Tommohawk

Thanks Craig, too kind!

I seem to remember you did some cracking planetary images with your Orion so will be interesting to see if you can squeeze even more out of it with your 1/30 PV secondary - watch this space as they say! Interesting to know that your primary is 1/10 wave, must make all the difference I reckon a quality 300mm F5 in a purpose built truss frame would suit me well, but I don't have my own workshop so have to use a bit of ingenuity and adapt stuff. The flextube idea worked well because it allowed me to coarse tune the scope length, but its a heavy scope and I couldn't do the same with a 300mm.

Thanks for that - somehow I studied the atlas for ages, and then about 5 mins after posting I figured it out. I used to work with low mag to start with just to plan what I was going to view/photograph and then switch to high mag. With the current setup that isnt practical - I have to start with high mag so getting oriented is problematic and a lot of the landmarks are washed out with full moon. Thanks Neil. The scope is a bit of an experiment - the idea was to use a fast mirror just to keep the scope length down to minimise momentum etc, and then use the powermate to give the higher EFL. (F20, 5000m EFL) Mounting the powermate/camera assembly on the secondary spider means there's less central obstruction - the Omegon OSC Velox 385 camera assembly is only 43mm diameter. It's based on a Flextube 250 housing a faster Quattro F4 mirror - see pic below. Long term I wanted to do the same thing but as a truss design so I could disassemble and travel with it easily. In general it works very well, is really easy to collimate and holds collimation very well too. The big issue is focussing - I've built in a helical focuser which works well, but it really isn't possible to focus on surface detail because theres just too much image movement - I prefocus with a Bahtinov which does the job nicely. I used this rig extensively with Mars last year, and TBH I don't think this gives quite the resoluton I was hoping for. My conclusion is that (cheap) fast mirrors suffer from aberrations such that any marginal gain from the reduced CO is lost. It would be interesting to repeat this with either a better mirror or maybe say an F5, although an F5 250mm would be about 1.5m long including the camera assy. and would probably need a bigger mount. Currently my HEQ5 just manages this with a stack of extra weights!

I was trying out my modified Quattro with 5x powermate last night but difficult to get much detail with the moon being pretty much 100% full. This was the only image worth keeping TBH, but I blowed if I can figure out where it is. This rig is a bit unusual - I use the camera at prime focus with no secondary - which means I think it has one less plane of reflection than a regular Newt. Maybe not. This region ought to be South West - but I can't see it on any atlas. Grateful for any thoughts! Edit: Byrgius maybe? Quattro 250 with Powermate x5 and Omegon 385C. Captured with Toupsky, 8 minute run at about 40 fps, best 5% AS!3 + Registax + PS

Thats very nice - is it a single sub or a stack?

Looks great, nice colour range and nice detail especially at the limb. Which scope was this I wonder?

I think you also need to consider that if scope has 10x aperture and same focal length then above is true. But if has same F value then it will have 10x the FL, in which case you don't have any extra light per pixel. Doubtless someone will correct me if I'm wrong!

Difficult to know whether your scope would be good for full frame - not sure what it is and what manufacturers claim - though might want to take this with pinch of salt anyhow TBH. I'd have thought that a full frame DSLR is going to be similar cost to astro camera, though not 100% up to speed with prices. I would say if you want to use the camera for non-astro too then go for DSLR maybe but you wont get such a good result as cooled dedicated camera. Also Canon software integrates better than Nikon.

Guys there is a risk here that we may generate more heat than light, focused or otherwise.😉 The question I asked is how can all of the various wavelengths comprising LUM be focused simultaneously in a system where RG and B have different foci, and I think the answer pretty unequivocally is that it's not possible. The LUM image will therefor be blurred to some extent and this can be minimised by restricting the bandwidth of the LUM filter. The sensitivity of the human eye is still relevant though because we use a human eye to evaluate the end result, and I think it must therefore be true that if the R or B is defocused, this will be less apparent to the human observer of the image, than if the G were defocused. But as I have said previously, for astrophotography this is not actually very useful, because there isn't much detail in the green part of the spectrum. In any event leaving humans out of it for a moment, the practical outcome is that LUM is of particular benefit in well corrected refractors and less so in scopes with poor colour correction. My experience is that LUM can actually be detrimental with systems with significant CA, especially with wide pass filters, and especially with sensors that have (relatively) high QE at the extremes of the band. In these cases a synthetic superluminance layer derived from combined (focused) RGB elements will work better, although the SNR benefit of true LUM is then lost.

Not sure I can run with your X-Ray vision analogy! But there is another issue here - we can look at how photographic sensors work with OSC or mono images using filters, their sensitivity at different frequencies etc OR we can consider what the eye sees, eg at the telescope eyepiece. We would agree they are two different things. But the third comparison is what the human eye sees when it looks at a sensor/screen generated image - then we have to consider both aspects. And with astrophotography this is mostly what we are doing .My point here is that if an image produced by a sensor/telescope/screen is wonderfully sharp in red and blue, but blurry in green, and the eye is most sensitive to green, then the screen viewed image will appear mostly blurry. I think this is the point Michael was making. In any event, the key point of my initial question was to establish whether a LUM image generated by telescope system having differing focal points for RG and B, will be defocussed, and I think this must be so. The RGB components derived from filters will be better focused, and so adding the LUM data may therefore spoil the image. A partial remedy is to limit the bandwidth of the RGB filters, and similarly limit the bandwidth of the LUM filter. The other partial remedy is to use a superluminance generated from the RG and B data, rather than straight LUM, but this doesn't have the SNR advantage that the LUM data has. My interpretation of Michael's point is that if the green is focussed in the final image viewed on a screen, the final image may not suffer too badly. My only problem with this is there isn't much data in the green. If this is so, we are all correct!

Re 2 previous posts...... this is exactly why I said previously "I need to think about it!" The thing is when making a call about how good the image is, "photographic CA" is in the end perceived and judged by the human eye.... .... I'm still thinking about it!

The trouble with small newts is you need disproportionately large secondary to get fully illuminated sensor. Below is the spec for an F4 100mm. Even with the camera and EFW fixed direct to the tube (focusing done at the primary a la SCT) you need a 32mm secondary to get fully illuminated sensor (of decent size) But this doesnt allow for a coma corrector - if you allow 50mm for a CC that doesnt block the light path then you need about 42mm secondary which is too big a central obstruction. Its not so bad with an OSC because the camera sits closer, but that would make for a very niche product. I've been looking at this on and off for some years, and the only design that would work properly is if one element of the CC is built into the secondary housing. Technically this would work well, but it such a niche market it would never fly. RE the SGL signature - yes am on desktop... I did something the other day and the signatures disappeared and now I cant see how to get them back!

Thats a good point Vlaiv - the 130PDS gets great results for a bargain basement price, and mosaics are becoming easier to do eg with NINA. The only box it doesnt tick unfortunately is the portability one. I typically fly to a kinder sky location a couple of times a year and although the 130PDS will go in a case (and frankly is cheap enough that it wouldnt be the end of the world if it got trashed) it does eat up a lot of luggage space. But your point about binning and achieving the same overall result is a good one. Having said all that, the Sharpstar 61 Mk1 - which has been referenced in a number of threads - is mechanically excellent, and the field is wonderfully flat with reducer and ASI1600 sensor. The only issue is that it isn't parfocal - the reduced bandwidth Astronomik RGB filters fix this nicely for mono work at least (apart from having to refocus between filters) and it just remains to see how good the L3 will work. I'll be sure to post when I have my next imaging session although of course this scope has now been superceded so not sure this will help others much.

Hi Craig - I'm onside with Newts for sure but no real options for a widefield newt sadly. There are a few 114mm but only with 1.25" focuser. I've tried designing from scratch but there are design limits especially if you need to get a coma corrector in the train. Thanks for the 3 dots tip - I tried that but on someone else's post so of course it dint offer edit! Whilst youre there - Ive also lost the signatures somehow - any ideas on that?

Hmmm - just done some reading and looks like microlensing occurs with mono too. I've done quite a bit of BB and NB and although I've had some issues I've never noticed this effect. Also i seem to have lost my edit post option - not sure why!

Sorry I mean mono version - I've heard of microlensing but assume this is down to RGB matrix, so mono should be OK?

Thanks Vlaiv - pretty much confirms my thoughts.

Shapstar 61 and ASI1600 cool. Re M45 I got a good result in RGB, and was trying to see if I could improve it with L - but it wasnt a fair test due to moon and slight high cloud so will have to wait for another chance.

Thanks for that Dave. One thing I should have made clear at the outset is that I expect the RGB filters are parfocal - it's the scope which is the issue. I agree with everything you say - and the proof will be in the results. Using FWHM is likely the best way of assessing the reality of the situation with the LUM filter. Partly the reason for asking all this is I've just changed my LUM filter for one with a slightly reduced bandpass (Astronomik L3) and am wondering what to expect. The previous LUM filter worked greaet with the Newtonian, but was unusable with the refractor - huge FWHM. So far I only had one evening with the new one which had 75% moon and my target was M45 which is a bit of an odd one as most of the signal is in the blue I think. Time will tell, but my thinking is that with a refractor that's not 100% chromatic, refocusing between filters fixes the RGB foci, but the LUM detail will likely be compromised.

OK Michael, now I see where you're at! I need to give that a bit more thought, but my instinct is that although the human eye is more sensitive to yellow/green, for astrophotography most signal is in the blue and red wavelengths. If the R and B signals focus differently, this can be fixed by refocussing when doing the colour (RGB) capture, but when doing the LUM capture they are captured simultaneously so will inevitably be mismatched for focus. This will cause blurring. If there was a big green signal then as you say that could outweigh the R and B - but I don't think thats the case. In any event, I think most folk process in such a way that the detail is all in the LUM, and if thats defocussed the image will suffer even if the underlying G signal was sharp. I'm expressing this as a statement, but it is all followed by a big question mark - sort of thinking out loud!

It's only that guiding is much more challenging with longer focal lengths. Your imaging scope has a 2000mm focal length, and so will be very sensitive to small deviations. You definitely want to guide for longer subs, I'm just pointing out that learning to guide is tougher with that imaging scale. But do go for it!

Well you're off to a good start - TBH I'm amazed you got such a nice result with just 90s subs. The SCT is a great scope but its F10 or F6 ish with reducer so you will need longer subs to get a nice result. Once you get guiding sorted you should be good for longer subs, though guiding at F10 can be challenging.

Michael I'm grateful for your input, and I'm sure you're right in what you say - but I'm struggling to follow! The transmission curves for example filters are below. If green light is 500-565nm, give or take, why is that 70% of the LUM is green?

Thanks for that.... though TBH I'm not quite sure what you mean. Looking at the transmission curves the luminance bandwidth should contain all of the R G and B wavelengths, no?

That looks great especially with the moon so close. BTW you say 0 darks in the write up - I'm guessing thats a typo?