oymd

yes, im using bin2 and median3x3 in PHD2 to get ANY sort of guiding going!

Thanks, I thought so. I just wanted to hear: Don't worry too much, its the C8, and its not meant for AP, which is what you summarised. I have an Edge 11HD, but I am first learning the SCT ropes with this C8. The Edge will definitely need a better OAG, and possibly a better guide camera. My current OAG is the OVL one, 8mm prism, 5mm opening in the OAG turret, which overall is a joke, and a PAIN to use. Its like threading a needle in total darkness!! I will also need a better mount!! I am happy to keep experimenting, as long as I am doing something very wrong. Since March there is hardly ANYTHING large to image, so the WO71 and Esprit100 are useless really, and its a good time to experiment with the1280mm FL of the C8 on the only thing out there to image these days, Galaxies. BTW, what is the BACKFOCUS OF THE NON EDGE C8 WITHOUT the 0.63x reducer? I'm contemplating trying 2030mm FL? The galaxies are small anyways, so will always have to crop, and edges are not an issue?

Yes, I have been noticing a wishy washy ugly dirty background pattern Its hard to give it a scientific name, or to describe it accurately, but it is like someone smudged the background. I think my skies have been terrible recently, and might be haze? Not sure.

Thank you for your reply. That's exactly the same purchase route I went through! I bought a Nexstar C8SE off Amazon in Xmas 2019, and initially was enjoying it for just visual, and learning the skies. In Feb 2020 decided to get an EQ mount, and bought an AZ-EQ6 Pro, and since then, it has been downhill!! I am a much, much poorer individual now...

Hello everyone. Have been in the hobby for 2 years, imaging with widefield refractors, mainly a WOStar71 and 294MC Pro, with good results. Image scale is 2.7"/pixel. With galaxy seson starting a month or so ago, I decided to give it a go with my Celestron C8SE, which I had bough a couple of years ago as a full setup. I used an OVL OAG and my 290MM Mini for guiding. I also used Celestron's 0.63x reducer. I set back focus at 105mm Collimation looks OK. I platesolved and checked on Astrometry, and indeed my image scale was 0.75"/pixel, confirming my FL was 1280mm exactly. I am well polar aligned, and PHD2 is showing RMS of 0.5-0.7, so overall good guiding. Looping on 2 seconds. I am using a ZWO EAF for autofocus, and its been working well, except for the fact that it seems focus changes a little whenever I am slewing around. I guess that is Mirror flop? Anyways, durimg imaging, I set the focuser to AF every 10 images. When imaging, and seeing the images come in on NINA, I can clearly see all around the periphery of the image that the stars look properly egg shaped, quite severe at the edges. Is this an inherent property of the SCT, especially that its not an EDGE? Since I have never imaged with an SCT before, I am not sure what to expect? This is from yesterday, 60 L of 300s, 20 Fs, 20DFs and 20Ds. Dithering EVERY frame. Compared to my refractor experience, images look very very soft. I had to use a LP filter, the IDAS LPS D2, as my skies and LP is terrible. Many thanks

OVL OAG: https://www.firstlightoptics.com/off-axis-guiders-oag/off-axis-guider.html

Here's a paragraph from the review of the OAG on FLO's website: The Turret is 45mm long, which might be the problem? The 90° prism was of high quality and no aberrations were visible when examining a daylight scene through it. The prism is held firmly in its socket at the end of the adjustable turret by a single tiny grub screw. There is no tilt adjustment for the prism but the 45mm long turret allows for plenty of adjustment in and out of the casing and this is secured in position by a single knurled bolt on its short side. I would have preferred to have had a second bolt working at right angles to the first to stop any chance of the turret ‘rocking’ in its channel but this is just in my personal wish-list as none of the commercially available OAGs have such a second bolt. I made up a tiny shim using kitchen foil to act as a buffer on one side of the turret and this ensured that there was no unwanted movement of the turret in any direction. The light port through the turret is 5.6mm in diameter which suited my 4.9mm x 3.7mm sized guide camera well but would cause vignetting with larger sensors – not a disaster but a shame as it could rob larger sensors of an increased guide star choice.

113mm!! And you plate solved at 1280mm?? Guess those 0.63 reducers are not very consistent?? But, here's a question though....I think I read somewhere that the FL of SCTs changes with change of focus? Maybe the variation is due to movement of the primary mirror?

The OVL's OAG stalk is 45mm. That might be the problem? Its this one: https://www.firstlightoptics.com/off-axis-guiders-oag/off-axis-guider.html

Ok, so here's my problem: I setup my imaging train as such: C8--> reducer--> 50mm Celestron T2 extension--> OVL OAG (15mm width)---> 10mm T2 extension---> 10mm T2 extension---> ZWO 11mm Spacer ---> 294MC Pro sensor recessed at 6.5mm My train is very very close to 105mm I get good stars, and I platesolved and Astrometry shows that my pixel scale is 0.745 arcsec/pixel, so my focal length appears to be bang on 1280mm. The problem is that I CANNOT push my 290MM mini far enough inside to get into focus. I can see very unfocused stars in PHD2 looping, but the guide camera is bottomed out against the OAG's shaft that brings in light from the prism. The only solutions I have are: 1- Moving the OAG farther away from the sensor: NOT POSSIBLE, AS THE CELESTRON T2 CONNECTION IS FIXED AT 50MM 2- Adding a 5mm T2 extension on the camera side, but this led to poor stars and my FL changed when I platesolved! I ended up swapping one of the 10mm extensions with a 15mm extension, and I could finally focus my guide camera on the OAG. Am I missing something? Mu current setup puts the OAG at about 43mm from the sensor!! That is just too far for the OAG, but anything nearer, and I cannot focus the guide camera.# Currently the guide camera is bottomed out completely against the OAG shaft in my helical focuser. I am using the OVL OAG, and attached to it is the ZWO non rotating helical focuser. P.S. Another crazy solution would be to chop off 5mm from the OAG's shaft that is holding the prism!!?? Or find a SCT T2 adaptor that fits to the reducer, but is smaller than 50mm?? Thanks

I platesolved my images that I took over the past 2 nights. I was using 105mm backfocus and indeed I calculated 1280mm focal length.

What I find confusing regarding using a guide camera with a LARGER sensor, or using an OAG with a larger prism, is the fact that the actual light beam coming through the OAG is barely 6 or 7mm wide? What would be the point of getting a larger sensor, if the actual opening in the OAG that send the image to the guide camera is only 7mm in diameter? It barely even covers the 290mm mini's sensor?

I get your point Vlaiv. But that will only work with 1.25" filters. My problem is that ALL my filters are 2"!!

Sorry Vlad, I did not follow that? I have the ZWO T2 filter drawer, which is 21mm. I plan to use the Optolong UV/IR filter in it as I have bad LP. Yes, I plan to go: Camera ===> 21mm Filter drawer ===> OAG, so that the OAG gets unfiltered light, but that moves the OAG AWAY from the sensor?

I will certainly try the flats trick!! I have a geoptic flat panel. Will experiment tonight. I am confused however regarding the back focus. As Vlad pointed out, TS optics clearly states that back focus is 85mm. FLO and several you tube videos advised that back focus is 105mm?

Yes, I definitely am using the standard 0.63 reducer on my NON EDGE C8SE OTA. I will try 85mm tonight. FLO definitely recommended 105mm though. I changed around my train, and now the OAG is IMMEDIATELY after the 11mm ring on the camera side. so around 11mm + 6.5mm from the sensor. But that will cause a problem when I factor in a filter drawer down the line?

Thanks Vlad The 105mm back focus was recommended by FLO when I emailed them? I have also seen several YouTube videos where 105mm was the recommended back focus? I will certainly give it a go with 85mm. With regards to binning, the options on the 290mm mini are either x1 or x2. Should I choose x2? What does that change? Many thanks Vlad. Here we go again, after 2 years happily imaging with the 70mm apo, I am back to kindergarden with the SCT!!

I consider myself a beginner in AP, but I have gained some good experience over the past two years using a widefield Williams Optics 71 Star Apo and an EvoGuide 50mm for guiding. I am fairly familiar with NINA, PHD2 and the quirks and problems with setup for AP. Last week, with galaxy season arriving, I decided to give it a go with my Celestron C8 and an OAG. Boy, this is a whole different level of DIFFICULT!! I need some advice regarding my imaging train, and what am I doing wrong. I setup the train for 105mm backfocus. No matter what I do, I cannot get any sort of stars in the view of my 290mm mini on PHD2. Just to check my imaging train is OK, I took 30 1 minute subs of M51 without any guiding. Understandably the image is soft and stars are not that great. Anyways, what am I doing wrong with regards to the OAG? My imaging train is as follows: C8 ===> Celestron 0.63 reducer ===> 50mm Celestron T2 extension ===> ZWO 42mm to 48mm adaptor ring ===> OVL OAG ===> 21mm ZWO extension ===> 11mm ZWO ring ===> ZWO 294MC Pro Total imaging train amounts to about 104.5mm, but I added 2 thin spacers. Issues I have: 1- There is obvious vignetting on the image window in NINA. Is this expected? Is it caused by the 0,63 reducer? Never saw that with any of my refractors? Here is Arcturus. I used it for the autofocus routine in NINA Why is there such obvious vignetting? 2- I imaged fine, but guiding was impossible. I CANNOT get the 290mm mini in to focus at night. During the day, I got it into focus on a very distant object, but for some reason, at night I just see GRAY NOISE? I tried Gain 0 and Gain 90. In PHD2 for the guiding focal length, I input 1280mm, which is what the 0.63 reducer amounts to. I realise that guiding is working at an incredibly small pixel scale of 0.5 pixels / arcsec!! 3- The OAG prism is correctly positioned, and I confirmed that by shining a light onto the scope. I placed the prism parallel to the long axis of the sensor, and is NOT covering the sensor. 4- Could it be that the OVL OAG i am using is not suitable for my imaging train? 5- I am aware of how small the 8x8mm prism is, and how borderline the 290mm mini is, and the 174mm mini is a better fit, but overall I am just trying to do a preliminary setup. 6- I am also aware that the Celestron C8SE OTA is not great for imaging, but as I mentioned, I am just dabbing into SCT astrophotography. 7- I have an EDGE 11HD, but I do not dare trying it yet. I want to familiarise myself with the C8 first. Many thanks

I Thought I would update this thread I started. I replaced the bearings on the RA worm shaft with SKF 608-2Z. I fine tuned the backlash manually on both axis while the mount was stripped, and did some further tuning with Guiding assistant. As of last week, the mount is behaving very well, with my guiding at 0.30 to 0.50 rms, constant, depending on the seeing. Very happy here... Especial thanks to @wimvb for all his help and patience in getting me over the line!! Here is my first project after fixing the mount: IC 2177, the seagull nebula, taken during full moon over 6 nights, with loads of subs discarded... 103L, about 8 hours.

Many thanks Can you please advise me how to populate my Dither settings to fix this issue?

Apologies.. here's the guide log and my Dither settings in NINA PHD2_GuideLog_2022-02-15_204257.txt

Yes. Sorry. I tried to upload it from my phone. will upload it from the PC once I get home.

I’m also attaching the PHD2 guide log.

Dithering is set to 5 pixels.

Thank you for your replies. Yes, the artefacts do not show up in the individual subs. I have been advised to generally Dither every 5 minute sub. When I use the L-eXtreme, I usually take 300s subs of Nebulae. I dither EVERY SUB. I used the same calculation when I was imaging M42. For the 2 minute subs, I dithered every THREE subs. for the 30 second subs I dithered every 10 frames etc, so basically dithering every 5 minutes or so. With the 10s subs, I dithered every 25 subs. Could be the effect of UNDER DITHERING? If so, how often should one dither with 10s subs?