What do you find easy and what is still causing you problems? - Forums

Topic starter

Jul 24, 2022

Just trying to work out why I find some things easy and others difficult and looking to see what you think?

* Balancing my scope is the least of my issues … a few post-its to mark the last position and then next time I do it it takes about 2 mins.
* Polar alignment … might take me ten minutes, but pretty straightforward with SharpCap
* Primary mirror alignment of my Newt .. takes only a few minutes with a well-aligned laser
* Secondary mirror alignment

I'm really struggling with this one and would welcome some suggestions
* Guiding optimisation: It's a black art, but getting there
* Backlash elimination: it's taken a long while to get there, but it is straightforward and now happy that I know what to do

So, Secondary mirror alignment and guiding optimisation are my real two challenges.

Andy Wray

Topic starter

Jul 24, 2022 · Edited: Jul 24, 2022

FWIW: I spent a few hours on secondary mirror alignment tonight. Things I found:

1) The secondary was a few millimetres too far down the tube (maybe even as much as 5mm). That may well account for my severe vignetting on one side of my images .. partially fixed
2) The secondary mount was quite heavily tilted, so straightened that out first before aligning it to the primary centre dot

The above two issues were probably due to me not having a clue when I tried to collimate my telescope a week after I had bought it four years ago.

Main lesson: don't touch the secondary at all until you know what you are doing.

Here are my flats (boosted autostretch) before and after adjusting the secondary ... it's better, but obviously I have more tweaking to do:

Any suggestions would be welcome. I think the top left dark area is mainly due to my OAG encroaching on the image path. I also wonder if my focus tube, which extends well into the optical path, is having an effect.

FWIW: Here are my FlatContourPlots. Looking rounder in the latter image, but still way off centre. The pre-collimation contours could well explain why my stars have had a slightly eggy look to them.

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andrea tasselli

Frequent contributor

Jul 24, 2022

It would be useful to you figuring out how much light loss you have in the corners as a % of the peak illumination. As for the draw tube I don't expect an effect on vignetting. To see how much effect it has on overall diffraction you would need an image of the Fresnel rings.

Andy Wray

Topic starter

Jul 24, 2022 · Edited: Jul 24, 2022

andrea tasselli:
It would be useful to you figuring out how much light loss you have in the corners as a % of the peak illumination. As for the draw tube I don't expect an effect on vignetting. To see how much effect it has on overall diffraction you would need an image of the Fresnel rings.

Thank you once again Andrea.

It's a 6% drop off in the bottom right and a 29% drop off in the top left after I collimated. It was 16% and 35% respectively before I collimated, so at least an improvement. I have no experience to know how bad those figures are. What would you suggest is a good target? I think I would be happy with 15% overall as that is manageable in post processing.

I can see that I have improved the dropoff by 5% overall on the left hand side of the image, without impacting the right overall. I can also see that the overall shape of the light distribution is getting more even (circular) which can only be a good thing. I will try again over the next few cloudy days and see if I can get all four corners with about the same dropoff (+/- 5%) whilst also keeping everything collimated. I am using 1 1/4" filters and I'm not sure how accurately the filter wheel positions them, so I may always end up up with one side being slightly darker I guess, but I will try and get the light distribution as central and circular as I can.

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Andy Wray

Topic starter

Jul 25, 2022 · Edited: Jul 25, 2022

Wow: that secondary mirror has far too many degrees of freedom! Anyway, I gave the collimation one last go and ended up with the FlatsContourMap below. It's not perfect, however probably about as good as I can get with my skills right now. The flats will easily take care of my vignetting in post-processing and I have checked as best I can with my cheshire and laser collimator that my image train is aligned. I clearly have some residual tilt as the gradients aren't quite circular, but way much better than before.

For those wondering why there is a chunk missing out of the central circle: that's a dust bunny that is hardly visible in the original flat.

I'm pretty sure the vignetting on the right is from my 1.25" filters and the dark corner on the top left is from my OAG. I could move to 31mm unmounted filters to improve things, but will live with it for now.

FWIW: here is my flat stretched to the middle of the spectrum (not too shabby) :

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kuechlew

Jul 25, 2022

Andy, I believe you answered your question in your other thread Stuff to do when the skies are cloudy - AstroBin too: Buy a Newtonian and spend the rest of your life to get it properly collimated.

Good luck and clear skies
Wolfgang

Andy Wray

Topic starter

Jul 25, 2022 · Edited: Jul 25, 2022

Andy, I believe you answered your question in your other thread Stuff to do when the skies are cloudy - AstroBin too: Buy a Newtonian and spend the rest of your life to get it properly collimated.

Good luck and clear skies
Wolfgang

Ha ha ... given the Newtonian as a birthday present by your wife means having to take the pain I'm afraid ;) Mind you, I am learning a lot in the process

She also thinks she gave me the best present ever as I have been playing with it for four years now.

Mind you: she also gave me a build your own 3D printer kit a few years before that and it is still going strong too with plenty of upgrades. I can't complain.

SemiPro

IOTD Staff

Jul 25, 2022

What do I find easy that was hard? Collimation and tilt/backfocus adjustment.

But because I image at F/2 what is still hard?

Collimation and tilt/backfocus adjustment.

Mina B.

Jul 26, 2022

Easy now: Getting very good guiding values - especially here: A good running mount doesn't need overengineering in guiding parameters. It just works.
-> How I got there? Fixing my mount (CEM25P) - RA was always good - pretty constant, got gradually better when I went initially from a findershow mounted guidescope to a ringmounted guidescope and then to an OAG - which now gives me RA values around 0.4" pretty consistentely, seldomly higher, sometimes lower. However, I was plagued with huge DEC backlash issues - which made guiding initially impossible, then I got it dialed in that I could at least guide unidirectional, then I finally remeshed the gears - and now, backlash is under 1000ms, easily fixable with backlash compensation. I figured, if you have a wellrunning mount, that's well balanced, you actually don't need to overengineer guide settings - default slightly adjusted usually works just fine.

Easy now: Polar Alignment - started with the PA alignment scope, and it was a PITA, nearly drove me insane - I think I even once aligned on the wrong star. Then I went to use Sharpcap pro, now the AsiAir Pro, which makes it even faster, because the Laptop in field is just inconvenient.

Easy now: Focusing - EAF fixed it for me, a Bahtinov is good to get it initially right - but dragging yourself out there to refocus every hour or two, depending on temps, is annoying.

Easy now: Balancing - you just get a feel for it after a while.

Still struggling: Post processing, especially Noise Reduction, my 80mm f/5.6 Apo and the ASI 183 MC Pro aren't the most forgiving combo here - I plan to use that camera either with a 5" refractor or 8" reflector in the future, more aparture would make the results a lot better, it's just a special use case camera imo.

Still struggling: Light Pollution, hard to fix, sadly.

Still struggling: making decent Flats - I sometimes still have issues of them not taking care of every dustspot, or dustspots moving - how do people nail this? Vignette is not a problem for me with a chip as tiny as the IMX183, but that pesky dust - it definitely is. No biggie if I go for targets like the Fireworks galaxy, and I tend to crop a good amount out - but definitely annoying if the target fills the sensor - the chance, that you have it on structures that you actually don't want to crop out is pretty high.

Still struggling: Sensor alignment. Combined two nights of NGC6946 lately, and I was amazed by how little overlap there was - I could have sworn it was the same alignment - but apparently it wasn't - as it was a good amount of time in between.

kuechlew

Jul 26, 2022

Mina, for noise reduction you may try NoiseXTerminator. I find it unbelievable easy to use. There is a free trial period. Only complaint is that I find the standard setting of 0.9 way too aggressive.

So my list now starts with:

Easy now: Noise reduction thanks to NoiseXTerminator by rc-astro
Easy now: Stretching thanks to Generalized Hyperbolic Stretch (ghsastro – astrophotography) and the great support by David and Mike

Still struggling: Sharpening
Still struggling: Not going over the top with my colors
Still struggling: Avoiding bloated stars

Still struggling: Deciding on a mount that is compact and lightweight enough to get carried into the field in a backpack. Currently Avalon EVO-Zero and iOptron HEM27 are my candidates but performance of both is still not reliably confirmed since they just appeared on the market.
Still struggling: Nailing the declination of my target with the iOptron SkyGuider Pro where you have to adjust the dec axis by hand. I'm getting better and I'm good enough for 1 session but reproducing the fov over multiple sessions remains a pain with a lot of back and forth.
Still struggling: Not messing up my polar alignment when manually fiddling with the dec axis of the SkyGuider.

Not struggling but still room for improvement: Improving my tracking values. Typical values around 0.8 - 1 arcmin in RA may not be bad for a tracker but sometimes I slip above 1 arcmin even with good seeing. With my current image scale it's not an issue but to me it's a matter of general improvement.

Clear skies
Wolfgang

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