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| Has anyone been successful in getting a descent set of round stars with a F5 GSO Newt? Ever since I purchased it I simply cant get round stars. I get tails everytime. Its no guiding or PA or Collimation. I have also unpinched primary. The only thing left is the secondary? Anyone who has specific experience with GSO Newts will know what Im talking about! |
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| Are you using a coma corrector? |
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| I was going through ask the same question as Kevin. Having switched from an SCT to a GSO Newt chasing round stars became an impossibility. I masked my primary, completely redesigned it's mounting in case there was pinching, collimated it to within microns, but all of this didn't completely solve it on my GSO 8" F4. It wasn't until I did an experiment with removing the GSO coma corrector that it appeared that this as the culprit. I've dialled out as much as I can and tried to remove any pinching there, which has helped some, but I think I'll be buying a different CC, probably a Sharpstar 1x |
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| I've done a lot with my fast newt. Still not perfect. My guess without seeing any images is either tilt somewhere in the imaging train or collimation off center in secondary from rotation or tilt (which can't be seen with a centre laser). You don't mention how you have done your collimation or ruled out mount issues. Assume center stars are prob better than corners. Have you also checked mechanical alignment (focuser centered and square, secondary centered, etc?) |
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They all took the words right out of my mouth. Coma corrector is first on my list, assuming you're not using one. I use the baader mark III multipurpose CC. beautiful stars edge to edge. Orthogonality of the focuser would be next. And of course, a good collimation tool, like "catseye". But you already said you've got that covered. My coulter f4.5 primary was very astigmatic and poorly ground. I ended up having to have it "touched up" by a professional optician. Hopefully you won't have to do that, but it made all the difference in the world once the strehl ratio went from 40 to 85! Stars are round but even then, my focus is soft. Like Matt said, never really perfect. Newts are a great tool and it's all I image with 99% of the time. But man, they have their quirks! Good luck! Let us know what you find. |
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Can you give more specifics about your newt? Got that it’s an f5 but what size and image scale are you working with? Is it a 6 or 8”??? I’m also going to suggest posting some pictures for us to look at as well as @Matt Balkham has mentioned (without seeing any images), this will help us diagnose and offer suggestions based on your image/images. This said here is my offered suggestions to look at based off my experience with newts from 6” - 12”. I have my 12” DBA Certified Newt that started out with all GSO parts. Everything in one way or another has been either replaced or modified. There are many things as many have mentioned above but I’m going to put an emphasis on collimation as well a good CC corrector. I highly recommend the Sky Watcher Quattro CC! https://www.skywatcherusa.com/products/sky-watcher-quattro-coma-corrector Its not cheap, but its the best one I’ve found for my setup. Another one that I’ve not tried but many I’ve read like a lot is the Starzona Apex but its a CC/FR so if your not looking to do any reducing (which I was not) then I’d stay with the Sky Watcher Quattro. Second is collimation. You say you’re in collimation and I’m assuming you have done some sort of analysis but what is your procedure and how are you determining you’re in good collimation? I used to say the same thing when I was having some of my personal issues as well but when I found out about “MetaGuide” I completely had my thoughts thrown under the table. It is a free software mainly for guiding like PHD2 but it has an excellent “collimation” tool in there that uses the airy disk to collimate instead of a defraction pattern like we use for visual inspection when we collimate on the monitor or PC. If your interested here is the link: https://smallstarspot.com/metaguide/ I will say that there is a learning curve to the software but once you get it figured out you will be very happy you did, believe me! There are many other things that will affect your stars as well, but my next thing I’m going to ask is how are you guiding? Are you piggybacking a guide scope or using an OAG? Again if we new your image scale/focal length of your setup this could change depending on the size of you scope. Im going to guess your using either a 6 or 8” based off your mount you have listed as I wouldn’t put anything over an 8” because of the capacity rating. While I used to use an 80mm ed piggybacked guide-scope with excellent guiding results I ended up going with an OAG for my setup. If your using a piggybacked guide scope, my recommendation would be to not use it on anything larger than a 10”. Dale |
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Sounds like the folks here got it covered! All I can suggest is what's already been covered really. Collimation is king. Assuming the use of a CC, poor/mediocre collimation will mask many other problems (mechanical and optical) that can lead to poor star shapes. I use the catseye cheshire/autocollimator combination for collimation (pretty much the gold standard). I can also testify to the skywatcher CC. I used that for a couple of years before upgrading to the Televue Paracorr II and it was great aside from no option for a threaded connection (compression rings are bad for introducing tilt). I've had a skywatcher 10" F4 quattro for some time now and have made many modifications to it over the years to bring it up to the standard needed for a fast newt so I'm was no stranger to some funky stars in the early days of owning a fast newt. Yes show some pics! Always helps. |
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| I hope i can get my GSO 6" F/4 collimated, because it is bad, i used same collimators i have for my Meade 8" F/5 but it didn't work, the problem is with the secondary mirror, while my 8" F/5 is nice collimated, but i didn't use it much yet due to other issues, and it has been really so long years now without using it and my 6" is still sleeping, so i am not sure what i can do with my Newt, i bought more refractors and last year i bought 10" RC, which means those Newt might see less lights, i won't give them up really, and i wish to find a solution for my 6" F/4 mainly, but even few tests i did with 8" before i never get round stars as well, and i ask myself, should i buy another collimators, should i buy TV Paracorr [i have Skywatcher Aplanatic F4 CC], i changed the focuser on 8" F/5. |
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Hopefully you won't have to do that, but it made all the difference in the world once the strehl ratio went from 40 to 85! Stars are round but even then, my focus is soft. Hey Tom, May I ask how you've measured your strehl? Cheers Mike |
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Hey Mike! Actually, the optician who reground the glass ran a battery of tests before and after he refigured it and measured the wavefront at that time. This was back in 2007 at a time when master opticians were vying for a place in the large aperture dobsonian world, before China was really making a big splash. Now, he's out of business and there are only a few like him left. Most will not even touch my glass because it is not annealed correctly and it would not be worth the time. The cost would almost be a new mirror! But there are a few companies who still may risk grinding glass that isn't theirs! Kind regards, Tom |
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Tareq Abdulla: Hey, Tareq. Sorry about that secondary! I know that mass produced secondaries are often a problem. I don't know how it affects star shapes, though. I just replaced my secondary from my 1980 Coulter 13" and it is tremendous, but costly (I could have bought three ZWO EAF's, haha!). It is an Ostahowski Optics handmade secondary and he takes his time making sure they are really flat. The coulter wasn't too bad, but undersized for photography. But it's taken me forty years to get to a point where I could buy it, haha! There are good secondaries out there: just make sure they are tested and whoever sold it to you will give you the numbers. Kind regards, Tom |
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Tom Marsala:Tareq Abdulla: I can find or get secondary mirrors higher or better quality but much cheaper than your Ostahowski one, in fact i found one brand they make high quality amazing secondary mirrors at whatever wavefront i need and size according to Newt design, but i am not sure if the problem is from the secondary mirror alone or whole system i mean the secondary and knobs and vanes, i can't assume that even primary is a problem but it isn't, so i will decide sooner or later if i will replace the secondary to a high quality one or another mass production one but better design one. |
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Hi all and thanks for reply. Its a 6 inch F5 GSO Newt. I tried with and without coma corrector. THe GSO coma corrector is not good. but even without the centre stars are still not round. I use OAG on a heq5 belt moded. I use a cheshire to colimate and also a lsaer to "check" here are some pictures of the results indicating the "tails". One out of focus to see the disk, the other focused.  |
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Coolhandjo: In the defocused star image, there is a very prominent gap at about the 8 o'clock position. Is there some sort of obstruction in the optical path (other than the spider of course). The secondary shadow doesn't appear to be well contered indicating possible collimation. But if the secondary is offset, then the secondary shadow will also be offset. |
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Jacob Heppell:Coolhandjo: *** Thanks for your response. All I can think it was the focuser draw tube but I thought i cleared that when I looked down *** |
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*** Thanks for your response. All I can think it was the focuser draw tube but I thought i cleared that when I looked down *** The draw tube shouldn't go that deep to the center. It looks more like a twisted spider arm for me. Cheers Mike |
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*** Thanks for your response. All I can think it was the focuser draw tube but I thought i cleared that when I looked down *** ***you know i had a dew heater strapped to one arm very thin wire. In my previous newt it made no difference to the image - but im beginning to think that might be causing some issues. So I have taken it off and will test. Thanks for your response. *** |
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Coolhandjo:*** Thanks for your response. All I can think it was the focuser draw tube but I thought i cleared that when I looked down *** It may cause some issues depending on how thick the wire is. I have a mate who bought a second hand scope that had an old plug adapter inside the tube that was protruding into the light path. It created a large flare emanating from the stars. Mirror clips also generate a lot of diffraction. I replaced mine with an aperture mask. I also recently replaced my 1.2mm thick wonky stock spider with 0.5mm thick carbon fibre spider, which also cut down diffraction noticeably. |
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Tom Marsala:Tareq Abdulla: Slightly off topic but for anyone looking for high end secondary’s I highly recommend Antares Optical out of Rochester New York. I always used there secondaries in my Certified line of Newts. In all cases you get an interferometer sheet with the P-V rating. https://antaresoptics.com/ Dale |
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Coolhandjo: I responded to this yesterday but for some reason it never went thru and didn’t get posted, but here is my take on your situation. 1st, You defiantly have collimation issues. Not only can you see it in the defocused image you can also see it in your focused image. This definitely needs to be addresses. 2nd, I also believe you have a pinched primary mirror. If you look at your defocused stars you can see they are slightly 3 lobbed which indicates a pinched primary mirror. 3rd, defiantly have coma and tilt. 4th, as @Jacob Heppell has pointed out a possible obstruction in the light path. So all of this will make a huge difference in your stars. The way I would approach this to correct it is as follows. 1) Remove the primary mirror cell and make an adjustment to the mirror clips. If you have any kind of adhesive tape holding the primary mirror to the cell I would absolutely remove it. This alone will give you issues! While you have the cell out I’d check to see how close your center spot is on the primary mirror and correct it if needed. 2) Once you get this done move to your initial collimation. When I have my ota’s off the mount working on them, I’ll do what I call bench collimation and make sure thats as good as I can get it. Tools range from a Cheshire to an auto collimator. Once thats done mount it and check collimation on the mount as it will change. Make the necessary adjustments. 3) Next get yourself a good coma corrector. My suggestion was posted above so I’ll stay with the Quattro because I use it and it’s done an excellent job for me. No matter what you did before this point if you don’t have a good CC you I will not see a whole lot of difference around the edges. Maybe the center part of the field. Back spacing will be important as well. My setup was 55mm and I added 1mm to compensate for a filter because I always shoot with one in place even if its a UV-IR Cut. 4) I would check to see about the obstruction in the light path. If there is something that can be done easily I’d suggest doing so. Ultimately as @Jacob Heppell said, you don’t want any obstruction in the light path if possible. Optional, If you really want to clean up your stars the next thing that I’d recommend is a mask to cover the mirror clips that hold the mirror in place. Its most noticeable in longer exposures but it will give you a much cleaner appearance for your stars. Hope this helps and good luck! Keep us informed on the progress! Dale |
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Dale Penkala:Tom Marsala:Tareq Abdulla: I know this site, i was going to order from them when i was planning to build a 20" dobsonian [and still in this plan hopefully soon in future], but i can't tell about this minor axis size i should choose and which P-V rating, i mean is it really necessary to go all the way to the maximum 1/30 PV one. |
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Tareq Abdulla:Dale Penkala:Tom Marsala:Tareq Abdulla: Hello Tareq, When I built my certified newts this always the company I bought my mirrors from. It really comes down to what you have to spend honestly, but first it’s been my experience that even the lowest P-V rating is significantly better than any stock mass manufactured scopes secondaries. I always put in the lowest wavefront secondary unless the customer wanted a better one. In most cases people would upgrade to the 1/16 - 1/20th wave. In my case for visual I went with 1/30th quartz but that was just me and my personal scope. When I upgraded my 12” I went with 1/15th for a couple of reasons, 1st cost obviously, second and probably more common sense related here in Michigan our seeing very rarely gets to 2” of seeing so really unless you have excellent seeing conditions your wasting your money at least in my eyes anyway so thats why I did what I did. Bottom line is if you bought their lowest wavefront secondary to replace a stock GSO, Syntha, etc… you will certainly have a much better secondary mirror. I just can’t recommend them enough. Thats why I wanted to post this for others. Not many know that they do secondary mirrors for the astronomical community. If memory serves me correctly they do a lot of government work. As for your 20” dob, what is the Strehl Ratio of your primary? If you really want to pull the best images from a primary with a high strehl then I’d move for something closer to the 1/18th wave but remember seeing will most likely be your limiting factor. Dale |
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Dale Penkala:Tareq Abdulla:Dale Penkala:Tom Marsala:Tareq Abdulla: Hello Dale, This is a big topic really, i watched this site for a while and i know they are making high end secondary mirrors and even lowest one is better than all mass production ones we are using no doubt, but i didn't upgrade as i had another issues with my Newt, once i fix those which i did to some points then i can think about upgrading the secondary or even primary mirrors, but those are for imaging so primary is doing fine for now. Regarding 20" dob, i still don't know anything, because i am not sure i will build or i buy one ready cheapest i can find around, i once talked with one guy who told me that he can build a structure only for me for that 20" design at very very low cost but good materials and quality, i am not after high end materials and design to pay fortune even for structure alone, and for mirrors i was planning to go with Antares optics secondary mirror and something like Zambuto Quartz primary mirror although bloody expensive, but at least i got something or started, now i feel like i will just buy any 20" dob i can find the cheapest even as mass production and then i replace parts including mirrors by myself later, pity that Europe and USA dobs designs are very expensive, the guy that talked with me about the design is Asian, so i assume i have to look at Asia craftsmanship who can design one for me at cheapest it can be with enough proper quality at least, i wish to make a Newt 20" so i can use it with EQ mount rather than AZ/Alt or EQ platform, but well, i also think about visual, that dob is actually mainly for planetary imaging, i don't want to use 11"-16" SCT for that at all, and 20" is another class for both visual and imaging anyway. About seeing condition, build me one 20" and then i will show you what is a nice excellent seeing condition means from here  Tareq |
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Dale Penkala:Tom Marsala:Tareq Abdulla: Yes I replaced my stock secondary with one from Antares Optics. Optically it was great but was way too thick, which meant I couldn't raise it high enough to be properly centered with the focuser (thickness wasn't stated on the website at the time so had no idea!). I've since replaced it with an Orion Optics mirror, which are optically great and a reasonable thickness. |
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Dale Penkala: Great explanation. This is more-or-less what I do when I'm helping local astro mates tune-in their newts. Problem is I'm not very good at explaining things on forums. So when mates in my region are explaining problems and need help I just tell them to come over to my place so I can actually eyeball what's going on as opposed to endlessly messaging back and forth. I also used to have minor problems with a pinched primary. A few years ago I siliconed my primary so I could get rid of the mirror clips but I soon noticed that the primary would pinch on cold nights (I probably didn't do the siliconing right anyway). When I found out about aperture masks, I un-siliconed the mirror and went with that option (see pictures) so now the primary sits freely in the cell and the aperture mask rests ~1-2mm above the mirror (supported by springs) to stop it falling out (also masks out any potential turned edge). Also, I found that the cork pads that support the primary occasionally would stick to the primary. Wiggling the primary would unstick it but it was nonetheless another source of pinching. Some flat nylon pads solved that issue. Newts (particularly fast newts) can be a challenge initially and the need for precision (collimation, centering mirrors/focuser, tilt, good CC, backspacing, focusing, etc) has been a turn-off for a number of people I know personally. But once you know your beast and get everything dialed in, the dividends a fast newt pays are more than worth it and will be a source of enjoyment for years (has been for me!).   |
