perform. It just took me some years to acquire the knowledge about Barlowed Newtonians, and to think through how to resolve the internal glare issue with this scope of mine, followed by nutting out how to push forward the mirror with minimal fuss and minimal added weight.

If I see a cheap second-hand short-tube or someone is throwing one out (many are upset with their image softness), I like to experiment with it, it will be fun.

Keep safe and clear skies! @@

Thanks and have a great X'mas break,

Bill

Yes I had many of these scopes from 114 to 130 mirrors , I think around 10 or so and only one was OK I also tried to remove the so called corrective lens and moving the primary forward , the image was ok but certainly not great , I just used long bolts
The biggest problem is the primary , you may be lucky but most are rubbish they made a small scope that can fit on a cheap mount/tripod and sell it as a equivalent to a small SCT at half the price or less , I certainly would not recommend it to anyone starting of .
As you mentioned that the so called corrective lens is a simple Barlow I tried one in a spare Barlow housing , after measuring it looked like a 2x barlow but was thicker then the original but managed to fit , I tried it in a 8 inch Dobs and was terrible with a 32mm eyepiece so perhaps not just a simple Barlow , I never bothered after that , but perhaps the housing I used was not correct size , but sure with a cheap scope like these it is just a cheap 2x Barlow , These scope give a bad name to the BJ scopes that were quality items long time ago

Thanks, Richard.

I share my humble thoughts here further.

As you can see from the photos, the "Barlow" on my scope is not thick (I was surprised too when I first removed it), and quite unlike articles I read/seen online from dis-assembled “Jones Bird” where these "Barlows" are thicker as you mentioned. Please see link: http://www.thescruffyastronomer.com/202 ... -bird.html

Therefore, I do not know whether it is because my "Barlow" is not as thick, which makes a difference in the outcome when I moved the primary mirror forward.

As I replaced the doublet Barlow with a singlet Barlow, and now that I am not reverting to the Barlowed configuration, the singlet lens now stays in the disassembled Barlow housing at the moment. The singlet Barlow lens was removed from those shorty plastic Barlow lens housing, and I have replaced it with the doublet "Barlow” removed from this short-tube and the plastic shorty Barlow with the doublet lens worked very well as a Barlow lens on my short tube frac, the modified F30070. viewtopic.php?t=17920

Therefore, to summarise, whether moving the primary mirror forward is worth it in a Barlowed Newtonian is dependent on:
1. the figuring of the primary mirror (but to be fair, a fast f/4 mirror will never be as crisp as its f/8 brethren in high powers).
2. the thickness or construction of the "Barlow" that is housed in these Barlowed Newtonians.

Hopefully someone who can get their hands on many of these Barlowed Newtonians can test them out and let us know here what works and what nots. But in my case, swapping with a singlet Barlow lens reduced the fuzziness on the outer 40% to around 25% with my scope, but again it may just be my case only because I have a non-thick "Barlow" to begin with.

Keep safe and clear skies! @@

Thanks and have a restful Festive break,

Bill

Hi All,

Update:
I did a torch-light test of both my modified 114mm short-tube and 114mm long tube f/8 long-tube reflector and found out the following:

1. By pushing the mirror forward, the secondary mirror chops off the outer part of the light cone, so the secondary mirror acts as an aperture stopper
2. As the aperture has decreased, the f ratio has increased, so the short tube is no longer f/4, so the image is dimmer than it should be and crisper too.
3. I have dipped the eyepiece into the focuser draw tube bracket when I removed the Barlow and draw-tube to see if the primary mirror provides a clear image to the edge of field and established that the primary mirror is a good f/4 mirror. It is a mirror that works better than having the original doublet or my substituted singlet Barlow lens.

Therefore, it is a balance/compromise of losing aperture as I push the mirror forward with my short-tube reflector by removing the Barlow lens, without changing to a larger secondary mirror.

When I have time later, I am going to reduce the push forward height by half and see how I go.

At present, I have sufficient room to reduce the push forward height and still achieve focus with the original rack and pinion focuser.

Happy and Healthy New Year and stay safe.

Clear Skies!@@

Bill

Hi Bill,

Have you thought about installing a different spider for the diagonal mirror? The only question I have if you go that route is, if the diagonal is correctly placed will the hole in the tube for the focuser now become an issue because the hole might need to be enlarged to handle the new light path?

Thanks JayTee,

My el cheapo short tube has the non-removable built-in spider similar to this one:
https://makezine.com/projects/gaze-acro ... telescope/

It is a case of changing the size of the diagonal instead if desired.

The hole for the focuser may need to be enlarged to allow for the light cone to not be cut-off with a larger secondary mirror, subject to:
1. the size of the secondary mirror
2. the location of the secondary from the primary.

I recalled Gary Seronik wrote a nice piece on the right size for the secondary mirror for a Newtonian, please see link:
https://garyseronik.com/sizing-up-the-n ... secondary/

Hope you find Gary's article useful, thanks.

Keep safe and clear skies! @@

Bill

Here is a photo of the "built-in" spider from my el cheapo short tube Newtonian, and with removal of the Barlow Lens with the primary mirror pushed forward.

Hi Bill,

Have you been to this website to determine the accurate numbers based on your design changes?

http://www.catseyecollimation.com/designie5.html

A couple of questions Bill,
1. How did you determine the 9cm distance for the move?
2. How do you collimate the primary mirror now in this configuration?

Thanks for the link, JayeeTee, I have not seen this link before.

Will have a closer look when I have time later. Looks very complex though, maybe beyond me.

Thanks, AstroBee for your questions:

1. The 9 cm was determined by marking the position of focus on the focuser drawtube with a 25mm eyepiece (with the Barlow lens still intact). I then removed the focuser drawtube (with the Barlow) and dipped the eyepiece into the focuser drawtube bracket to see how far inward travel to achieve focus. In my case, it was at least 7 cm, if not more. I allowed a bit more outward travel as the focuser drawtube will allow outward travel to achieve focus, with the added bonus that the focuser drawtube will not block light when achieving focus. On the other hand, if I do not push out the mirror enough, I do not have enough inward travel to achieve focus.

I was not doing any precise calculation or measurement to determine the 9 cm, as the scope was really an el cheapo unit including EQ mount, costed less than AUD $16. It was a project for me to learn about modifying Barlowed scopes, i.e. whether to flex the mirror to parabolic or just use 'as is' as a rich field scope after removing the Barlow. Though Galileo would have given an arm for it if we could time travel and passed it to him. We live in lucky times, with mass-produced "rubbish" scopes which I hope in this case, I can give it a breath of fresh life with these short tube Barlowed Newtonians.

2. As the Barlow lens is now removed from the focuser drawtube, the el cheapo scope is now just a normal Newtonian. Collimation is either with a Cheshire or laser collimator.

Hope that helps.

Keep safe and Clear Skies!@@

Bill

Barnacle wrote: ↑Thu Jan 13, 2022 10:19 pm Will have a closer look when I have time later. Looks very complex though, maybe beyond me.

I have a link to a website with a simpler approach. When I find it I will post it here.

AstroBee, I may have misunderstand your question earlier.

Collimation of the primary mirror is not affected by the push forward, as it is really just an extension of the base platform.

The base platform bracket is still able to be tilted by the 3 adjustment screws at the base of the scope.

Hope that helps.



Stay safe and Clear Skies! @@

Bill

Yes, That makes sense.
Thanks for the clarification.