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

On 24/10/2021 at 17:28, Astrobits said:

Having more than three points of contact will almost certainly distort the mirror. If it eliminates some astigmatism then it is very lucky that the mirror was pulled in the right direction!  Also, make sure the mirror is about 3mm away from the backing plate. That way it will be easy to remove it when necessary and provides a cushion against any distortion on the backing plate.

It is not necessary to have three blobs on the secondary. Mine has only one small central blob and it has been that way for nearly 20 years.😀

Nigel

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?

3 hours ago, Chriske said:

The two attempts, described higher up, to glue that sec. mirror are not 'standard' procedures. (in fact completely wrong, sorry)
During course we always glue the sec mirror with only three small dots of silicone(near the edge). Distance between mirror and holder about 2mm, to allow better and evenly cooling of the mirror.
During our nearly 40 years of course we've glued literal hundreds of sec mirrors. never had any issues.

Look at the shape of the defocused stars and look at the shape of the silicone ring...  Seems familiar...?
 

2 hours ago, Peter Drew said:

I agree with Chriske's comments and would try the 3 blob silicone approach at the next attempt.      🙂

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:

1 hour ago, Peter Drew said:

I don't really know to be honest, small stalks seem to be commonplace these days with no general problems reported.  I've always preferred to make backplates the same size as the secondary mirror, specially large ones.    😀

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?

14 minutes ago, markse68 said:

A bigger stalk would offer a few advantages i think- more evenly distributed support at the very least but i’m worried about your dew heater- looks like it’s heating the stalk so you’re going to get uneven heating of the secondary. Glass isn’t a good conductor of heat which will lead to physical distortion. maybe that’s the issue here?

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...

6 minutes ago, Peter Drew said:

Looks like classic astigmatism to me rather than any collimation error.  If you use a ring of RTV to attach the flat you should provide a hole or gap in the ring, otherwise differential cooling of the air trapped by the ring can cause distortion of the flat.  Regardless of differing opinions about OOUK I think it highly unlikely that they would supply an astigmatic flat, as they are easy to judge for flatness against a known reference.

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?

1 hour ago, markse68 said:

You can check flatness with an optical flat by looking at the interference fringes produced in monochromatic light when you lay the optical flat on the secondary. I bought a russian flat for this purpose and plan to check my old mirror with it. But i’m not sure i’d want to do it on an expensive new mirror- you have to be really careful to place it and remove it without moving it sideways and the surfaces should be scrupulously clean and completely free of dust. And optical flats aren’t cheap.

Mark

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...

45 minutes ago, markse68 said:

What are you using for collimation and are those imaged through a CC? The 90deg thing is an indication of astigmatism- it's what I was getting and in my case I'm pretty sure was caused by the secondary not being perfectly flat but having a slight curve along its long axis. Can you check again with less defocus? I was seeing it clearest just a tiny bit either side of focus looking at how the stars just started to grow from a dot.

Mark

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...

55 minutes ago, markse68 said:

HI Kari, I'm having secondary issues at the moment too   (but hopefully soon fixed :))

Do your irregular stars elongate with 90 degree separation as you go intra/extra focus? It seems quite normal nowadays to support big mirrors on a small stalk like that but maybe gravity is distorting the mirror? Did you let it acclimatise long enough and are you using the dew heater when you notice this?

Are these images from centre of view? Are you using a coma corrector and if so have you tried without?

Mark

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?

27 minutes ago, Merlin said:

Maybe the flat itself has stress lines in it due to poor annealing. 

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

9 hours ago, skybadger said:

I find that it depends....

Typically it's always clear below -18C 

It's always cloudy above 5C

It's in between where it can be tricky. I've had nights where the sky temp has been above zero and they were still clearer than other nights where it was below -5. 

I believed it was due to the direction of the prevailing wind at the time, bringing different bulk masses of air at different temperatures and humidities. 

If the air temperature is warmer than usual due to say coming up from the Sahara, it can still be clear and quite warm. If it's from the north or north east (for the UK) the air tends to be quite cold when it's clear. Which is when the -18 figure might be the useful figure. 

I have encoded this into my safety driver but what I really want to finish is my star detecror/cloud coverage estimator on the all sky camera.

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?

On 05/10/2021 at 19:24, pjsmith_6198 said:

Excellent work.    Upgrading a focuser can be tricky, especially if you have to perform surgery.   I'm glad it worked out so well.

 

Phil

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.

57 minutes ago, CraigT82 said:

Illuminating from the inside of the tube like that to line up the template is an excellent idea... I'll steal that next time I upgrade focuser... if you don't mind! 

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..

2 hours ago, Captain Magenta said:

Perhaps the website is not returning L1 and L2 as I have defined them, but some other dimensions? I can't check as I can't open your spreadsheet, it's a format my computer doesn't recognize, so I don't know what input values you've used: i.e. distance of mirror-interception from focus etc.

Cheers, Magnus

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

31 minutes ago, Captain Magenta said:

Re the adding up the components you need to multiply the holder diameter by sqrt 2 to make up the major axis length

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!