kbrown

Good job! I've done something similar with my QHY5L-II and a CS mount CCTV lens attached to it. I used oacapture to view the video feed. It has a few different style reticles you can overlay on top of the image but nothing like in your software. Curious if you're planning to add support for any commercial astro cameras?

Still haven't re-attached the secondary as life's gotten in the way. However out of curiosity I tried testing the flatness of it using a second hand optical flat I got off of fleabay and a 50mW green laser diffused through a ping pong ball and reflected off a white card. Never done this before so I have no idea if I've done something wrong or not. I basically cleaned the surfaces as good as I could then placed the mirror on the table, a piece of optical cleaning tissue on top and the optical flat on top of this. Then I slid the tissue out whist making sure the optical flat didn't move. This is what I got time after time: I can see a little bit of waviness on the fringes but I don't know whether this is due to my method or perhaps because my optical flat isn't big enough to cover the entire mirror. I'd be grateful if someone more knowledgeable could give me some pointers and/or feedback. Clear skies, Kari

Thanks Nigel. I'm curious about how big is your secondary? Do you have a matching size backing plate? How big is the blob of silicone you attached it with?

Thanks guys. I will try that next as well. The idea of the silicone ring came from this thread. Someone said they fixed astigmatism by doing so but maybe in my case it's doing the opposite.

Cool. I shall see if I can actually pull it off... Something like this:

I have a desktop CNC router I could potentially use for making a backplate of matching size out of aluminium or even stainless steel sheet. I could then drill and tap say three M3 holes into the stock stalk and attach the backplate on it with counter sunk screws secured with loctite and then use a larger silicone ring (with gaps) to attach the mirror to it. I might be even able to put my dew heater on this new backplate instead of having it on the stalk. Does this sound like a good idea?

Yes it's heating the stalk. Not ideal I know but I never had an issue with it with the stock 58mm mirror and it effectively kept the mirror dew free. I'll keep it off next time I'm testing...

Thank you Peter. I'm pretty sure (and really hoping for) it is my own fault and I can somehow fix it. I will "mind the gap" in the silicone next time. Do you think I should aim to have a larger stalk to hold the mirror as well as it has grown from 58 to 75mm?

Thank you. I found an 80mm russian optical flat on ebay for reasonable price so I thought I'd just get it out of curiosity if nothing else. But it's coming from Ukraine so god knows when it's actually here. For now I'll continue testing without it and maybe try to come up with an alternative way of holding the mirror on the spider...

Yes the images are through a CC but I did try with another camera without the CC and any filters and still got similar results. I will try again with less defocus as soon as I can. For collimation I used several things. I 3D printed sort of a ruler/template which allowed me to centre the secondary in the OTA. I then put my small QHY camera in the focuser with a CCTV lens that allowed me to visually centre the mirror with the focuser tube. Then I used a laser collimator to adjust the beam from the secondary to hit the centre of the primary and also roughly align the primary as well. Final primary adjustment I did with a cheshire eyepiece. Then I took some flats to check for even illumination. I iterated this whole thing a few times to get everything agreeing with each other. I wish there was an easy way to check the secondary mirror on its own without having to go through all this as it's very time consuming...

Slightly inconclusive as I don't get distinct ring(s) when at extra focus but I think they are in 90 degree separation. What does that tell us? Here's intra vs extra focus by the same amount: Not 100% sure but I think I was actually using a dew heater as it was pretty humid. I pretty much used it always with the old mirror without problems. I will try without next time it's clear and make sure everything is acclimatised properly. My gut feeling is that it's not related to this though. Do you think I might need a larger stalk to hold the mirror? I certainly hope not. But I have no idea how I could test this?

Hi, As some of you might already know I recently replaced the stock 58mm secondary mirror with a 75mm one on my Sky-Watcher 250 newt. I've attached it to the stock mirror holder twice now and still cannot get round stars. I have double and triple checked it's not the primary mirror by rotating it in the OTA by 60 degrees and re-collimating everything (the distortion did not change). I also rotated the imaging train in the focuser which did affect the orientation of the deformation. Also tried with another camera without any filters etc. Still the same. So with this in mind I can only conclude the problem is with the secondary mirror. Here's what the defocused stars look like before and after rotating the primary mirror (they look pretty much the same to me): Initially I used a patch of double sided VHB tape to attach the secondary: I didn't press it down to the mirror. I just let gravity to do the work for me: Then after the horror test results I took the mirror off again and re-attached it with a ring of RTV silicone: I 3D printed this two part spacer / template tool so that I get the holder in the right place and also act as a spacer while the silicone cures: After about an hour I took the spacers off and let it cure for several days before putting it back in the scope. To my horror this did not resolve the issue. The distortion is pretty much exactly the same (size and even orientation) as it was before. So my questions would be: 1. Is there something fundamentally wrong with what I've done above? 2. Suppose the silicone method is better in theory? 3. What would I need to be able to test the secondary mirror for astigmatism on its own before even attaching it to the stalk? Just to rule out I didn't receive a lemon from Orion Optics... Any help would be appreciated. Thanks, Kari

That's very useful information. Thanks for sharing. I suppose sky will always be brighter than usual when there's clouds around at least in light polluted areas like where I am (Bortle 8).

Hi, So I have had already quite some time a TSL237 light to frequency sensor and also an MLX90614 IR thermometer in the view to make sort of a DIY SQM meter and cloud detector. Finally I've actually hooked them up to an Arduino Nano in a 3D printed enclosure for testing. I'm not really trying to copy one to one what Unihedron has done nor trying to match their readings. I just want useful relative data to compare the conditions from night to night. I'm thinking of making a device that sits on top of the scope looking at the same direction and focusing on what the sky conditions are there. So with this in mind I'm thinking of omitting the hemispherical lens in front of the TSL237 and just simply rely on the lens that it already has. However I'm not quite sure should I still add an UV/IR cut filter in front of the TSL237? Here's the normalized responsivity at wavelengths from 300-1100nm from the datasheet. It seems to be peaking at the red end of the visible spectrum and is somewhat responsive to NIR region but not so much at UV region. Here's the vertical and horizontal angular responsivity. Still pretty large FOV so I suppose it'll do just fine without an additional lens? In regards detecting clouds, what is the general consensus? Is there some kind of a formula that tells what sky temperature vs ambient temperature corresponds to a certain level of cloudiness?

Thank you! I've now put the rest of it together as well and have been able to test it. Good news is that I'm able to reach focus with plenty of spare inward travel and the focuser tube well outside the light cone. The bad news is that it seems like I have managed to pinch the secondary mirror with the 3M VHB tape I used to attach it to the stalk. I spent quite a lot of time diagnosing this and really was hoping it wasn't the secondary but I think it is. I did what I could with the primary mirror including loosening the clamps as much as I dared and rotating the cell in the OTA but it just didn't improve the situation. The stars just are not round. I did my best to centre the secondary with OTA and the focuser and collimated everything with a laser collimator and a cheshire eyepiece and re-collimated after each change. So I'm going to have to re-attach the secondary. I'm hoping some gentle heat will soften the tape enough I can remove the secondary without too much trouble. I'll then try again with some RTV silicone instead.

Of course. I just had to come up with something as I couldn't follow Baader's instructions to the letter and use the old screw holes as the guide. Figured I'd make use of the template I printed in the initial post like this.

New focuser in place finally. Used a printed a template again to aid drilling the holes in the right place.

Got the mirror cell holes done. Using the old holes I measured the distance between them along the circumference and printed matching templates then cut and taped them together: Then I simply taped the whole strip onto the tube then punched and drilled the holes. I drilled them slightly (0.5mm) over size to allow some wiggle room. Got them close enough. With a bit of gentle persuasion the screws went in nice and tight. In case you're wondering I've replaced the stock M4 screws with M5 socket head ones and also installed V-Coil thread inserts on the mirror cell which makes it a lot more durable than just screwing into the cast aluminium.

Finally got around to do something about this. I decided to cut 35mm off the tube length as suggested above. I did it using a Dremel tool with a cutting disc. I designed and 3D printed a flange guide that allowed me set the cutting depth and to run it along the inside of the tube. Took me about 20-30 mins to do the cut and I had to change the disc twice as it wore down quite quick. Anyway, I think it's accurate enough. Had to sand down the rough edges and then vacuum the inside of the tube afterwards as it got a bit messy (especially the velvety flocking material). Still need to drill new holes for the mirror cell etc etc...

Hi All, Just wanted to let you know I've now posted a blog post about how I assembled this thing together. You can find it over here. Do let me know if something doesn't quite make sense Cheers, Kari

Just attached my new secondary. Turns out the holder was actually 32mm (not 35mm) on my SW 250P so I had to re-calculate the offset again. Drew an printed a template in LibreCAD, then cut and taped it on the back of the mirror with a hole for the holder. Was a bit tricky to get the holder in the right place as I've added a heater and temperature sensor around the holder. Didn't want to take those off as it would have most certainly damaged them. Hope it's in a good enough position. All done... For now..

No, it was my error. I was using 35mm for both minor and major axis of the holder diameter. After changing the major axis to SQRT(2) * MinorAxis I get to the same ball bark as you with your values and my values add up too. L1 = 22.64mm L2 = 33.86mm Holder Major Axis = SQRT(2) * 35mm = 49.5mm L1 + L2 + MajorAxis = 106mm I tried to save my spreadsheet as Excel file from LibreOffice Calc which I use. Attached below. Hope it works. secondary_mirror_offset.xlsx

Ah. This is because it's cut in 45 degree angle and the cross-section is an ellipse? Still don't understand why I get so different results? EDIT: I think I understand now... I had the holder major axis the same as the holder minor axis in my spreadsheet... If I multiply that with sqrt 2 I get to the same ball park! New version attached to this post. secondary_mirror_offset.ods

Hmm... I'm by no means trying to invalidate what you've done here but I'm a little confused. I'm about to replace my secondary too on my SW 250p. I used this to calculate the offsets: http://www.deepskywatch.com/Articles/replace-offset-collimate-secondary.html I'm getting quite different results from your spread sheet vs the method in the above article. I'm getting these values using the above article: L1 = 29.89mm L2 = 41.11mm With your spreadsheet I get: L1 = 21.28mm L2 = 35.28mm Also your L1 + L2 + Holder Diameter does not add up to the major axis of my secondary => 21.28mm + 35.28mm + 35mm = 91.56mm while the actual major axis of my secondary is 106mm. I'm attaching a spreadsheet I made based on the above article. EDIT: Attachment removed as it had errors in it. See below... Cheers, Kari

Pawel Soja at the INDI forum is definitely onto something here: https://indilib.org/forum/astroberry/8142-asi178-camera-performance-on-rasperry-pi-4.html He has been developing an alternative library for ASI cameras and getting some impressive results. Unfortunately it doesn't seem to be in stable enough state for everyone. I managed to get about 51FPS at full resolution with his library remotely via INDI/Ekos/Kstars. However I did get quite a few random crashes as well. I didn't get this working with oacapture, firecapture or planetary imager though. Not quite sure if it's even possible without modifying the source code of those apps. I shall jump onto the above thread and post my findings there. Hope this develops into something great!