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

First Light(s) - Tuning a GSO 150mm f/4 Newt for Imaging

  • Please log in to reply
56 replies to this topic

#1 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 26 March 2023 - 04:42 AM

I’ve recently picked up a new Newt to add to my collection… a 6” (150mm) GSO f/4 model. This is to complement my existing 8” (200mm) f/5 Newt.

 

Basic Modifications

(NOTE: Technical details are up front - hopefully some of which may help anyone working with this Newt. Imaging results in post 3 and 4).

 

I started looking at adding a new OTA to my collection in December, when an elbow injury made it very difficult to safely mount the 8” Newt for a while. I ended up sticking to using my small refractor for a few weeks while I recovered, but that got me wondering whether it was worthwhile adding a new imaging OTA to my collection that slotted in between the nimble 72mm refractor (435mm at f/6) and hefty 200mm reflector (1000mm at f/5). I, of course, immediately started looking at 4” triplets, and mentally justifying the need for one, but then I spotted that a local GSO distributor had a “double discount” on their GSO Newts, and I could pick up a 150mm f/4 unit for an astonishingly cheap AU$350 (US$230). Brand new.

 

This seemed like it was worth a shot at that price. I’d paid more for my coma corrector. The focuser, tube rings and dovetail included with the unit added up to more than half that price if bought separately. However, I also knew this unit was going to be a bit of a project. I’d spoken to a local reseller about it previously, and despite being a huge GSO fan, he’d openly described the 6” unit as “challenging”. I figured I was up for that challenge, particularly when the cost of getting it wrong and making a total mess of things was minimal. I’d enjoyed tuning my 8” Newt, and was satisfied with the results. Worst case, I’d have a spare dovetail, focuser and finderscope, and learn something along the way.

 

Newt.jpg
Figure 1: Product image of the GSO Newt. Note how far the focuser is down the tube on the new model!

 

So I placed the order and got ready to get to work on it. I read Gianluca Rossi’s article on tuning his GSO Newt, but noted it was an older model, with a shorter tube and the old-style GSO secondary holder. Some of his issues may not apply here. I read bokemon post on the issues with the mirror cell, and started noting down what I’d copy from his ideas.

 

The unit arrived, and I immediately went open it up, just to loosen off the primary mirror clips. They’re always tight on Newts when shipped, but these were tighter than I thought was possible. Tight enough to prompt me to do a web search for “can overly-tight primary mirror clips damage a mirror?”. Relieved and reassured by the results from that search, I went to reassemble it.

 

Reassembling it was easier said than done. The primary mirror cell for the 6” Newt is entirely unlike the 8” unit, and has some “interesting” design choices. Unlike the 8” unit, which has collimation bolts, the 6” cell has collimation nuts. To remove the mirror, you loosen off these nuts, then pull out the mirror, which is simply sitting on a thin backing plate, like this:

 

Backplate-stock.jpg

Figure 2: The mirror backplate. A really simple design.

 

Once you want to re-insert the mirror “cell”, you push it, mirror and all onto some bolts that protrude from the rear of the tube. Hopefully avoiding scuffing the mirror surface on these bolts in the process. You have a millimetre or two around the edge of the mirror to avoid scraping it. Also, not shown in the image below, there are springs that operate as the “push” of the push-pull alignment mechanism. There is so little clearance for these that they scrape on the side of the mirror as you insert it. During my tuning of this OTA, I’ve had the mirror on and off a dozen times, and those bolts worry me every time. I’ve taken to mentally calling these bolts “the spikes of doom”.

 

Bolts1.jpg

Figure 3: The spikes of doom.

 

Really, this is a bizarre design choice. Inserting the mirror into this OTA is the closest I’ve ever felt to Indiana Jones avoiding deviously-designed hazards… warily avoiding spikes and springs, ready to trap me and impale me (or the mirror) if I lose concentration for a second. It’s like a game of Operation except the sound of failure isn’t a buzzing, it’s the sound of your mirror coating being scraped away. But so far, I’ve successfully navigated this hazard.

 

Once the backing plate is on, it’s all fine. Except the backing plate has 7mm holes. Yet the bolts are 5mm. So there’s a huge amount of play and scope for lateral movement:

 

M5-7mm.jpg
Figure 4: Not exactly fitting like a glove.

 

Tightening each collimation bolt ends up moving the centre-spot in an arc as the mirror shifts laterally while the bolts are tightened. Dialling in perfect alignment is an exercise in frustration. So the mirror cell came off again, and I followed the tip from bokemon’s post above of adding sleeve washers to bring the holes down to 5mm. Except due to machining tolerances in the backplate, I could only do this on two of the three holes if I wanted to get the plate back on again (so, for the third, I stuck on a plain nylon washer to equalise heights). This was sufficient to stop lateral movement and make collimation straightforward.

 

I also inspected the cell for other issues while doing this. I noticed when rotating the mirror in its cell that it was scraping on the backplate. The supports for the mirror clips had some rough edges that meant they made contact with the mirror from the underside, rather than it simply being supported by its cork pads. This small amount of contact probably wouldn’t cause any issues with astigmatism, but I didn’t like the sound of it. So I propped up the mirror slightly by adding a few thin layers of non-stick tape to the cork pads. This resulted in silky smooth rotation of the primary in its cell.

 

I then took the opportunity to replace the comically weak springs on the spikes of doom with new ones. These needed to be very thin to avoid scraping on the mirror’s sides, but rigid enough to actually provide some “push”. I found some fairly rigid ones, but they still felt a bit squishy when installed, and I doubted their ability to hold the mirror perfectly in place without using locking screws (which I loathe). So I replaced the locking screws with some nylon alternatives I could easily cut down to short stubs, and pushed an extra set of springs onto the ends of these. Not ideal (it would be better to have just three strong springs, each located on the bolts supporting the cell), but it does seem to help collimation stability by adding a bit more “push”.

 

BackPlate.jpg
Figure 5: The abused and modified primary cell with sleeve washers and springs

 

I also replaced the secondary holder adjustment screws with thumbscrews. I’d rather not be sticking a screwdriver down the front of an OTA, no matter how cheap it is.

 

In adjusting the secondary, I tightened down the spider vanes as best I could, and noticed the tube starting to deform around the spider vane adjustment nuts. The fact that the spider is so far down from the front end-ring meant that the steel tube didn’t have much support at that location and flexed easily. I added some washers to the spider vane adjustment nuts to help avoid dimpling, but figured I’d need to think of a better solution. This flex didn’t bode well for focuser stability either, but that was a problem for later.

 

Secondary Screws.JPG
Figure 6: I keep some cheap M4 thumbscrews (purchased in bulk from eBay) for replacing the usual Phillips head screws.

 

To round things off, I attached a ZWO EAF to the GSO Dual-Speed Crayford, using BuckeyeStargazer’s excellent 3D-printed attachment.

 

During this process, I’d been assessing collimation stability and doing tests with an artificial star. I wasn’t really happy with the star tests (particularly the astigmatism and turned-down edge that dominated the test), but this Newt was now at least stable and I could collimate it. So it was time for first light.

 

... to be continued.


Edited by AaronH, 26 March 2023 - 05:36 AM.

  • psandelle, elmiko, photomagica and 4 others like this

#2 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 26 March 2023 - 04:45 AM

First Light

First light (i.e. the first clear night since I’d bought the Newt) unfortunately coincided with the full moon, so I decided to choose a target that was away from the worst of the moon glow, relatively bright, and could be used with my dual-narrowband filter. The Tarantula Nebula seemed perfect.

 

I’d ordered a cheap fan to aid in cooling, so I velcroed it to the back of the Newt, ready to go outside and cool down. This fan needed to be a small 80mm model to fit on the tiny backing plate, and the best option I’d found (with quick shipping) seemed a bit flimsy and had no fan grille, but I figured it would do for now, as long as I didn’t do anything silly like stick something into it. As a bonus, it came as a package deal with two for the price of one.

 

So, I set the Newt up on my EQ6-R, let it cool down, and then went to collimate it. Secondary collimation was holding fine after being brought outside, despite the worries about tube flex, but the primary adjustment needed some tweaks. So I reached around the back to the collimation adjustment nuts while keeping my eye on the Cheshire and went to tweak one of the nuts. Then heard a cracking sound and felt a pain in my fingers.

 

After collecting the fan blades that were now scattered around the yard and swearing a bit, I continued collimation. Luckily, the fan was flimsy enough that nothing had been actually damaged, including my fingers. But I made a mental note to order a better fan. And a grille. Meanwhile, I went inside and grabbed the second fan and attached it. Lucky I’d bought two.

 

I inserted my QHY 163c OSC camera and Sharpstar 1x Coma Corrector into the focuser, dialled in rough focus, polar aligned, and was ready to go.

 

It’s worth noting here how far the focus point is out from the side of the tube on this Newt. I ended up moving the focuser to the midpoint of its range, then pulling the coma corrector a full 40mm out of the tube to reach focus. This isn’t ideal in terms of tilt, and it’s certainly not ideal for achieving full illumination from the rather small secondary.

 

Imaging the Tarantula

Regardless, I started the imaging process, and found that the autofocus curve was hardly the “V” shape I’d expected. It was more like a “W”. Had I done something wrong in attaching the ZWO EAF? This was possible, but it had worked fine with the artificial star inside. Was my seeing really that bad tonight? Were the thermals so problematic for this OTA that I was seeing the effects of tube currents? Was this fan just so flimsy it wasn’t helping? Was the secondary, sitting so far down in the tube, sitting in a stew of turbulent air? Something was really off.

 

I collected data anyway, then decided to come back the next night to see if seeing had improved. It hadn’t. So I reversed the fan direction to see if it helped improve the thermals and tried again the next night. No luck.

 

Long story short, after a few nights of investigation, reversing fan directions, and general confusion, I spotted the issue, and it had nothing to do with the OTA. I was using a full-field focus routine in Ekos, where it takes the average HFR from all selected stars. So far, so good, as this generally works for me. However, there are many small, round pockets of nebulosity in the Tarantula Nebula that look a lot like defocused stars when the nebula is in-focus. The focus routine, as it approached focus, would start picking up these pockets of nebulosity as defocused stars, and the average HFR of the full field would increase accordingly. This created the bizarre “W” shaped focus curve. So I switched to single-star focus and finally had a textbook focus curve.

 

However, staring at this single star going in and out of focus revealed just how bad the astigmatism was in this OTA. I’d clearly seen this in my star testing and isolated it to the secondary mirror, but hoped it wouldn’t be such a big deal in practice. It wasn’t really visible with a focused star, but it was severe enough that any defocus at all led to star elongation. Unless I was willing to re-focus constantly, I’d have to do something about it.

 

At this point, I decided to just collect the data I had of the Tarantula Nebula and stack it, to see what I could get out of it. The end result wasn’t terrible when zoomed out:

 

Tarantula_500k.jpg
Figure 7: Tarantula Nebula. 500kB, 1500px.

 

But a full-resolution crop of the stars wasn’t as pleasant. There was still work to do.

 

Tarantula_stars.jpg
Figure 8: Tarantula star crop.

 

... more to come


Edited by AaronH, 26 March 2023 - 04:59 AM.

  • AndreyYa, psandelle, mikewayne3 and 3 others like this

#3 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 26 March 2023 - 04:48 AM

Further Adjustments

I decided the secondary needed to be re-mounted. The astigmatism was just too bad to use it like this even temporarily.

 

So I grabbed an X-ACTO knife and proceeded to cut away the ludicrous amount of adhesive that bonded the secondary mirror to its holder. The mirror was really firmly held down from edge-to-edge, with no allowance for thermal expansion. Once I’d freed it, cleaned it up, and remounted it with three blobs of RTV silicone, the star test was satisfying. Astigmatism was now negligible.

 

I proceeded to do some more testing outside. After being bitten by the Tarantula Nebula, I decided SH2-308 would be a great, somewhat ambitious, choice of target. It was pretty high up in the sky, away from murk, and would allow me to assess whether there was any remaining eccentricity in my star shapes now that the secondary astigmatism had been addressed.

 

As the results rolled in, I could see the astigmatism was definitely gone, but I still wasn’t happy with the erratic FWHM I was seeing, and brighter stars still had some less than pleasing fan-like diffraction patterns. Stars were also more misshapen and eccentric than I’d like. That was the next thing that needed to be addressed. But what to do?

 

Firstly, I flocked the tube to avoid stray light bouncing about. Then I made up a circular baffle from corrugated plastic and flocking material to mask off the mirror edge. I’d done something similar for my 8” Newt, and it had cleaned up the image nicely. By making it wide enough that it just fit inside the tube, it also helped direct airflow across the primary to disrupt the boundary layer. However, installing it had been simple with the 8” Newt. I simply attached it to the mirror clips. This wasn’t possible due to the way the primary cell is constructed on the 6” Newt. The spikes of doom and locking screw plates are in the way. So I ended up needing to affix it to the interior of the tube instead.

 

Baffle.jpg
Figure 9: Handcrafted, artisan baffle.

 

This cleaned up the diffraction artifacts and star shapes somewhat. But the image was still just wrong. FWHM values weren’t what I would expect, the star shapes weren’t as clean as I’d hope. The image wasn’t as bright as I’d expect at f/4 (even allowing for the slight reduction in aperture from the baffle). Only one thing explained all this. The secondary was just too small for the distance to the camera. After all, the focal plane was quite far from the mirror, and even on-axis, it was struggling to illuminate the sensor. This led to decreased illumination and brought diffraction artefacts from the secondary edge into play.

 

The focuser and secondary had to move up the tube. There was no way around it. I did calculations, and found that with my rather long coma corrector, shifting the focuser by 40mm would be ideal. At focus, the end of the coma corrector would be sitting right at the edge of the light path. Not intruding, but as close to doing so as possible.

 

So, all I needed to do was drill four holes to relocate the spider, cut a new hole for the focuser, patch up the old one with a metal plate, and make sure I didn’t destroy the tube in the process. A simple process for a handyman. Unfortunately, I’m not a handyman, I’m a clumsy academic more comfortable with computer bits than drill bits. So I just took it slow, using a whiteboard marker on the tube to draw it all out (I presume that’s why Newtonian tubes so often have a white glossy finish), measuring everything ten times, and being careful with the drill. To cut the hole, I decided a hole saw would be unadvisable on the smooth, rounded surface, so I just drilled a succession of holes where I wanted the new focuser, following the lines I’d drawn on with the marker, then filing out the gaps in between to remove the metal, and finally finishing it all off with a large, semicircular file. The final result may not have been the most professional finish, but all the bits and pieces ended up where I wanted them… to the millimetre.

 

To patch the old focuser hole, I cut a sheet of aluminium to the right size and shape, drilled some holes and painted it. I was really happy with the smooth finish I managed. To attach it, I decided to make use of the left-over RTV silicon in addition to screws, so I ran a bead around the old focuser hole, attached the plate with some screws, clamped it down with a tube ring, and waited for it to set. Unfortunately, my lack of handyman expertise showed through here. The paint felt dry, but it clearly wasn’t, so the tube ring clamp made a mess of it when removed. I begrudgingly resprayed it with the plate still on the tube, with a much less professional finish. But it will do the job.

 

Plate.JPG
Figure 10: After this, I’ve decided not to switch careers to being a metalworker.

 

I also attached some embroidery rings to the tube to add rigidity. This is a trick I also use on my larger Newt. I can’t quantify how much it helps, but it seems to reduce focuser flex on these steel tube Newts.

 

While it was all apart, I blackened the edge of the secondary mirror, and reassembled it all, including adding a good-quality 80mm “maglev” fan to the back.

 

A Dolphin's Head

 

I went back to imaging SH2-308, and was immediately pleased with the results. Stars were rounder, tighter, and illumination was better. The day spent with a whiteboard marker, tape measure and drill was worth it.

 

So I decided to combine the data for SH2-308 and make an image of it. This wasn’t going to be perfect, given the numerous changes made during the imaging process, but I had around 10 hours of data to work with. I removed the stars using StarXTerminator and just used the ones from my final imaging session after the focuser had been relocated. It’s not bad.

 

SH2-308_500k.jpg
Figure 11: SH2-308: 400kB, 1600px. Larger Version

 

.. but it still needs a "proper" test.


Edited by AaronH, 27 March 2023 - 04:39 AM.

  • AndreyYa, geminijk, psandelle and 9 others like this

#4 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 26 March 2023 - 04:52 AM

The Final Test

Now it was time to really test out whether this was all worth it. Had it all really come together?

 

Why not choose a showcase object? So I picked NGC 3372, the Eta Carina Nebula. They don’t get much more showy than that, and it has plenty of detail to pick through. I collected three hours of data, and unlike the previous imaging sessions, there weren’t many frames that needed rejection.

 

I’m happy with the result. Not bad at all for an AU$350 reflector that’s been manhandled by a clumsy amateur.

 

NGC_3372_500k.jpg
Figure 12: NGC 3372, 500kB, 1400px. - Larger Version - Full-Res Version

 

NGC_3372_stars.jpg
Figure 13: Star crop. These seem round enough for me, and there’s plenty of detail to be seen.

 

This image did reveal one issue. The screws for the plate were a bit long and were adding an extra diffraction spike off-axis, but that was easily fixed by replacing them with shorter ones. I also added bigger, grippy M5 nuts to the back plate, much like the thumbscrews on the larger GSO Newts.

 

I also noticed the spider is ever so slightly off-parallel, creating a slight double diffraction spike. But that’s a minor tweak for another day.

 

After all that, I now have a very usable scope with a useful focal length, and that is extremely portable. I learned a lot, improved my handyman skills, and ended up with a credible little Newt for not much money and a few weekends of educational effort.

 

Side1.jpg
Figure 14: Not pretty. But it works!

 

[FIN.... and thanks for indulging me]


Edited by AaronH, 26 March 2023 - 06:43 PM.

  • psandelle, mikewayne3, elmiko and 15 others like this

#5 maxsid

maxsid

    Apollo

  • *****
  • Posts: 1,493
  • Joined: 11 Sep 2018
  • Loc: Sunnyvale, CA

Posted 26 March 2023 - 05:22 AM

That's a lot of effort but very informative/helpful.

Thank you very much!


  • AaronH likes this

#6 mayhem13

mayhem13

    Mercury-Atlas

  • *****
  • Posts: 2,549
  • Joined: 10 Jan 2021
  • Loc: New Jersey

Posted 26 March 2023 - 06:27 AM

Thanks for sharing…..these little cheap scopes CAN be great if one is willing to spend the time/effort to tune them up. The location of the focuser is a compromise in that these scopes can be used with DSLR cameras……one would assume that the entry level astrophotographer likely working on a budget to even consider this scope would be using a DSLR instead of an Astrocam 2,3,4,5,6x the costs of the scope itself.

 

I wonder if removing the entire primary assy including the tube adapter would have made that spike of death thing less anxious as the spiked portion could then be lowered over the mirror with more vision and maneuverability instead of negotiating the heavy mirror blindly?


  • AaronH likes this

#7 Mert

Mert

    Voyager 1

  • *****
  • Posts: 11,108
  • Joined: 31 Aug 2005
  • Loc: Spain

Posted 26 March 2023 - 07:16 AM

At the end the result is very good!
Well done :waytogo:
  • AaronH likes this

#8 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 26 March 2023 - 07:18 AM

Thanks for sharing…..these little cheap scopes CAN be great if one is willing to spend the time/effort to tune them up. The location of the focuser is a compromise in that these scopes can be used with DSLR cameras……one would assume that the entry level astrophotographer likely working on a budget to even consider this scope would be using a DSLR instead of an Astrocam 2,3,4,5,6x the costs of the scope itself.

Agreed. This approach is only for crazy people who derive enjoyment from making things work. Now it's in-shape, I'm kind of feeling lost and wondering what else I can do to improve it.

 

I wonder if removing the entire primary assy including the tube adapter would have made that spike of death thing less anxious as the spiked portion could then be lowered over the mirror with more vision and maneuverability instead of negotiating the heavy mirror blindly?

I read during my research that the nuts that hold the end-ring on aren't captive, and will fall off if you do this. But, you know, despite all the messing about I did, I don't think I actually checked if that was really the case. Next time it's apart, I will.


Edited by AaronH, 26 March 2023 - 07:23 AM.


#9 arbit

arbit

    Apollo

  • -----
  • Posts: 1,365
  • Joined: 19 Feb 2012

Posted 26 March 2023 - 09:39 AM

Thanks for the detailed post.

I have one of these and it's my never used scope because it was so frustrating to use, especially once the coma corrector was installed. Some of the other posts assumed a handyman skill level which is definitely beyond me :-)

So this is now a project for the rainy season!

Btw, did you add the plastic washers to stabilise the secondary mirror collimation screws? Another source if frustration at least for me.

Sent from my SM-S908E using Tapatalk
  • AaronH likes this

#10 ValeRyo

ValeRyo

    Vostok 1

  • -----
  • Posts: 129
  • Joined: 23 Feb 2022

Posted 26 March 2023 - 05:37 PM

Congratulations! This is awesome! I own almost the same scope only that the focuser is in the position shown on the first image, so very close to the opening. I already changed the springs, painted black the side of the secondary and the primary too. I'll go for the nylon bushing for the "spikes of doom".

 

I sometime feel that the image is not properly "detailed" but a bit "soft" on details, do you think that the baffling on the primay can help on this?


  • AaronH likes this

#11 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 26 March 2023 - 05:41 PM

Btw, did you add the plastic washers to stabilise the secondary mirror collimation screws? Another source if frustration at least for me.

The plastic washers under the spider vane nuts are there to stop the tube from dimpling when I tighten them. This allows me to tighten the spider vanes more than I otherwise would, which does help quite a lot with maintaining stable secondary collimation (but don't go ridiculously hard or the tube can deform).

 

I also added a large metal fender washer on top of the secondary holder stalk, right on top of where the recessed holes for the screws are located. This has an interior diameter as small as I could find, and an exterior diameter just large enough to cover the secondary holder stalk, including all three recessed screw holes. This fender washer isn't really there for stabilisation, but mainly so I could rotate the secondary freely to place the reflection of the primary precisely in the middle of the secondary when looking through a sight tube. This ensures there's no light falloff to one edge.


Edited by AaronH, 27 March 2023 - 01:31 AM.


#12 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 26 March 2023 - 06:00 PM

I sometime feel that the image is not properly "detailed" but a bit "soft" on details, do you think that the baffling on the primay can help on this?

 

Mine had that softness until I baffled the primary and moved the focuser. The edges on these cheap mirrors aren't finished perfectly (but the mirrors are otherwise just fine), so my guess is that the light scatter introduced by the edge plays a large part in the soft images and "furry" stars. The effect of the turned down edge on a star test was plain to see when I first tested the Newt.

 

With an undersized secondary, that scatter could be introduced by both the primary and secondary edge. Baffling the primary will help with the former, but not with the latter. For that, you just need to get your coma corrector as close to the secondary as possible, without intruding on the light path. Blackening the secondary edge could help too, but probably only marginally.

 

Definitely try baffling the primary first. I picked up a sheet of corrugated plastic from the hardware store (but any semi-rigid plastic material will do), drew two circles on it  of appropriate diameter with a compass, carefully cut it out with an X-ACTO knife, then covered it with adhesive flocking material (that I'd bought for flocking the tube anyway). Carefully applying the flocking material smoothed off any rough edges on the plastic ring, and the result is satisfactory. It was all done in an hour or two.


Edited by AaronH, 26 March 2023 - 06:58 PM.

  • mayhem13 likes this

#13 arbit

arbit

    Apollo

  • -----
  • Posts: 1,365
  • Joined: 19 Feb 2012

Posted 27 March 2023 - 02:34 AM

The plastic washers under the spider vane nuts are there to stop the tube from dimpling when I tighten them. This allows me to tighten the spider vanes more than I otherwise would, which does help quite a lot with maintaining stable secondary collimation (but don't go ridiculously hard or the tube can deform).

 

I also added a large metal fender washer on top of the secondary holder stalk, right on top of where the recessed holes for the screws are located. This has an interior diameter as small as I could find, and an exterior diameter just large enough to cover the secondary holder stalk, including all three recessed screw holes. This fender washer isn't really there for stabilisation, but mainly so I could rotate the secondary freely to place the reflection of the primary precisely in the middle of the secondary when looking through a sight tube. This ensures there's no light falloff to one edge.

Thanks. Also, would you mind sharing how you got the 40mm number for moving the focuser?



#14 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 27 March 2023 - 03:25 AM

Thanks. Also, would you mind sharing how you got the 40mm number for moving the focuser?

 

Indoors, I fully inserted the coma corrector and noted that it protruded about 3mm past the end of the drawtube (GPU-style coma correctors are very long). I then removed it, racked the drawtube in until it started to intrude into the light path (I simply looked through my Cheshire eyepiece to see this). Then I racked it back out 3mm to account for the coma corrector’s extra length, then another millimetre or two to be safe. I reinserted the coma corrector and looked through it as best I could to see that there was no drawtube intrusion into the light path. I then checked the focuser position (18mm if I recall correctly) to make sure it didn't shift during the next step.

 

I then took the Newt outside, put it on the mount, and pulled the coma corrector (with the rest of my imaging train now attached) out of the focuser until it reached focus. I measured it was 40mm from the coma corrector flange to the focuser. That means it could theoretically move 40mm in and not intrude on the light path when focused.

 

I then did a few experiments, setting the coma corrector at exactly 40mm out (using a 2" parfocal ring) and checking an autofocus routine landed at 18mm. Then doing the same with it set at 30mm out (it landed at 28mm). Essentially just making sure that no matter how I sliced it, or how I measured it, the focus point could travel in by 40mm while not exceeding that 18mm mark on the focuser where it was safely outside the light path.

 

Of course, this means that it's tuned for the specific GPU-style coma corrector I'm using. If I was to swap to another model, it may not be optimal. But I'm happy with my coma corrector (it cost me significantly more than this Newt!), so I'm satisfied with this tradeoff if it maximises the illumination of the secondary.


Edited by AaronH, 27 March 2023 - 07:54 AM.

  • mayhem13 likes this

#15 Dunc

Dunc

    Explorer 1

  • -----
  • Posts: 96
  • Joined: 02 Oct 2015
  • Loc: Manche, France

Posted 27 March 2023 - 04:56 AM

Great write up. bow.gif  Love your images, you've got a great choice of targets down there! I hope others have the same success as you improving their scopes.

 

I thought I quickly share my experiences in this area (don't want to hijack your thread).

 

I've got a TS1506UNC 6" f4 newt from Teleskop Service. It's almost identical to your scope except it's black and cost 3x as much (me muggins?). It's got exactly the same primary holder with it's peculiarities and a GSO mirror and a very similar if not exactly the same  secondary holder which is way down inside the carbon fibre tube. There is quite lot on CN about the primary holder frown.gif

 

So far I've printed an edge baffle for the primary, placed thin nylon sheets under the mirror at 1/3 distance replacing the cork so the mirror can expand with less impediment and shimmed the edge of the mirror without pressure so it can't slide up to 0.5mm sideways.

 

I've also 3d printed bushes for the holes the collimation bolts go through. These act to stabilise the mirror holder, act as washers for the knobs and centre the springs which stops them fouling the threads on the bolts. I glued the bolts in as they were prone to becoming loose in the badly cut threads allowing them to go off square. Stronger springs as well.

 

Back of the primary/scope is covered with a steel bin lid which is flocked inside and has a dew heater around the rim as well as well. Cooling down is not a problem where I live and the scope is always at ambient. The scope has been repainted inside with Matt Black Krylon.

 

I've also rebuilt the focuser (v-Power) as the drawtube was not square in the body, I thought initially it was the whole focuser.

 

Next I've got to address the secondary holder as it rotates when the knobs are adjusted. I'll try the fender washer over the knob holes trick, I've put a stronger centre spring as well.

 

Duncan


  • mayhem13 and AaronH like this

#16 elmiko

elmiko

    Fly Me to the Moon

  • *****
  • Posts: 6,855
  • Joined: 27 Jul 2009
  • Loc: Arizona

Posted 27 March 2023 - 10:49 AM

Excellent job Aaron! That's a hell of a scope now! It's producing images like a scope many times more expensive!


  • AaronH likes this

#17 Mike in Rancho

Mike in Rancho

    Mercury-Atlas

  • *****
  • Posts: 2,847
  • Joined: 15 Oct 2020
  • Loc: Alta Loma, CA

Posted 28 March 2023 - 02:13 AM

Nice work!  bow.gif

 

I use the Orion 6 which is the same, except Orion adds a metal reinforcement plate inside the tube under the focuser, and has a 13-inch bar so the OTA rings straddle the focuser.  And the front ring in fact sits ahead of the spider vane knobs.

 

From trying to remove the primary "cell" in a more normal fashion for my very first mirror cleaning, I can in fact confirm that a non-captive nut will fall out.  Clink.  Rattle rattle.  Oh oh.  So you have to then pull the backplate out past the spikes of doom, find the loose nut, and then use a thin needle nose or something to to hold it while you tighten the screw back on.  Unless you can get it to just sit there in place until the screw grabs.

 

I've thrown a decent number of mods at mine for imaging, but you have some nice ideas too.  I do use the lock screws, but some properly-sized nylon inserts are still probably a good idea for the gap around the spikes, and maybe new springs as well.  Re-mounting the secondary might be useful also.  I did do the big metal washer mod for that tri-legged dimpled Y shape of the secondary holder, along with Bob's knobs.  For proper smooth motion, plastic disks cut from a water jug (called the milk jug mod in the reflectors forum) too.  I also used the same plastic to create a short cylindrical sheath to prevent the secondary spring from falling askew and snagging on the washer.

 

It's a pretty fun scope.  The focal plane is way out there, but the drawtube has 50mm of movement, and I wouldn't want it sticking into the light path.  Considering I hang a 2600, 7x2 EFW, and Sharpstar CC off of it, I don't think the stock focuser is too bad at all.  Except for GSO's all-in-one drawtube and visual back, preventing threaded connections or even a click-lock.

 

Just never use a Baader MPCC with this thing, unless you have no alternative.  The difference between that and a good CC is quite striking for both star shape and center-image details.


  • ntph, mayhem13 and AaronH like this

#18 ValeRyo

ValeRyo

    Vostok 1

  • -----
  • Posts: 129
  • Joined: 23 Feb 2022

Posted 28 March 2023 - 03:12 AM

Mine had that softness until I baffled the primary and moved the focuser. The edges on these cheap mirrors aren't finished perfectly (but the mirrors are otherwise just fine), so my guess is that the light scatter introduced by the edge plays a large part in the soft images and "furry" stars. The effect of the turned down edge on a star test was plain to see when I first tested the Newt.

 

With an undersized secondary, that scatter could be introduced by both the primary and secondary edge. Baffling the primary will help with the former, but not with the latter. For that, you just need to get your coma corrector as close to the secondary as possible, without intruding on the light path. Blackening the secondary edge could help too, but probably only marginally.

 

Definitely try baffling the primary first. I picked up a sheet of corrugated plastic from the hardware store (but any semi-rigid plastic material will do), drew two circles on it  of appropriate diameter with a compass, carefully cut it out with an X-ACTO knife, then covered it with adhesive flocking material (that I'd bought for flocking the tube anyway). Carefully applying the flocking material smoothed off any rough edges on the plastic ring, and the result is satisfactory. It was all done in an hour or two.

 

Can I ask you exactly which size did you realize the primary baffle? inner and outer diameter, thanks!



#19 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 28 March 2023 - 03:38 AM

Can I ask you exactly which size did you realize the primary baffle? inner and outer diameter, thanks!

I actually can't remember! I was eager to get it done, and didn't take any notes.

 

I'm a bit reluctant to pull it out and measure it now, since it's very safely taped in (using a mix of foam double-sided tape and flocking material).

 

However, I can remember the process of creating it. I measured the diameter of the tube as best I could and used that as the outside diameter. For the inside diameter, I just wanted to mask off the outside few millimetres of the mirror, so I recall settling on ~144mm.  Once I drew the circles, the ring was much thinner than expected, but it cut out fine.

 

Yet by the time I'd finished applying the flocking material to the ring, the inside diameter ended up a bit smaller than expected. I hadn't really accounted for the thickness of the flocking material itself. The finished baffle now just barely covers the mirror clips when looking straight down the tube. I had actually intended to mask off just the mirror's edge and not quite fully cover the mirror clips (to maximise aperture). But the result is good, so I'll pretend I intended it to cover the clips all along.

 

So if you can end up with a baffle that just barely covers the mirror clips, and has an outside diameter roughly the same size as the tube, you'll be fine. If in doubt, you could just make a test version with cardboard first, overlay it on the mirror cell to see how it compares to the clips, then see how it fits into the tube. Just don't forget to account for the thickness of the flocking material if you're using it.


Edited by AaronH, 28 March 2023 - 04:59 AM.

  • mayhem13 likes this

#20 ValeRyo

ValeRyo

    Vostok 1

  • -----
  • Posts: 129
  • Joined: 23 Feb 2022

Posted 28 March 2023 - 05:30 PM

What do you think about this as baffle / improved primary retain? can it be a good solution for reducing image "softness"?

 

https://www.thingive...m/thing:5024632



#21 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 28 March 2023 - 05:46 PM

What do you think about this as baffle / improved primary retain? can it be a good solution for reducing image "softness"?

 

https://www.thingive...m/thing:5024632

That looks like an elegant solution (provided it is made of soft material that isn’t going to scuff the glass). Certainly more professional than my handmade solution. It should also protect the mirror somewhat when inserting it.

 

The only advantage offered by a “full” baffle that reaches the edge of the tube is that it can help divert some of the airflow from a rear fan across the mirror to disrupt the boundary layer. But that’s probably only a marginal benefit with such a small mirror.

 

If you go for the 3D printed option, let us know how it goes. It looks tempting, purely from the perspective of protecting the mirror, let alone the benefits in optimally masking the edge.


Edited by AaronH, 28 March 2023 - 05:54 PM.


#22 ValeRyo

ValeRyo

    Vostok 1

  • -----
  • Posts: 129
  • Joined: 23 Feb 2022

Posted 28 March 2023 - 05:57 PM

ok, now I only have to beg my brother in law for printing it then... lol.gif lol.gif lol.gif



#23 TareqPhoto

TareqPhoto

    Cosmos

  • *****
  • Posts: 9,989
  • Joined: 20 Feb 2017
  • Loc: Ajman - UAE

Posted 01 April 2023 - 08:17 AM

I will never "Frankenstein" my GSO 6" F/4 that way or that much, but i will accept adding things to it to make it great, i will never drill or hole anything in all of my scopes, i prefer to have them with issues than doing some surgery to make them perform, pity why the manufacturers never polishing them enough to be ready and great for use without any modification or tuning, but i will see what i can do because the main issue with my GSO for now is kinda a "Blocked" collimation.  



#24 AaronH

AaronH

    Viking 1

  • *****
  • topic starter
  • Posts: 863
  • Joined: 20 Apr 2021
  • Loc: NSW, Australia

Posted 01 April 2023 - 07:50 PM

I will never "Frankenstein" my GSO 6" F/4 that way or that much, but i will accept adding things to it to make it great, i will never drill or hole anything in all of my scopes, i prefer to have them with issues than doing some surgery to make them perform, pity why the manufacturers never polishing them enough to be ready and great for use without any modification or tuning, ...

 

Totally understandable. I remember how tentative I was when I first drilled a hole in my 8" Newt for its secondary dew heater wire, I was having heart palpitations.  However, I guess once I got past that, I stopped worrying, and bought this OTA with the full intention of "Frankensteining" it if necessary.

 

It's a shame that was needed to get the most from it, but I can totally understand why this Newt was built like this. It was to allow plenty of in-travel for DSLR users. Unfortunately, that makes it less than optimal when using a coma corrector and cooled camera.

 

The fact that the spider and focuser are so far down the tube is curious, though. It's almost as if they intended to allow users to do this kind of mod. The previous version didn't have this, and users ended up needing to do things like add extra tube to the back end of the OTA.

 

 

 ... but i will see what i can do because the main issue with my GSO for now is kinda a "Blocked" collimation.

 

What is the "blocked" collimation? Perhaps you could make a post about this and we could try to assist?


  • elmiko likes this

#25 arbit

arbit

    Apollo

  • -----
  • Posts: 1,365
  • Joined: 19 Feb 2012

Posted 01 April 2023 - 08:44 PM

I will never "Frankenstein" my GSO 6" F/4 that way or that much, but i will accept adding things to it to make it great, i will never drill or hole anything in all of my scopes, i prefer to have them with issues than doing some surgery to make them perform, pity why the manufacturers never polishing them enough to be ready and great for use without any modification or tuning, but i will see what i can do because the main issue with my GSO for now is kinda a "Blocked" collimation.

I dont know if "Frankensteining" is the right word.

The GSO 6-inch F4 is very poor mechanically, and I never really used mine because of this. The GSO CC actually increases focal length (to 687mm in my case), so the imaging train is an extra 87 mm out. And at low latitudes because the focuser is so far back from the front, the train has to be on top when mounted, which is a pain from the torque and balancing pov.

If it were possible, it would have been sold by now.

It's actually encouraging to see these kinds of mods working, and personally am looking forward to trying it out during the rains.

Of course, this is fine as a Newt is a simple design - doing this on a triplet or an RC would be a different story

Sent from my SM-S908E using Tapatalk
  • AaronH likes this


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






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics