andrew s

How can first thoughts take 134 ish posts and 6 pages. It should have been built by now ? Regards Andrew

Springer seem to price all there technical as opposed to popular science books at ridiculous prices. May be we could form a club to get a copy and pass it round. A book tontine! Regards Andrew

Yes, welcome to the fascinating realm that is spectroscopy. The key to the Universe ? Regards Andrew

Many a true word spoken in jest. Nice scope. Regards Andreqw

I looked at this idea some while back. For me the difficulty of collimation and focusing made me stick with a secondary mirror. It can and has been done before. Regards Andrew

You could (given your perma-cloud) simulate this by taking two lines then convoluting them with a seeing functions of various types and see if you can recover them. You might be surprised at how broad some lines can be say in hot A stars where there is considerable pressure broadening - in which case they are very non-Gaussian Regards Andrew

I compare my spectra with reference spectra but you need to filter them to the resolution of your spectra otherwise the fact that lines are resolved in one but blended in the other can cause issues. I tend to use rectified spectra to avoid issues with the continuum. Regards Andrew

Interesting project. If you google "diffraction grating sheet" you may be able to get what you propose making very cheaply. What processing is advisable depends on what you what to measure. While processing may make the line clearer and while the apparent resolution may improve information on line shape maybe be lost. As a rule of thumb I find measuring the wavelength of a single line should be accurate to 1/10 of a resolution element (~lamda/R) and with cross-correlation about 1/100 This depends not only on the resolution but on the accuracy of the wavelength calibration which can be difficult in an objective grating system. However, my experience is with slit and fibre fed spectrographs. With a slit spectrograph you can (in theory) measure the instrument PSF and then use deconvolution to improve you spectra but I don't see how you can do this with an objective system. If you were doing a search for emission star, for example, sharpening might make them "jump out" as in this case it is the discovery that matters not the details. I will be interested to see you results Regards Andrew

All these Newts are far to pretty. This was my work horse for many years but has now passed on. Regards Andrew

If it's too tall for you wee Scots use the video camera! With the VS focused it will be very top heavy. Why not sell it and get a smaller one? Well done a magnificent effort on your part. Regards Andrew

Contacted FLO about my MBox not working 09:41 they had replied offering a replacement by 10:19 and the no cost order placed at 10:18. Oh yes and stamps to return the faulty one. Can't beat that for service. Regards Andrew

I used thin steel rod rather than coins but the principle was the same. When removed they created the gaps! Regards Andrew

Don't blame Jim for the mirror attachment he got it like that. The secondary holder is hollow if I recall correctly so you would have to remove the mirror and add a solid layer to bond it to in the manner proposed. In the past when I researched this area I found a wide range of "best" options all quite different. I have made 5 or 6 Newtonian telescopes with the secondary attached this way without any cool down or optical issues. I would try it as is before fixing a possible problem. I have always found a thin sharp blade .e.g. a craft knife cut the mirror from the holder without any problems. Regards Andrew PS I should have said I used 3 arcs of silicone with gaps between them.

Jim, I don't think the gauge will be an issue if you can get a high tension in the spider vane (hope the welds hold). An additional option is to counterbalance the weight of the mirror and holder to remove the torque. Regards Andrew

I think you will find is that what you are seeing is due to astigmatism. It is common to see this with an RC at the edge of the field even when in focus https://starizona.com/acb/basics/equip_telescopes_ritchey.aspx . From this it may be the collimation and or primary/secondary spacing are out or the mirror set is astigmatic. Sorry but I don't have any practical advice. Regards Andrew

George you need to keep the grating square to the incident light (i.e. the telescope) and then angle the camera to the first order diffraction angle as in your Mk3. Regards Andrew

Nice job looking forward to seeing the results. Is the grating blazed? If it is it would be worth angling the camera to center the first order. This is what I have done on my fiber fed low resolution spectrograph. It has extra lenses as the grating is in a collimated beam it also does not need to be attached to the telescope. Regards Andrew

"Under certain conditions an observer in a stationary or moving reference frame may not have to apply the principle of addition of velocities from the Galilean or Lorentz transformation equations to the propagating sound wave. " Assuming you mean inertial reference frames: What conditions ? Stationary or moving with respect to what? The equivalence of physical laws in inertial frames has been verified by countless experiments do you have an experimental result that contradicts it? Regards Andrew

It is certainly true that light in a medium does not travel at the vacuum speed c and that the relative motion of the medium and the observer effects this. However, I don't think this invalidates special relativity and the use of flat Minkowski space-time. General relativity is much more complex but even in GR nothing can out run light. (It is complex as in general you can't simply compare or add velocities of widely spaced points in strongly curved space-time.) On a cosmological scale simple ideas like distance and time between events is difficult to get to grips with due to the metric expansion of space-time. The best that can normally be done is to consider observers co-moving with the metric expansion (i.e. they see the CMB as homogeneous and isotropic - give or take the minute fluctuations) as a time base and ask how they would measure the distances when their CMB clocks align. Other than that you get (as in special relativity) observer dependent answers. Having said that SR and GR are the best tested theories we have in their domains and I see no reason to give them up. Regards Andrew

Absolutely right. In addition the speed of sound varies even within an inertial frame, it changes literally with the weather. Regards Andrew

Ok as it was my fault for introducing the term optical distance so I will redefine it here as it is now being mixed up with the geometric distance. The geometric distance is the distance you would measure with you ruler, calipers etc. (I am here ignoring the effects of special & general relativity Olly!) The optical distance is the integral (i.e. sum) of the geometric distance multiplied by the refractive index n. So if you have 20mm air (n~1) then 3mm of glass (n ~1.5) then 10mm of air the the geometric distance is 20+3+10 = 33 while optical distance is 20*1 + 3*1.5 +10*1 = 34.5. Don't forget though adding the 3mm of glass instead of 3mm of air also changes the focal length. Normally in optics it is the optical distance ( referred to as the Optical Path Length) that matters but if the medium is air then they are about the same. What was to be my last post on this above gives my best view on the issue of adding a filter to an existing system. I hope this makes it clearer. I have also looked up the design of some reducers/field flateners and as far as I can tell they are designed assuming only air between them and the focal plane. I can see no way they could account for all the specific designs of CCD with there differing distances window & cover glass thicknesses. My conclusion from this would be to not modify the system when adding a filter and see if it is ok in use. If not and it is simple to add a 1mm spacer try that next and only if it gets worse try reducing it or do it the otherway round if it is simpler. I have a strong suspicion we are all dancing on the head of a pin but none of us are angels ! Regards Andrew retired.

I hesitate to post again on this but... ...I agree Astrodon and QSI are in agreement. What they are saying is consistent with the diagram I posted. That is adding a filter pushes the focus out by ~ 1/3 thckness i.e. 1mm for a 3mm filter. This means that camera plus filter consumes 1mm less of geometric backfocus than the camera alone. What to do if you have a corrector/reducer depends on if you want to maintain the geometric distance or the optical distance. If you want to maintain the geometric distance add 1mm as John proposes if you want to maintain the optical distance subtract 1mm and refocus. Given that CCD will have a window and cover plate then the optical distance is probably ill defined anyway! I now retire from this thread. Regards Andrew