andrew s

No withstanding @pete_l comment on theory the idea of the device makes good sense in control theory. If you are trying to accurately control say a flow of liquid and have a slow sticky valve you can add a small fast smooth valve in parallel to do this. It was a common practice when I was a control engineer. These units seek to do the same thing. So probably best for lower class mounts. Regards Andrew

Thanks that's what I thought. In the end for however good the feedforward control or feedback control of secondary variables ultimately you need to close the loop on the target variable (star) for ultimate precision. Not that that is always required. Regards Andrew

Somewhat puzzled by this Dave. What do you see as causing the slow wandering that the DDM fails to correct for? Regards Andrew

Reminded me that NASA was given two spare Hubble class mirrors by the US military. Although they might have a tendency to point the wrong way. Now if anyone has a spare... Regards Andrew

The relationship between aperture and seeing might be worth repeating to aid this discussion. When the aperture is smaller than the typical size of the turbulence cells in the atmosphere the light tends to pass through one cell at a time causing the image to sift location but remin reasonably coherent. When the aperture is much larger than the typical turbulence cell the image shift less but breaks up incoherently as it passes through multiple cells. While it varies the typical transition aperture is 300mm so stopping down to below 300mm shifts you from one regime to the other. Regards Andrew

For visual and normal imging yes. For lucky imaging the larger aperture allows shorter exposures and increases your chance of being lucky! Regards Andrew

We all start at zero and go up from there. It is easy to forget how difficult the basics can be when they have become second nature. Regards Andrew

Yes, but still you prostrate yourself at the feet of his creations in pursuit of yours. If that's not masochism what is? Regards Andrew

@JTEC masochism takes many forms and is not restricted to any one branch of our hobby. Regards Andrew

Or as the saying goes "old chemists don't die they just fail to react". Regards Andrew

Masochism. Regards Andrew

Does it have a screw focus Iock on the back? If so I am the Smith on the certificate and it was the first carbon fibre one they made! Very nice scope either way. Regards Andrew

Ok, I have done some tests and research. As @Adam J noted the killer is the nature of the amp glow. So I will have to do sets of darks for each exposure. My results showed there were very small changes in bias +- 2 adu in different sessions including camera and PC booting after power off. Darks were never below Bias with exposures in the range 0, 0.01 0.1,1,2...8,16,64 and 128s and progress linearly. Subtracting pairs of bias or dark frames showed just well behaved noise with no structure across the frame. Apart from the amp glow I found nothing to worry about. Regards Andrew

Don't be fooled the software for controlling a mount is non trivial given all the possibilities in mount orientation and errors, sky pointing - catalogue position v actual position, hardware limits, location on earth and northern v southern hemisphere, etc etc. In many ways the hardware is in comparison trivial. Regards Andrew

Not sure on the chemistry but given modern safety rule, that have banned many garden products, I am sure it will be ok. I kept a roll in an enclosed garage without obvious problems. Regards Andrew

You could do the split thing but as I pointed out you would need a guide module. Doable but adds cost and complexity. Interestingly The Sky X can plate solve directly throught the SA200 as the Source Extractor selects the stars and not the spectra. Robin's Aply200 is a good example of what can be done with a low res slit system. On the lenes you need to consider not only the focal length but also the apertures to avoid vignetting and limiting the field. You need fast camera lenes to obtain enough aperture. Your calculation is correct. My design was for a specific program. All spectrographs really need to be designed for the telescope (scales with the diameter of the scope) targets and intended resolution/measurement. Regards Andrew PS This shows the current field 33' by 25'

Thanks Robin, yes an almost direct copy but with a few extra bits and a faster camera lens to better match the small pixels of the ASI. The 40mm projection eyepiece is ideal for matching the various aperture stops. I have to admit shamelessly stealing your basic design from the link you posted! Expanding the image to read the writing on the lens. I did manage to grab some spectra the other night and I am getting about 7.4 A per pix. The clear filter in the filter wheel allows easy plate solving and the rotator allows aligning the spectra to avoids overlapping other stars so automation is easy so much so I can get packaged software to do it rather than use Python. Regards Andrew

I will investigate all these issues and see what happens then report back. Regards Andrew

Yes thanks, but I did mean dedicated astro cameras specifically ASI 1600 MM Regards Andrew

Thanks @Adam J that gives we something to investigate. When you refer to powering the chip I assume that is via USB as I can run it without the 12v needed for cooling. I don't think amp glow is not an issue for me as I use max 20s exposure and the science image is on a quite small area near the centre of the chip but will test. Regards Andrew

Several threads on here have commented that in effect that bias frames with CMOS cameras can give problems. I have started a series of tests on my ZWO ASI 1600 at -20c. So far the statistics have remained stable and subtracting pairs of bias frames gives a sensible looking histogram. I am keen to uses scaled darks as my observation program requires a wide range of exposures and doing darks for them all is possible by messy. Does anyone have any data on the issue or can point me to any papers which discuss the issue scientifically rather than anecdotally. Thanks Andrew

I accept you are not anti-science Olly. It is often an issue when a too simplistic theory is applied or a good theory applied without an understanding of the assumptions and constraints. The Nyquist criteria is a prime example even in pro circles! What does surprise me is, that given how easy it is, that owners don't characterise their own CCD and CMOS cameras. It is easy to check linearity, gain, read noise, hot and warn, pixels etc. I know you review cameras and if I were doing it I would make these measurement. I assume there is no call for it. Maybe none if this matters in artistic as opposed to scientific imaging. Both, in my view, have a legitimate place in our hobby. However, I find the more I understand the easier things become and the less often I get caught out. Regards Andrew PS I almost said "pretty picture" imaging but that might have sounded derogatory and in no away would I wish to imply that to your and others skill and creative excellence.

My sympathy and recognition. I now tend to walk a session through hour by hour with The Sky X which has a realistic horizon set to avoid any mishaps. Regards Andrew

Here is the data: At -10c and at -20c others found a similar effect see here http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1919 Sometime science does work @ollypenrice Regards Andrew

I suspect, as you do, the increased in thermal noise at -8 is your issue. I have the SX 694 which has the same chip as the Atik 460. I measured the number of hot/warm pixels as a function of temperature by changing the cutoff level for what counts as a hot pixel at each temperature. This showed the chip had in addition to permanently hot pixel a significant number of warm pixels which increased as in number with temperature. I will try to find the data and post it. All darks, bias or real, will add noise however, the more dark frames you use the less it will be. Regards Andrew