symmetal

Getting the field flattener back-focus spacing correct is more critical the larger the sensor. For the small sensor 178 getting the spacing within a few mm would be sufficient but for the 1600 it needs to be set more accurately. Incorrect spacing will produce increasing star distortions the further their distance away from the centre of the image, like you've shown.

As Michael says your original images don't seem to show such distortion, though their small size may be masking it.

Alan

2 hours ago, 04Stefan07 said:

Ah, so thats what the flat darks fix in the images! Thank you

No problem. 🙂

Though it's the flats that correct the vignetting in the image. The flat darks reduce the extra noise introduced by using the flats during calibration. 😊

Alan

That's vignetting which is quite normal on optical systems. Using the smaller sensor size of the 178 it was outside the area covered by the camera so wasn't noticeable. The larger sensor on the 1600 shows it up.

To correct for it you need to take some flats and associated flat darks and use them in calibration as mentioned here.

Alan

1 minute ago, tooth_dr said:

I've not really noticed this statistic on a scope before, not sure what it means.

It could just be the distance an object at infinity would be brought to focus without any correctors etc. in the way. The corrector back focus spacing is the critical distance to maintain, the scope back focus would then be modified by the corrector anyway, with the focuser itself correcting for the modified scope back focus. As long as you can focus the object then the scope back focus distance is met by default.

Alan

What might be useful Alan is to obtain one of the desiccant tubes if you can, which were included with early 1600 cameras, and maybe others. They were dropped from later camera shipments as they weren't too successful. In reality two dry desiccant tablets in the tube sharing the enclosed air with the four saturated tablets in the camera meant at best they would all end up at 66% saturation.

If the tube was left permanently attached to the camera you could change the two tablets in the tube every month or so, or before they all ended up saturated, and repeated changes should then bring the internal humidity down each time.

ASI071 cameras do have the tube access port sealed with a screw and neoprene ring, on the side of the camera under the plastic sticker, so the desiccant tube will screw on.

Maybe asking on 'Wanted' whether anyone has one they don't need may be successful. I looked on ebay, but none were advertised apart from being included with complete camera packages.

Here's the tube components and the port location on the camera. FLO do sell packs of spare tablets.

Alan

20 hours ago, Mike73 said:

Definitely worth seeing full size!

What a beautiful image, worth all the hard work congratulations. 🙂

20 hours ago, carastro said:

Some excellent work there.  I like the orange and blue SHO version best.

Carole 

18 hours ago, tooth_dr said:

Very high quality work Alan, definitely click and zoom in to get most out of the superb images

Thanks @Mike73 , @carastro , @tooth_dr and others. 😀.

Yes, there's a lot of extra details that come out in the full size version which also isn't so noticeable in the HaRGB versions. I agree Carole that the orange version enhances the details more. 🙂

Just a pity about the OIII halos though I suppose I'm stuck with them unless buy an even more costly filter. 🙁 I thought £421 for the Astronomik was enough. 😬

Alan

This is a good explanation to using wavelets in Registax.

Here is a good tutotial on planetary imaging using Autostakkert, Registax and WinJuPos.

Alan

Here's my attempt of the Sadr Region with 5 hrs of Ha and 3.7 hrs each of SII and OIII using an ASI6200MM with Astronomik 6nm filters and FLT98. Ha was taken in April with the rest in July without full astro dark though I made sure the Moon wasn't around.

Stacked in Astro Pixel Processor, and processed with Startools binned 50%, with final touch-up in PS. The new Spatially Variant PSF deconvolution module in Startools does a good job in reconstructing the star shapes over the whole image. The OIII has some halos which are non-concentric the further from the image centre but aren't too bad when looking at the image as a whole. The halos do dominate the bright star colour so I didn't process the star colours to mono as it wouldn't have made a big difference and blue or yellow stars don't look out of place even if they aren't the correct colour. 🙂 

The Inchworm Cluster above Sadr stands out nicely. Click for full size images.

For the colour matrixing

Top SHO has Red = 40%SII + 60%Ha, Green = 40%Ha + 60% OIII, Blue = 100% OIII.

Bottom OHS has Red = 100% OIII, Green = 50%Ha + 50%OIII, Blue = 50%SII + 50%Ha

Alan

Well done.  It's certainly very colourful. 😊

Alan

7 hours ago, tooth_dr said:

Thanks again Alan. Going to take time to read this properly later. The Tak doesn’t have a built-in internal flattener, it’s screwed into the focus tube but is removed for collimation.  It’s spacing is in fact is very critical, with a quoted spacing of 55.2mm required! 

I'll have to retract the earlier Oops. 🤭 I thought it was like the Newtonian equivilant of their refractors. All the other info should still apply though. 🙂

Alan

2 hours ago, tooth_dr said:

The camera is screwed onto the flattener, which in turn is screwed into the focuser tube.  Is it possible the camera chip could be tilted?

That's often the case, but I've found in that situation with a flattener you end up with the star shapes being different from one corner to another and altering the spacing distance you can make one corner or side look good, but the other corner/side has elongated or other misshapen stars. You aren't able to get all four corners looking similar.

It's hard to tell exactly from your images but your star shapes are fairly round in all corners and not too misshapen, just well out of focus on one side compared to the other which is what made me think the tilt is not between the flattener and the sensor.

Oops, I've just noticed your using the Tak Epsilon 180ED which doesn't have a field flattener as such as it's corrected internally for a flat field, so camera spacing is not critical I would have thought as long as you can focus. This is why your stars have the same shape across the image which is good. The focus difference does indicate that the camera sensor is not square on to the optical axis so tilt correction is required.

2 hours ago, tooth_dr said:

Is it possible to adjust the front plate of the QHY268M, I thought it was but cant seem to find any info.

From pictures it looks like the front plate of the camera is adjustable for tilt by the three sets of push/pull screws around the front plate. These are difficult or impossible to adjust without removing the camera each time. Check that the tilt plate is parallel to the camera body and tight and hasn't got some tilt set by mistake.

There seems to be two different front plates on the images for the camera on QHYs website. The first one has what look to be tilt correction screws as I've indicated, while the second has three knurled screws which may just do camera rotation. Which one do you have or are both included?

I should have mentioned on the CCDI 3D plot the curvature shown in the plot doesn't mean the sensor appears to have that curvature compared to a flat plane. The best focus/shape stars are the lowest Z value so at the bottom of the plot, coloured black, while poorer stars are plotted with an increasing Z value, and brighter colour, depending on how bad they are.  Stars in front or behind the focus plane due to tilt will give an increasing Z value hence the bowl shape plots you tend to get.

Also the line just away from the centre cross on the CCDI curvature plot is an indication of collimation error, the further from the centre the worse it is. For my refractors this line coincides with the centre cross or is a pixel away. Yours is showing a larger error which may be significant. Here's the CCDI manual page on collimation.

Quote

Precise Collimation

Unlike collimation procedures of the past, CCD Inspector provides a revolutionary new way to collimate a compound-optics telescope.

Collimation makes a huge difference in the quality of image and resolution that can be achieved. With CCDInspector, a collimation error of 10 arcseconds can produce as much as 1 arcseconds increase in FWHM of a star. This means that a good 3.0 arcsecond FWHM image can become a 2.0 arcsecond image with proper collimation! 

By measuring the exact displacement of the optical center from the physical center of the imaging configuration, CCDInspector is capable of detecting the smallest collimation errors with your CCD still attached to the telescope, and with telescope well focused! This is the best possible way to collimate, since:

•  The optical train is not disturbed by removing an eyepiece and replacing the camera after collimation
   
•  Focus position will need only minor adjustments to get to best focus after collimation is completed
   
•  What's more, the collimation can occur right on, or very near-by to the field you will be imaging. This may be the best way to collimate a telescopes with significant mirror flop
   
•  Since collimation is done on hundreds of stars, there's no need to re-center anything after adjusting collimation: just take the next image, and keep adjusting.

With CCDInspector version 2.1.0, we introduce another breakthrough innovation: Single Defocused Star Collimation method. This method will help you achieve the same spectacular results as with the original collimation routine, but using only one bright star centered in the field of view.

 

Differences between Multi-Star and Single Defocused Star collimation methods

• Multi-star method can be used in perfect focus. This is beneficial when the focuser mechanism can introduce changes in the collimation of the system, or if you want to do a quick collimation check without losing focus
• Single-star method can be used with a single bright star. There's always a bright star somewhere in the sky, but sometimes, there is not a rich star cluster, such as is required by the multi-star method
• Multi-star method doesn't require re-centering after making collimation adjustments, Single-star method does
• Single-star method is not as sensitive to tracking errors because a bright defocused star can be used with 1 second or less exposure. Multi-star method can require much longer exposures that are subject to tracking errors

Which method should I use?

That depends on what's most convenient at the time. Decide based the differences listed above. Either method will help you achieve excellent collimation.

[NOTE: CCDInspector also provides a simplified Collimation Viewer display that is even easier to use for real-time collimation]

To facilitate achieving perfect collimation, CCD Inspector provides two sets of crosshair on the screen: the larger one marking the physical center of the chip, and the smaller one marking the current optical center of collimation. By making collimation adjustments to move the small crosshair to the physical center of the chip, the best collimation is achieved. To help, CCD Inspector also provides a numeric reading at the top left of the Curvature Map Viewer window that shows the distance between the two crosshair in arcseconds or pixels (based on the choice made in the main CCD Inspector window).

The procedure to adjust collimation is fairly simple:

1. Find a reasonably crowded star field of reasonably evenly-spread stars, with no extremely bright stars in the field of view. Just point somewhere near the Milky Way and you'll likely see many hundreds of stars in one shot.

2. Start CCD Inspector, and open the Real-Time Curvature Map window from Real-Time/Curvature Map... menu.

3. Start taking exposures using the camera's main chip. The following are some guidelines, actual settings will be different for each individual setup:

•  30 to 60 seconds exposure should be sufficient for best S/N and for collecting enough stars to measure field curvature.  A longer exposure may be necessary with really long focal length telescopes, or if a well-populated star field is not available.  The goal is to see at least 100 or more reasonably bright, but not saturated or bloomed, stars in the shot. More stars are better. A large concentrated star collection anywhere in the image will distort the measurement (such as a globular cluster, for example).
   
•  Bin the chip 1x1 for best results with shorter focal length systems. FWHM of an average star in the field should be around 2 pixels or more. If it's less, the FWHM measurement will not be as precise, resulting in a less sensitive curvature computation. If the seeing is exceptionally good or the image is really undersampled, it would be desirable to defocus the image a bit to achieve the minimum of 2 pixels FWHM.
   
• If your image scale is 0.6 arcsecs/pix
   
•  CCD Inspector will detect conditions when there aren't enough stars in the field of view, or they are not evenly spread out, and will display a message indicating this may not result in an accurate measurement. If you receive this message, it's usually best to stop, and adjust the parameters to get more stars in the field of view (increase exposure, or move the telescope).

4. After each exposure, check the curvature map to see how much, and in what direction to adjust collimation.  Just like in standard collimation techniques, the closer you get to perfect, the smaller the adjustments needed.

5. Make an adjustment, take the next image, and repeat with step 4. Keep doing this until the crosshair at the center overlap, and the error is indicated as only a few arcseconds. At this point, you're in excellent collimation!

Some additional hints to help with collimation:

•    To help see the direction and distance of the collimation error, right click on the field curvature map. From the pop-up menu , select Magnify Crosshair

You can also zoom-in further from the pop-up menu to see the error magnified. You may need to scroll the image to the center to see the crosshair, if the zoomed-in image does not fit completely on the screen.

•  Remove as much  tilt from the camera as possible to achieve best collimation. Use screw-in connectors exclusively, and if your focuser has collimation adjustments, use them to square the camera to the optical train. You can use the CCD Inspector Tilt measurements in the real-time curvature map view to help with doing this.
   
•  Make sure the telescope is cooled down and equalized with ambient air: if not, you will see strange artifacts in the curvature maps that are due to air currents as the OTA cools.

Alan

Hi Adam,

Sorry, only just noticed this thread. I've run them through CCDI and all three indicate significant tilt top to bottom. Either the top or bottom edge stars are out of focus depending on the image orientation. The star shapes themselves don't look too different top to bottom so I would say the spacing distance is consistant top to bottom, it's primarily a focus issue which would imply the main tilt is before the flattener, as if the focuser is drooping. Here are the CCI images. I had to rotate some of the 3D images about the x axis to get them to fit in the window. The absolute values of FWHM shown are incorrect as CCDI made a guess on image scale, as it couldn't get it from a fits header. The relative values give a good indication. I have no idea either what the percentage (compared to what?) tilt values mean either as the third image has over 100% tilt  Again just treat it as an indication. Tilt below 10% is generally a reasonable value.

As I've also found when using autofocus, if there's tilt it tends to determine the best focus is near an edge which makes the difference in focus across the image more pronounced.

Veil

NGC281

IC5146

Alan

Hi Luca,

Don't worry, there's nothing to fix. This is a feature of One Shot Colour cameras which includes DSLRs to get a colour image. All the camera sensors are monochrome so to get colour, coloured filters are placed in front of the pixels in what's called a Bayer Filter Array. That's what you're seeing as the white dots. The software you use to process the image should have options to de-bayer the captured image to create the actual colour image. 🙂

Alan

For the inaccessible screws you could cut a spare allen key down so that the short side will fit under the focuser shaft body, assuming the allen key hex cross section is still present at the cut down end. 🙂

Alan

As John says it will be the ones around the base. The large screw A in the diagram above is the 'pull' screw and clamps the focuser to the body like any fixing screw. The grub screw B next to A pushes the focuser away from the body at that point.

You start off with no tilt adjustment by having all 3 A screws tight and all 3 grub screws B loose or lightly tight, so B screws do nothing. That just clamps the focuser 'flat' against the body.

If you want to apply tilt then initially with all A screws tight screw in the B screws until they are also tight. Don't overdo it though. Then for the location of the focuser you want to apply some tilt to, loosen off the corresponding A screw and screw in the B grub screw by the amount of tilt to apply. Say a quarter or half a turn to start with. This pushes the focuser away at that point. Then tighten up the A screw to pull the focuser tight again but it's still pushed away by the grub screw B from lying flush against the body.

To save working out which screw you need to adjust by analysing the images, just give each tilt screw pair a large adjustment, say a full turn of B and see what effect it has on the image. Reset for no tilt, and then repeat with the other pairs of screws. This gives an idea of what each does to the image.

As you say some screws are inaccessible which seems to be a common feature of basic tilt adjusters. Those push/pull adjusters you can buy to put in the image train often end up with the adjustment screws too close to the filter wheel to allow adjustment without some disassembly. Zwo cameras with tilt plates on the front can be impossible to reach if the camera is fixed to the filter wheel by 3 screws rather than the T2 or 48mm thread etc. Hopefully the adjustment you want is using the accessible screws. 🙂

Alan

Excellent result with lots of fine structural details. 😊

Alan

No problem blinky. 🙂 If you think it's just the camera and you can get the tilt adjuster fitted in the imaging chain the quick way is to just give a large tilt adjustment on one of the screws and see the effect on the image. Likewise repeat with the other two screws in turn on their own. You hopefully will find one, makes a good improvement in the necessary corner and you can then fine tune it if you can.  When you've tightened up the grub screw on the section you've adjusted the large screw should end up flat with the tilt adjuster face so shouldn't push against the filter wheel case.

It would be handy if there was the option of getting these adjusters with the screws on the other side so that they can be more accessible in some situations.

Alan

Your latest image does look like the corners are better though there is trailing in the centre stars so your alignment or tracking was out during the exposure which has affected all the image giving trails in one direction making it look worse. The bottom left though looks worse as you say.

Determining what corner of the image corresponds to a physical point on the camera body requires some mental gymnastics. It's best to start with the long image axis being parallel to RA direction. When the camera is pointing roughly to the South the camera writing on the rear is either the right way up or upside down depending on which side of the meridian you are. Choose the side of the Meridian where the image you capture is the same orientation as if you were looking at the target visually. This is for a refractor without a diagonal as usually used for imaging.

However some capture/display software may flip the image vertically for display. (reflection about the horizontal axis) I believe it's whether they adhere strictly or not to the fits protocol where coordinate 0,0 is at the bottom left for a graphical image type display. Camera image coordinate 0,0 is at the top left. Astroart and Fits Liberator do flip the image while SGP which I use doesn't. I suspect most don't flip the image but you need to check.

Also the orientation of the writing on the camera is no guarantee of what is the image top. I have several Zwo cameras, and for the same image orientation some models have the writing the right way up while others have it upside down. I can't remember which way up the 1600 writing is.

If you always have your camera in a fixed rotation position for imaging you can try adjusting tilt with the tilt corrector. If you often rotate the camera orientation then you need to determine where in the image train the tilt occurs and fix it there, as the tilt corrector may then only fix it in one orientation. It often is a sloppy focuser which flops to one side when imaging near the Meridian as the scope is then on its side and the focuser tension screws which are now on the side don't inhibit up/down focuser movement. Check this is all tight with no movement before proceeding with tilt correction. If in all orientations of the scope, including both sides of the Meridian the images look the same with the star errors in the same corners you can use the tilt corrector.

Having taken a test image in a southerly direction, see if the image matches the view in Stellarium which is what you would see visually. If it is, or is rotated 180 then your capture program/viewer hasn't flipped it which is good. Choose the side of the Meridian for testing where the image is visually the right way up.

Now the bottom left of the image corresponds to the top right of the camera looking from behind the camera at the target. The image is naturally rotated 180 degrees through the imaging train, for a refractor without a diagonal.

As you've found the low cost tilt correctors are awkward to use with the screws, often inaccessible without dismantling. On one of mine I've managed to put the required FF spacing spacer between the tilt corrector and the filter wheel just allowing access to the screws with an allen key. These push/pull screw tilt adjusters are awkward to use and it's difficult to achieve a repeatable screw setting.

I've ended up using one of these Gerd Neumann M48 tilt adjusters on one of my setups as they are much easier to adjust with just three screws on the outside rim of the adjuster which enables fine repeatable adjustment as the plates are pushed together by a strong spring pressure so the two screw push/pull arrangement isn't needed. However they are rather pricey particularly the larger one.

This lengthy diatribe shows that tilt adjustment isn't straightforward and may lead to frustration. 😬

Alan

You've measured from the front of the OAG to the front of the camera as your main measurement. It should be from the rear flat plate of the FF to the front of the camera. There looks to be a gap of 3mm or so between the FF rear and the OAG front due to the threaded section on the front of the OAG and a bit of exposed thread on the FF rear section as the OAG thread is not very deep. This is part of your FF spacing distance so needs to be included in your main measurement.

Add the 6.5mm camera 'back focus' to this main measurement to get your final physical spacing (1). Add your 1mm or so filter/window distance correction to the 56.8mm specified for the FF to get the actual back focus distance spacing needed (2). The value (2) - (1) is then the setting you want to set on the FF adjustment.

There does look to be some tilt on your test image with the top left OK and the other 3 corners showing noticeable spread of the stars. You may find after resetting the spacing distance as I just mentioned that the 3 bad corners are better and the top left is worse due to tilt. See how you go with another test image. Correcting tilt is another whole can of worms.

Alan

1 hour ago, Shimonu said:

Are you saying the FF might need to be set differently depending on if you have  DSLR or astro camera? I've just got a mono camera and wondering if I should set my Flat73 differently. The WO instructions feel a bit unclear.

The quoted FF back focus distance is the same, no matter what type of camera is used, it's the distance where the FF will present a flat focus field for telescope/FF combination. The camera sensor needs to be placed at that distance. (Note the effect of filters etc. on this distance below.) The FF instructions example for setting up the FF back focus tend to assume it's a DSLR hence the FF adjustment quoted being the extra required on top of the DSLR with T-Adapter 'back focus' of 55mm to achieve the required FF back focus distance of 56.8mm.

Astro cameras have a smaller front plate to sensor distance, also confusingly termed 'back focus', to allow filter wheels and/or tilt adjusters etc to be included in the camera train and ensure the total optical distance of these items is less than the FF back focus. The difference then being made up with T2 spacers of the appropriate length. Having adjustable field flatteners means these extra T2 spacers can be standard lengths, and the FF adjustment can conveniently make up the difference to get the exact FF distance.

In the reply to @blinky I forgot to mention that if filters, or any extra glass elements, are present between the FF rear and the sensor, approximately one third the total thickness of the glass elements should be added to the quoted FF back focus distance for calculations. The sensor protection window also needs to be included as a glass element so for example, if a 2mm thick filter and a 1.5mm thick sensor window are present around (2.0 + 1.5) / 3 or 1.2mm needs to be added to the 56.8mm FF back focus. In this case the FF back focus used for calculations should be 58.0mm. This is due to the difference in refractive index between air and glass causing the light path to be refracted when passing through the glass/air boundaries.

For a smaller sensor this difference may not make any noticeable image difference, but for larger sensors it needs to be taken into account to avoid distorted stars towards the corners of the image.

Alan

The FF back focus distance is 56.8mm. A DSLR has a front plate to sensor distance of 44mm, The DSLR T-ring adapter has a thickness of 11mm so the 1.8mm added on the FF adjustment totals 56.8mm.

The 1600 has a front plate to sensor distance of 6.5mm (this distance is also called back-focus hence the confusion that arises.) The filter wheel is most likely 20mm so your total distance at the moment is 6.5 + 20 + 9 = 35.5mm. To reach 56.8mm you need to add an extra 21.3mm. The FF adjustment may only go up to 15mm, so a 10 or 15mm extension T-ring needs to be added and the remainder of the 21.3 set on the FF adjustment.

Alan

If you have a field flattener in the image train you can get this effect with incorrect flattener sensor spacing distance, generally this effect is with the distance too great.

6 minutes ago, Rustang said:

Camera is definitely secure. If it happens to be the pola aligment thats out (i cant see how though) could that cause it even with guiding?

Yes if the polar alignment is significantly out, you will get visible field rotation in the image and it will rotate around the guiding star.

Alan

Your FOV (1.25 deg x 0.94 deg) is considered as being the longest side of the image for non-square images so the H17 database would be fine. As indicated on the useability chart the H17 works at FOV greater than 0.9 degree which still covers your short side image FOV.

Alan

Glad it's all working fine now Pete. 😀 It's possible the Pi power supply was giving slightly over 5V leading to the Pi itself running a bit warmer.

Having no switch mode power supply powered from a 2-pin mains plug in the system means there won't be any AC leakage tingles so earthing the Nevada isn't required for that, but it's beneficial having exposed metalwork like your mount earthed in case of any mains mishaps, so there's certainly no harm in bonding the Nevada -ve to the Earth terminal with a reasonably thick wire. 

I assume you have an RCD in the mains supply feeding your extension cable anyway for safety. 

Alan

Pete,

From your description the Rasp Pi EQMod cable 0V wire should be connected to the mount 0V which should connect to the Nevada -ve terminal via the mount power cable. You can check this continuity with your multimeter on ohms, with all the power off.

However it's always best if using separate power supplies to have a permanent solid connection connecting their negative terminals together, if all your voltages are positive relative to ground as in your case, to avoid the voltages floating apart by unplugging a cable to the mount. A permanent connection from the Pi GPIO 0V pin to the Nevada negative would achieve this.

As the Nevada linear supply outputs are also floating this wouldn't cure the AC leakage problem from the Pi power supply, so connecting the Nevada -ve to the rear Ground terminal as mentioned previously should fix this issue. These bonding cables between power supplies and Ground should be as thick as is practical to use, and not using very thin wires, especially if the supplies are supplying higher currents of a few amps or more to avoid currents in these cables causing slight voltage variations between supplies due to any current flowing in the bonding cables.

Avoiding using the Pi power supply altogether, and using a 12-5V buck converter like this or similar, from the Nevada output would also fix the issue and be more elegant. The buck converter I indicated can be used via the screw terminals, or the USB power output can be used to plug into the Pi power input with a standard USB cable.

Alan