angryowl

It’s been a while since I’ve posted on here, so here goes. I’m currently using a RASA 11” with the Baader F2 HighSpeed filters (which I believe to have a FWHM of 10nm) and while they’re fine, I am based in Bortle 8 skies and have found that they pass just too much of the light pollution as well. This means I need rather high integration times to reach an acceptable SNR and even then, with so much LP the integrated masters are usually low in contrast. In doing research for a new set of narrower filters, I’ve discovered that finding filters with a small enough FWHM and a high enough transmissivity is difficult for such fast f ratios due to the centre wavelength shift at those extreme angles. While I believe most of the standard filters with a bandpass between 3 and 6nm for instance would produce an image, from my research I’ve come to believe that the signal loss caused by CWS at the steepest light rays would effectively mean one was no longer taking advantage of the telescope’s full aperture and would now operate at a higher focal ratio. I’ve narrowed it down to Chroma’s 8nm and Astronomik’s MaxFR 6nm filters. I spoke to Chroma and they’ve been very helpful and even sent me spectra graphs of their 3, 5 and 8nm filters at the 12.5-degree incidence angle for the RASA. The 3 and 5nm filters definitely suffer considerable transmissivity loss but the 8nm filter doesn’t look too bad. After seeing the graphs, I was seriously considering buying an 8nm Chroma filter to try out but then I found Astronomik’s new MaxFR range which are specifically designed for very fast optics. On the Astronomik website they claim their 6nm filter has a guaranteed transmissivity of about 90% at F2 and the 12nm one about 85% at F1.4. After speaking with them they confirmed that the filters are preshifted and backed up the transmissivity claims. The thing is I’ve only found about two people using the 12nm MaxFR filters but no reports or comparisons to other filters. However, I haven’t found anything on the 6nm ones. So on to my question, is anyone out there using the 6nm filters? How are you finding them, any issues? I’m not too concerned about reflections but rather performance differences in terms of SNR when compared to other filters such as the Baaders on fast scopes. I’m really tempted to just buy one and drop it in my filterwheel next to the Baader one and compare them during a session, but given their current price I’d like to hear from anyone that is using them before doing that. Any input is greatly appreciated as always

Thanks for the reply Mark I must admit I hadn’t thought of that but now that you mention it, it makes sense. I know I read somewhere that NB filters nowadays use several coatings sometimes more than ten per filter and the idea that one or multiple of these coatings are reacting differently to light emitted from this particular star is indeed interesting. Learn something new everyday...

Hi all, Whilst capturing data over the last clear nights on my latest object, NGC 7822, I noticed a star with an odd profile in the OIII subs but put that down to cosmic rays or some other weird artefact. But after my usual preprocessing, I’m still seeing this strange star in all of the OIII subs and much clearer in the stacked image. The OIII data is a stack of 54 300s subs making for a total of 4.5 hours and similar integration times for Ha and SII. What’s strange is that this only shows up in the OIII data and not the other channels. As can be seen, Ha and SII look completely normal. At this point I’m trying to understand what might cause this behaviour only in one channel. I have thought about things such as collimation, tilt, debris on the corrector plate, tracking/guiding errors, dew, shooting through branches and filter reflections. However, none of these fit as the star in question is roughly in the center of the FOV and there are other stars around it similar in size and brightness (judging from the other two channels) which logically should show the effects as well. These were captured using a Mesu 200, RASA 11”, Atik 414EX and Baader F2 High Speed filters. The subs were dithered and 2X Drizzle integration was used. I also rotate my exposures through the filters every imaging session. No other processing done apart from the usual preprocessing steps and an STF curve. Having a look at the Simbad, 2MASS, Gaia DR2 catalogues reveals the star has designation [D75] 7p (Simbad) and is close in magnitude to the star at the bottom of the images. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=[D75]+7p&submit=SIMBAD+search Can anyone shed some light on what might be happening here? Is this an artefact due to some component in my imaging train or something that I’m missing entirely? As this doesn’t show in any of my other stars in this image, or any other image I’ve taken with this setup before, I’m not concerned about this being an issue with my equipment at this point. I’d simply like to find out what might have caused this out of pure curiosity.

When I tried the OAG lent to me by @souls33k3r I did manage to get the spacing right and guiding gave me pinpoint stars in all exposures. So far the OAG is the only thing I tried that actually works and currently I’m in the process of building a DIY one. The reason I’m not going with a commercial one is that the guide camera body and USB cable get in the light path of the scope and cause weird diffraction spikes whereas with a DIY one, I’m hoping to avoid that. Chip moving inside the camera…wow that’s scary! Glad you got that one sorted as I can see it being hard to solve. Seeing as the images in my OTA and guidecam drift the same direction and amount I’m inclined to believe the guidecam/chip is securely attached and not moving relative to the main OTA. At least not by any significant amount anyway. But a really good suggestion, thanks.

Oh yeah, that I will do given the opportunity. Perhaps when Ahmed gets his one finished he'll let me throw it on top for a quick test ?

Managed to get out during a clear night last week and tried guiding with the refractor piggy backed on top of the RASA and also with the 60mm guider clamped onto the bottom dovetail of the OTA. I also cleaned up the residual adhesive on the tripod head and sanded the bottom plate of the Mesu down which resulted in a much larger contact area with the tripod head. The results are not really what I expected, but a lot better than before and guiding is more consistent. That being said, guiding with either the 60mm guider or the refractor still doesn’t produce round stars. The elongation is much smaller than before and looks like guiding is now actually correcting for most of it, but still not completely. Not sure why this is as the elongation direction and amount are now very similar in both guide and main images. The guided subs are now almost usable, but I’d still very much like to get as close to perfectly round stars and just don’t see this happening with the current guider. Therefore I’ve started designing an OAG using a right angle prism and will be using the bare CMOS board from a QHY5L-II-C camera to avoid obstructing the light path with either the camera body or the USB cable. I have most parts and it’s just a matter of finding a way to mount them. I won’t be troubleshooting this any longer as I’ve spent far too many clear nights on this and I now know that an OAG will certainly solve the problem. I really wanted to get to the bottom of this but it just seems like it’s not practical at the moment. Will update this thread once the OAG is complete Some examples of what elongation looks like now

Hi Jakub Welcome to SGL, you’ll find the very helpful and experienced people here will provide invaluable assistance and guidance when dealing with issues such as yours. Don’t yet have a final word on this, but so far it seems like the PTFE pads were only part of the issue and although I can now guide most of the elongation out, it’s definitely still present in most guided subs. In response to your question, I’ve tried anything from 1 second exposures all the way to a minute in most directions in the sky and no elongation whosoever was present in these short exposures. I find that in subs up to 3-5 minutes I usually see no sign of elongation wherever I’m pointed. That being said, the first thing I’d suggest is rotating your camera 90 degrees to see if the direction of elongation changes by 90 degrees. If it doesn’t, that could point to the camera not being completely orthogonal. This would explain seeing elongation during such short exposures and also remember when pointing at the zenith the gravity would no longer be pulling the camera/imaging train to one side to such an extent and would explain why stars come out fine. Apart from this, we’d need to know a lot more before anyone on here can offer any meaningful advice on troubleshooting this. Please let us know how you get on with this as I’m very much interested in what this’ll turn out to be.

That makes a lot of sense now, but didn't know any better two years ago when I built the tripod. Looked at the EQ8 and saw that the tripod they sell had PTFE pads and the mount is rated at 50kg, I thought adding 0.5mm thick pads on mine would be a safe bet. However, I'm thinking the EQ8 might have more contact area with those pads than my Mesu did which would make a difference.

Hehehe, thanks mate. Don't want to get too excited as I said it could be only part of the issue, but further testing should confirm.

Forgot to mention the subs above were taken with the mirror unlocked.

Thanks for the suggestion.I mounted the guidescope as close to the mount as I could during one of my latest sessions and it drifted in the same direction and the same amount as the main OTA. Turns out this may have been caused by the PTFE pads between the tripod and mount, but further testing is needed to confirm this.

Some more progress on this. Piggy backed the refractor on top of the RASA using a very solid adapter plate made of steel profiles welded together. I’m pretty sure the plate does not introduce any flexure in the mix. Started by guiding the RASA using the refractor and QHY camera. I also guided without sending corrections to the mount and just monitoring the guide star in PHD and this showed the star drifting in the exact same direction as it did in the main OTA. I wasn’t able to get a single sub to guide properly using the refractor as all subs showed substantial drift. I also ran the other test of taking short exposures with both refractor and RASA at 10 minute intervals to see the direction of elongation and amount in both scopes. The direction was always similar in both scopes and the drift amount in arc seconds was always lower in the refractor. At the same time I also had the 60mm guide scope mounted on the bottom dovetail bar of the OTA. This is as close I could get it mounted to the mount plate and still have the full weight of the RASA on the mount. Again I measured the drift which was the same in direction and amount and tried guiding through it with no success. I then moved the Atik camera to the refractor and took some unguided exposures and these also show elongation even at 5 minutes. It was clear that the issue was now pointing at either the mount or tripod. I started looking at the tripod. As I have the two plates on top, separated by six M10 bolts (not the best design I know) I thought I’d eliminate potential flexure coming from the bolts by sandwiching some washers between the two plates and tightening everything up. Although I tried this some months ago, I also placed some large wooden boards underneath the 70mm diameter tripod legs as I was thinking the legs may be sinking into the ground due to the additional weight of the OTA. Neither of these made any difference and the drift was very much the same. I took things inside and securely attached a small laser module on the mount head plate with the RASA mounted and balanced. Then I pointed in some positions I know the drift occurs and marked the laser position accurately as it was projected on a wall. After about 30 minutes, in almost all positions I tried, there was a relatively small but visible movement in the laser position. I started pressing and tugging on different parts of the mount and tripod and to my surprise the only thing that moved the laser point on the wall considerably was me squeezing the bottom plate of the mount to the top plate of the tripod. The only thing that would allow this compression is the 0.5mm thick PTFE self adhesive pads I had on. Upon inspection of the mount bottom plate and PTFE pads it was clear that the contact area of the mount plate is actually rather small only around the edge as can be seen in the images. Plus, when inspecting the PTFE pads, the self adhesive glue was almost completely dry and I was able to remove them easily with a thin blade. Additionally, it looks like when the adhesive dried the pads started lifting up slightly in places. After taking the pads off, I remounted the RASA and pointed the laser in the same positions only to find that even after an hour the position of the laser didn’t move at all. It was clear for a few hours last night so thought I’d test again and I only had time to take a few subs before the clouds rolled in. I still observed some elongation in unguided subs, but mainly when pointing West and the good news is I was able to easily guide it out using the 60mm guide scope mounted on top of the OTA. Also my PHD RMS numbers are higher now at about 0.7” compared to the 0.3” I was getting constantly when guiding before and still getting the drift. I sort of rushed things last night getting out so didn’t have time to completely remove the residual adhesive on the tripod head and I think I’ll also look at sanding the mount bottom plate so it has more contact area with the tripod plate. As it stands now, it seems like the issue could have been the PTFE pads all along. I added them when I built the tripod to allow for easy azimuth adjustment when polar aligning, but during PA last night I didn’t observe any difficulties or significant stiction when making adjustments. I’ll do more testing after cleaning the glue and levelling the mount plate but hoping that the ageing adhesive combined with the low contact area were the root of my problems. The PTFE pads Mount bottom plate Adhesive residue Unguided/guided 600s exoposures pointing West Unguided/guided 600s exoposures pointing SE

This is how I see the drift moving: Pointing South elevation 50 degrees E side of pier – both RASA and guider drift east similarly Pointing South elevation 52 degrees W side of pier – both RASA and guider drift west (although RASA drifts up in Declination a bit too) Pointing SE elevation 44 degrees W side of pier – RASA drifts mainly north and guider drifts NE Pointing West elevation 40 degrees W side of pier – RASA drifts north and guider drifts NW Not sure what to make of this as it could point to differential flexure or flexure in the OTA when guiding, but still doesn’t explain the drift during unguided exposures. What’s interesting is that the guider most times drifts roughly in the same direction as the main scope. Don’t know what else I could do to eliminate the mount as the test with Dave’s refractor proves that the mount can definitely produce good results unguided for 10 minute exposures. At this point I honestly don’t see the mount being the culprit here. Also, the arc shaped stars seen in one of my earlier tests and this time in the 5 minute shot pointing near Polaris are what concern me the most. @michael8554 – Thing is I too never had to search for a guide star and never needed to adjust them so the guider screws are screwed in as far as they would go effectively squishing the Teflon at the tips. But you’re right, even so flexure could still be introduced by these and I'm not ruling it out.

It just occured to me that I've not added any images of my setup so here are a few. I've removed the motorised focuser seen on the back of the RASA a few months ago as I thought that may have been causing the elongated stars due to radial imbalance. Also the 3d printed parts on the guider are there as I wanted the flip mirror functionality, but a few months ago I took them off and replaced them with the metal parts that came with the guidescope as I thought I couldn't guide properly due to flexure in the PLA parts. This wasn't the case and I got exactly the same results and graphs with the same amount of elongation in the main imaging camera.

Sunday night was clear but just managed to find the time today to sort through the images and results. apologies in advance for the lengthy post. With @Davey-T's refractor mounted on the mount, I monitored both servo motors when slewing and foung the average current draw to be 0.2A for both Ra and Dec. The values varied a bit when seen on the ServoConfig screen, with the minimum being 0.1A and manimum 0.5A. With the RASA mounted the current draw was pretty much identical with an average of 0.2A for both axes and never exceeding 0.5A. After slewing around for a bit I touched the servos and both were just really cold. Next some guided and unguided shots with the RASA and guider. The Atik was oriented as follows while on the East side of pier: South/up, East/Right And on the West side of pier: North/Up, West/Right The QHY5 was oriented as follows while on the East side of pier: North/up, East/Right And on the West side of pier: South/Up, West/Right Pointing South elevation 50 degrees, scope on E side of pier (600s exposures, first unguided, second guided) - forgot to lock the mirror during these shots Guiding enabled and disabled graphs Pointing South elevation 50 degrees, scope on E side of pier (5s exposures unguided 10 minutes interval) - forgot to lock the mirror during these shots RASA Guider Pointing South elevation 52 degrees, scope on W side of pier (600s exposures, first unguided, second guided) - since a meridian flip was done, both guide and main image are now flipped 180 and follow the orientation listed above Guiding enabled and disabled graphs Pointing SE elevation 44 degrees, scope on W side of pier (600s exposures, first unguided, second guided) Guiding enabled and disabled graphs Pointing West elevation 40 degrees, scope on W side of pier (600s exposures, first unguided, second guided) Guiding enabled and disabled graphs Pointing West elevation 40 degrees, scope on W side of pier (5s exposures unguided 10 minutes interval) RASA Guider Next with the scope pointing at the Zenith I racked the focuser up and down to try and measure the backlash in arc seconds. I did this from both directions, clockwise and anti clockwise several times. I measured an average movement of 64 pixels in Dec and around 7 pixels in Ra. Ignoring the Ra movement and using my image scale of 2.15 arc seconds per pixel this gives a total of 137 arc seconds. Looking at values others with SCT scopes have, I get conflicting results. Some sources say up to 30” is considered acceptable whilst others say up to 100” or even more is normal. Dave, I tried to do what you suggested in your earlier post, but as I said before I never saw any elongation during exposures lower than 180 seconds so I ran the tests you suggested, but using 60, 300 and 600 second exposures. Pointing South 20 degrees elevation (can’t go lower than this) exposures are 60s, 300s, 600s Pointing North 32 degrees elevation exposures are 300s, 300s, 600s Not sure what to make of the arc-shaped stars in the first 300s exposure near Polaris. As a final test I proceeded to point the scope west and run 3, 4, 5 and 10 minute exposures consecutively with the mirror unlocked and then locked to see where elongation really starts showing. Looks like in both cases elongation becomes apparent around the 5 minute mark.

Appreciate the reply Steve. Just though I’d check with you first as Celestron might have had some tests they wanted me to run. I can understand that there’s not enough information and other potential causes eliminated at the moment to be able to say the OTA is to blame here. That’s not coming off as being evasive and I suppose I’ll have to go through more tests to eliminate as many things as I can and hopefully I’ll be able to do this some night this week. Will post results once I get some clear skies.

Thanks for the suggestion and I did consider this at one point but because the dual chip self-guide models available are physically large they would end up obstructing more of the light path than I’d like.

@FLO – Any response from Celestron? Is this considered normal behaviour in the RASA? If not any suggestions for any tests I can perform or do they need more info? If this is normal and just regular SCT behaviour, then I suppose the only option is to go the OAG route and try to find the smallest camera I can and figure out a way to mount it all so that it doesn’t cut into the light path. With a bit of DIY this just might be doable.

Was planning on having an imaging session last night as the forecast looked promising, but the conditions turned out to be very different so not had a chance to test anything else yet. In addition to the tests and checks suggested in the thread, been thinking of a few more: With the refractor mounted, check the current draw on both RA and DEC servo motors in the SiTech controller. If the mount is struggling with imbalance or the weight once the RASA is mounted, hopefully this’ll show up. The mount has a very distinct sound to it whilst tracking and I’ll also keep an ear close to the mount during a few subs for any other sounds, like the servo struggling or slipping. As the elongation shows up in both RA and DEC then it seems logical that both axes would have to slip at the same time which I find hard to believe. With the mount tracking and scope pointing near the zenith, I’ll rack the focuser up and down and take exposures to try and measure how much the mirror shift is in arc seconds. Because the scope will be pointing up where the elongation never shows, I’ll be looking purely at how much the mirror is free to move in its carriage whilst racking the focuser up and down. Align the guide camera to the same orientation as the camera on the RASA and point scope in different positions in the sky. As I can’t take long exposures on the guide camera due to having no filter in, I’ll simply take some short exposures on both cameras every 10 minutes and check if they both drift in the same direction. I did this before, and although the cameras didn’t have the same orientation, I remember working out the drift was in the same direction on both. If this gets me nowhere, I’ll put together a metal mounting plate and piggy back Dave’s refractor on the RASA as he suggested earlier. Another test I’m thinking of is pointing the RASA in a position elongation occurs, start tracking unguided, take a 10 minute sub and stop tracking. I’ll let both mount and scope just sit there for an hour or so, then without touching anything, start tracking and take another exposure. If there’s anything moving inside the OTA, then as @Astrobits puts it, it should stop moving in that hour and the second exposure should show no elongation. Would an hour be enough or should I wait more? I’ll also run another test to see exactly at which point stars start to elongate. I’ll start at 30s and work my way up 30s at a time. All of the tests above will be done with the mirror locks fully engaged. Looking at most images taken with the RASA on AstroBin, and I’ve found only a handful of images using 5 minute exposure times but the norm is 3 minutes. The 5 minute ones on AB show no sign of elongation whatsoever and although my tests were done at 10 minutes, I can definitely see elongation even in 5 minute subs. I can get away with 3 minute guided subs, but I’d like to go longer than that with narrowband due to the read noise of my Atik 414EX being 5-6 electrons.

I was simply referring to the common SCT mirror backlash/slop while turning the focuser in and out. This shows what I mean better. The mirror shift then is more than a few arc seconds I would image, yet the telescope is still in collimation. I was replying to what you said in regards to the movement visible in my four 300 second shots should have resulted in an out of collimation situation if only one component such as the mirror moved. No observatory unfortunately, just a DIY tripod on top of which the Mesu is mounted every night. Just as a side note, there’s no chance of the tripod legs slipping into the ground as they’re 70 mm diameter steel plates. Some good advice regarding the mount and food for thought. The Mesu is a friction drive mount so is always engaged but you are right, there could always be slippage somewhere even with this design I suppose.

Dave, keeping a constant eye on the forecast and when I get a clear patch of sky, I’ll give the tests you mentioned a go. The mount also has a feature that lets you see the amount of current motors on each axis are actively pulling from the controller. Next time I mount the heavier RASA on the mount I’ll keep a note of these values and compare with others I’ve seen that are considered normal.

Not able to do that Dave as I don't have the means to mount it properly on top. Could try a DIY solution of mounting it, but I fear it wouldn’t be sturdy enough and might introduce flexure which will affect the results.

From what I understand, I’m not sure the scope can come out of collimation so easily. For instance, the backlash in the focusing mechanism is there due to not being able to have the tightest tolerances for the shaft that the main mirror slides on and this small gap is filled with grease. When focusing and looking at a star and going through the entire backlash I can definitely say that it shifts the star’s position more than the movement seen during those four 300 second exposures you’re referring to. Does that mean that the scope should be out of collimation after focusing or reversing focuser position? If I’m completely wrong here and it’s something in the mount itself then how did the refractor worked so well on the mount prior to mounting on the RASA? I balanced both scopes equally well, I know the Mesu is more than capable of handling the weight of the OTA. I didn’t see anything out of the ordinary happening with the mount when the RASA was mounted after the refractor, I’m just at a loss as to what it could be if it’s something with the mount.

Well for one I didn’t consistently monitor one particular object or position in the sky for hours on end. I never really spent more per night on an object than say three hours during which elongation always occurred. Then two, if we’re entertaining the theory others have suggested here, that something is loose and moving inside the OTA like the mirror, then are we not talking about minute movements of the component here? Like arcseconds of movements in those elongations which in turn wouldn’t translate into a large measurable movement in the component itself? That way, at the RASA’s focal length, a millimetre of movement/slop in the mirror or any other component, could translate to several hours of movement visible in the stars? I could be wrong here, but this is how I see this play out. I welcome any suggestions, like looking at the drive train of the mount again. But what exactly do you mean by that? As I’ve said previously, the mount is rock solid and I could not induce any movement anywhere including the front saddle. Balance has been checked so many times and always found to be spot on so the mount couldn’t be slipping in any direction. I now know the mount tracks well as it can produce pinpoint round stars at 900mm focal length at 600 second exposures.

This makes sense and logically thinking if we're talking about a sloping component it should eventually stop. Immediately after the telescope is pointed in a new position the trailing begins in the direction it wants as that depends on the position of the OTA. The trailing or slopping does indeed seem very regular and one would think that if something were sloping, over time, in a few minutes or an hour at most say, it should sort of stop. But through all my tests it didn’t.