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I was almost ready to do some real imaging, but a new problem seems to be my last necessary obstacle to overcome. Basically I was about to go outside in my backyard, but then at the doorway, I had a feeling I should first check if I can reach focus; thankfully, I didn't ignore this feeling, and I found out I couldn't get enough inwards focus travel, but I was like an inch away from reaching focus. To @V.M Legary, and others who are interested, I got the UV-IR filter installed, and my Askar OAG; both look high quality. One thing that was exciting was that I was even able to see colors in the defocused star, which I could have never seen without my UV-IR cut. My only complaint (which is not very important) about the OAG is that what holds the camera and camera holder from wriggling off the prism and hitting the ground is a tiny black screw, that is a little tough to tighten maximally with just 2 fingers, but I'm able to do it; 2 screws would've been safer in my opinion. And, I did not buy the filter drawer, because I thought I might get away without it; with a filter drawer, my focusing problem would've actually been worse. This is the "accessory train" and the accessory lengths (in mm) from the tip of my focuser to my sensor, maybe someone could help me cut down on the adapters somehow, or, recommend a more low profile focuser: 0. The length of the focuser, excluding the focuser tube protruding into my newt - about 64mm length 1. Large male thread on the focuser tube itself, and I know it's bigger than M54, but I don't know what size it is; manual doesn't say - 0mm length when connected to No. 2. (I contacted Celestron about the thread size of this "large male thread", but they haven't responded yet.) 2. Focuser to 2 inch eyepiece adapter/holder - about 33mm 3. M48 spacer with 2 inch filter installed, inserted into 2 inch eyepiece holder - 0mm 4. M54 male to M48 female OAG adapter - 0mm 5. M54 OAG - 18mm 7. M54 male to M42 male OAG adapter - 0mm 8. ASI533 M42 black ring - 11mm Hopefully someone can mentally visualize that  If a photo is required for something, I'll take one.So, the total minimum length from the outside of the Newt to my sensor: about 126mm, which is too much for me to reach focus; I would say that needs to be reduced by about 25-40mm. Also, another problem is, if I remove my M48 spacer, I will not be able to use my filter, so then I also need to add in a filter drawer?  I was this 🤏 close from reaching focus!  It looks like this hobby is filled with seemingly unavoidable and perpetual problems. It's like you lockpick a locked door, and it takes you forever, then once you finally open the door, you see another door in front of you, and this repeats for 10, 20, 30, 40 times, some doors harder to pass than others, and at the end you finally reach a room called perfection, or is this room reachable? Are the doors infinite in number? 🤔 But by far, the first 5-10 doors are the most necessary to pass, if you know what I mean... Pardon me, I always have to type something that sounds unordinary in many of my posts, and this is mostly an unconscious decision. I have been eyeing the Baader Diamond Steeltrack focuser, as a possible fix to my problems, but I will wait for advice. |
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| You are running a f/5 newtonian *without* a coma corrector???? That's gutsy but not really brilliant. Same with the filter holder. At any rate that newton has very short backfocus and adding an OAG was, IMO, a bad choice. More so one with a M54 flange. A short, better focuser is highly recommended. |
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I was almost ready to do some real imaging, but a new problem seems to be my last necessary obstacle to overcome. Hey there, thanks for asking the questions, I got you covered. First off, I would absolutely not advise you to use the filter's threads as a connection point. They are not load-bearing, and will eventually fail. I would buy a filter drawer without looking back. Additionally, it can introduce tilt into your image train due to short threads. The issue here is you're measuring your backfocus from the tube outer diameter vs the focuser draw tube's exit. The image train is all you need to worry about for backfocus. You need only make sure that it is... well, I don't know the metric for C8-N off the top of my head (and internet isn't helping), but assuming it's similar to a Quattro, or pretty much any newtonian made in the 21st century, it should be about 55mm +/- 5mm from the end of the draw tube. Exact distance (unless you use a coma corrector) doesn't matter as long as you actually get into focus, as (a perfectly collimated) Newtonian has a fixed focal point vs an SCT (like my EdgeHD) which have dynamic focal points that require precise backfocus measurement. I would recommend you measure the outside diameter of the "Large Male Focuser Thread", and find a proper adapter for that. Remove that focuser to 2" adapter if possible after that, and replace it with the adapter, then put the OAG there, and then your filter drawer, then the camera. Total backfocus as it looks right now is 62mm, which is close, but not quite there. Also, with the focuser, the shorter the better. |
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Thanks both of you for the answers to my questions. Looks like I got to get the Askar filter drawer, and a shorter focuser. What short focuser would you recommend for an 8 inch newtonian? I saw that the Baader steeltrack one is commonly used as a replacement to the stock one on the C8-N, plus, there is an adapter that Baader sells that puts male M48 threads on the focuser; that would simplify everything for me. EDIT: however I'm not exactly sure the focuser would be short enough for me to reach focus, because of the filter drawer. Maybe I got to look for a focuser that's very low profile and just get a coma corrector with it? |
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| is it good? are there better ones? |
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| That's good enough. I have one of those. |
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Is this all I need? Any adapters or something I'm missing? Couldn't believe tax from Teleskop-Express was 19%; that makes my total cost about $970 Hopefully, this is all need. Ohhh! 🤦♂️ I forgot the 1.25 inch IR pass filter! Alright, so basically $1000 total  |
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| You shouldn't pay German VAT on imported good into the US, only local taxes if applies. And, while the GPU is a very good CC it will most likely stick out from the focuser into the light path. Given that you have a f/5 you are better served with the TS MaxField or Baader Mk.III. The TS MaxField (or Sharpstar equivalent) is the better one but the Baader is cheaper. |
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andrea tasselli: Alright, I'll get the Maxfield. Thanks. |
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Update: Finally got all the parts I need (sharpstar .95x, filter drawer, IR pass 742nm 1.25 filter, OAG, UV-IR cut 2 inch, asi120mm mini, asi533mc pro), and assembled it, and it looks nice! If I never knew this much about this hobby, this thing would look to me like some scientist's strange heavy-duty professional device, whatever that is. My only problem now is that the mounting holes on my new TS-optics focuser don't match the ones on my OTA's tube, so it looks like I gotta do some drilling on the tube. |
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Update: Little drilling aint never hurt nobody, just make sure to plug the old holes to prevent light leak. |
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V.M Legary:Update: I did it! The old holes were covered with electrical tape, the new focuser and everything else is attached, and I recollimated, and I'm ready to go! Now let's hope I reach focus... |
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Updates: Okay, I was able to reach focus:  And here I got a 1 minute (processed) image of (supposedly) Capella:  The star colors are much much better, and the diffraction spikes are perfect, plus it looks like there's no coma anymore. However it was not easy to remove this halo thing: (no calibration bias, flats, darks)  With 120 darks, 200 bias, 20 flats (a little more stretched than the uncalibrated one):  And this is what my master flat looks like: (but a single flat frame is more contrasty than this)  And I have other problems, like: 1. Always losing guide stars no matter what, even without the IR pass. This was slightly improved when I moved the prism more in the path of the main sensor, but I still wasn't able to guide for more than a few seconds. 2. When I was able to guide, at one point, for a few seconds, it was supposedly 0.4" accuracy, then afterwards, didn't get any better than 2.50", which is worse than when I used a guide scope, and at one point, was like 2400" or some big number like that (after recalibration of guiding, this specific problem went away.) But I was doing imaging in my doorway (not outside), so I'm contributing these problems with disturbed air (hot inside temps and cold outside temps mixing around). You might ask why I didn't go outside; well, I was originally just testing to see if I can reach focus (but I then decided to try guiding, and then got curious and started imaging), and, going outside would not really be worth it, because a lot of clouds came soon after. And FYI, my PA accuracy was within 30" in one axis, and 50" in the other. And I was lucky to get around 2.7" guiding accuracy when I took the star image. So next time, I will have to go outside, and wait for my scope to match the outside temp, and if at that moment I'm not able to guide with at least 0.6", I might make another snot nebula for y'all!!! (just kidding) If it doesn't work, I'm just gonna miss using my guide scope; but then I wont want to use the guide scope because I already spent a ton for the OAG. |
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So, can someone tell me what that halo thing is, and what I need to do to remove it? I think that thing probably only appears when bright stars are in the frame. |
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| Since isn't shown in the flat I'd surmise it is a poor illumination effect due to the fact that scope was half-in half-out. IOW, stray light getting in the way. |
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andrea tasselli: Ah, okay, thank you. Next test will be done completely outside. |
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Big update: This image explains everything https://www.astrobin.com/xagdtg/C/ I was able to get 0.55" constant guiding, and some small spikes up to around maybe 1.4"; in the end, only like 3 frames out of 70 were discarded because of star elongation, and another 4-5 were discarderd because they were taken when dawn was approaching (I left my telescope running outside all night, for the first time) So I was able to get 5 hours 25 minutes of useable data. And I didn't use the IR pass filter for the guide cam; it was making the view too dim, and it was HARD to (almost manually) find a bright star to focus my OAG, even without the IR pass. Maybe next time I'll try the filter, since I'm now focused. If not, then I'll use the filter for planetary stuff. And I'm very lucky that there are no internal reflection problems with my setup! And I bought PI, NXT, and BXT, so a huge difference in processing power. I didn't get SXT, but I can still use Starnet. |
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| Oh, and I also used ASIair's Meridian Flip function for the first time, and it worked! |
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| All well what ends well... |
