andrew s

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

As Merlin 66 says it does. With the case of an just an objective you don't have any option but to refocus to account for the focal shift due to the filter. With a reducer/corrector you have two degrees of freedom. You can adjust the reducer/corrector distance and refocus. This allows you to bring the reducer/corrector to ccd distance back to the design distance and then refocus to account for the focal shift. In the case of the objective only it also cause a shift from the design optical distance and that is why a filter in a converging beam introduces aberrations. As light relief you might like to ponder the following. 101 optics says that for a simple converging lens rays parallel to the optic axis (and close enough to it) go through the focal point. Now why is this so? Well one way to explain it is that all the rays have the same optical path length. The ray through the center of the lens goes through the thickest part of the lens but takes the shortest distance through the air along the optic axis while a ray through the edge of the lens has the least glass to go through but the longest route through the air from the edge of the lens to the focal point. In fact you find t*n + r*1 = constant ( where t is the thickness of glass gone through with n it's refractive index and r the air distance with refractive index~1). If you think about it it also explains why a simple lens has a curved focal surface! This is an example of the Principle of least Action applied to optics, It led Feynman to his formulation of quantum mechanics QED the subject of another post! Regards Andrew PS I am sure you will be all pleased when I get some clear skies and so post here less! Got any free slots at your place Olly? PPS John posted while I was typing but I think I covered the point. I am now worried my diagram will cause you all to do the wrong thing.

I think I understand you now but don't agree with your conclusion. A corrector works by adding a specific optical path difference into the light path to bring the light rays to, say, a flat focal plane at a specific optical depth (distance) from the corrector. As air has a refractive index of ~1 the optical and geometric distances will be the same e.g. your 55mm. If you place another optical element after the corrector e.g. our 3mm filter and as its refractive index will be ~1.4 you no longer have the matching of optical and geometric distance so to get back to the correct optical distance of 55mm from the corrector to ccd you need the geometric distance to be ~ 54mm ( 51 mm of air and 3mm glass as 51*1 + 3*1.4 ~ 55). This effect was use in the past to design very fast F1 Schmidt cameras which were solid glass with a cutout for the film! Regards Andrew

Ken - I am not sure I fully understand your post but I would say that the design distance is most probably the optical depth (geometric distance * refractive index) so if you place a 3 mm filter in the optical path it adds to the optical depth so you need to reduce the geometric distance to compensate. I agree with you earlier post on this. The two issues I discussed above are not mechanical v optical but about two different questions you can ask both of which can have mechanical v optical distance issues. Regards Andrew

Hi John - I think the confusion here is because there are two different case and different definitions of back focus. Case one what does adding a filter do to the focal position if you add a filter without changing the geometric distances- in this case as my diagram show it moves the focal point further away from the objective i.e you add ~ 1/3 thickness to get the focal point. Case two what to do if you place a filter between a corrector and ccd. In this case you what to maintain the optical depth between corrector and ccd and you need to subtract ~ 1/3 thickness (actually t(n-1) where t is the thickness and n is the refractive index of the filter) from the spacing between corrector and ccd. You will also need to refocus. Regards Andrew

Not wishing to complicate things but I have been pondering what one should do if you have a corrector that is required to be a certain distance from the detector and you place a filter between them. If you assume that corrector is designed to have a certain optical path distance ( i.e. geometric distance * refractive index) from the corrector to CCD then if you place a filter of say Xmm thickness and of refractive index n between them, without increasing the geometric distance, it adds X*(n-1) to the optical path length - assuming the index of air to be ~1. So the geometric distance between the corrector and CCD needs to be reduced by this amount does it not so the total optical path length stays at the design distance! Refocusing will also be required. I still stand by what I posted before but for this particular point. Regards Andrew

I knew I had it somewhere - attached from Telescope Opitcs Rutten & van Venrooij - Andrew

I am never clear what adding or subtracting backfocus means. I still believe the glass plate results in the focus being further from the objectve. I attach a sketch that show the effect. While not accurately to scale the key point is the light is refracted towards the normal as the light enters the plate and away when it leaves. This means the light converges less quickly when in the plate. Regards Andrew

If the filter is between the flatner and the chip I would increase the flatner to chip distance to account for the filter as specified above in a previous post.Regards Andrew