First light with Meade 14" ACF - some questions

[gorann]

This spring just after astro-darkness left me up here I bought a second hand 14" Meade LX200R (later renamed ACF). SInce I had invested in a Mesu200, my old EQ8 was just taking up space in my garage and I though it could be a good match for the 14" SCT, at least for visual use. It also meant that I had to spend the summer building a second obsy for the large SCT which would not fit under the roof of my old obsy and it is too heavy to be taken down every night (weighing 40 kg).

Now yesterday it was dark enough to allow me to test if I could use this set-up for imaging. The imaging train consised of a Meade Electronic focuser, a Lepus 0.62 (reducing the FL from 3.5 to 2.2 meters and giving f/6.2), a ZWO OAG with a Loadstar X2 (run on PHD2), and for the night I used my Canon 60Da as camera (if it all works out I probably end up using my ATIK 460 or ASI1600). There was no filter in the train. The image is 5 x 3 min at ISO1600. Stacked in PI and no stretch done.

It was meant as a test to see if I could satisfactory guide this beast on my EQ8. With regard to guiding it went suprisingly well (RMS around 0.6"). I was mainly interested in star shapes so I had the scope pointed where it ended up after 2-star alignment, in this case Deneb. That created some apparent reflections (possibly in the reducer) and I do not worry so much about those since I will not normally point it at the brightest stars in the sky.

QUESTIONS: My main worry now is that there is some chromatic abberations in the stars, particularly red appears to be a bit shifted. My experience with SCT is very limited and I would be very happy if anyone can tell me what the cause of the chromatic abberation could be. Could it be collimation? Or is it the reducer? I did do some primitive star test and it looked fine to me (rather symmetric rings around a defocused star) but I could play a bit with Bob's knobs. What is the best way of collimating a SCT? Any comments on how I could improve the performance of this set up are most welcome.

[kirkster501]

Good luck with this Gorann.  I have a similar set up and only used for visual thus far.

You collimate these with the screws on the secondary.  Defocus a star and then get use the three screws to get the defocused star symmetrical.  TINY movements, we are talking eighth turns. There are tons of online guides and videos on this.

Might be worth removing the reducer for now as a temporary measure to see if you have this issue with red channel then....

Edited August 26, 2019 by kirkster501

[gorann]

Thanks for the input! I will try without the reducer and have a closer look at defocused stars tonight.

[kirkster501]

15 minutes ago, gorann said:

Thanks for the input! I will try without the reducer and have a closer look at defocused stars tonight.

You need to be sure the collimation is right before any other step.

Collimation is critical with these scopes.  They hold collimation very well night to night but still worth doing a quick check before an imaging run.  You can do this defocused star test with your imaging camera focused on a bright star like Vega.  Loop exposures every second and 3x3 binning and look at the pattern of the defocused rings.

Some imagers like Damian Peech do a collimation check even when pointing the scope in a different direction in the same night since they say the mirror can move minutely when the scope slews.

EDIT: I'd be interested in your thoughts about the ACF on the EQ8.  I am thinking of doing this too.

Edited August 26, 2019 by kirkster501

[vlaiv]

Well, something is off, that is for sure. I suspect some sort of misalignment, but can't be sure if it is collimation.

Examine stars in different parts of the image and you will see different things with them:

Bottom left corner:

Stars look almost ok, I would say that there is slight elongation, but can't be sure what is the cause of it - might be periodic error / guiding - what sort of target spread did you have on PHD - was it elongated in one direction or circular?

This one is close to center of the field:

Clearly shows defocus, maybe a bit of miscollimation because bottom left part is brighter.

Top right corner:

again, stars are tighter - but this time clearly elongated.

Top left / bottom right corner show in principle the same thing:

a bit of astigmatism and lateral chromatic aberration, but otherwise almost fine.

You should probably try without reducer first to see how well optic is collimated. Then check spacing on reducer as well as possible tilt (rotate camera + reducer, and camera with respect to reducer to see what happens to star shapes in corners).

I just searched online for any clues, and found some, it appears that Meade ACF design uses hyperbolic surfaces and is therefore much more sensitive to collimation. It also has thicker corrector plate than Celestron SCTs and this leads to more lateral chromatic aberration.

 

 

[alan potts]

Owning a slightly smaller one I would say remover the reducer, double and triple check collimation with increasing eyepieces until at least X300 mag if you do it that way and then try again. I often top out at 500X when doing this.

Alan

[Chris]

The thing about very large SCT's is the larger you go, the more mirror flop you get! and I'm confinced the flop on the mirror effects both corners and collimation. At 14" I think you would need to check collimation after every big slew to a different part of the sky. Not sure if the Meade has mirror locks like the Celestron Edge? If so this will help. 

No doubt CA is coming from something added to the optical path, I've owned a bunch of SCT's and there isn't any CA typically. 

Here is a good guide for collimation:

 

Edited August 26, 2019 by Lockie

[gorann]

Thanks guys a lot for all your suggestions! I will for sure check collimation thoroughly using an eyepiece rather than just the live-view of the camera. The scope do have a mirror lock so I do the fine adjustments of the focus with the Meade electric Craywford focuser (sits first in the image train). I will also try without the reducer. Still I am generally rather pleased that things did not look worse after the first try.

[kirkster501]

Just now, gorann said:

Thanks guys a lot for all your suggestions! I will for sure check collimation thoroughly using an eyepiece rather than just the live-view of the camera. The scope do have a mirror lock so I do the fine adjustments of the focus with the Meade electric Craywford focuser (sits first in the image train). I will also try without the reducer. 

That is exactly the way to do it.  Leave the main mirror alone.  In fact, you can generally lock this in mid-focus and pretty much forget about the main mirror focus knob and just use the Crayford.  Unlock the mirror and run it through its range of motion every six months or so to keep the grease spread along the spindle the mirror is supported on.

[Datalord]

I had a thread with a cmos and my RASA showing similar issues. I settled on atmospheric dispersion, which moves the red light different from blue. I suspect this effect is increased with aperture, so your 14 will be more susceptible. I had a whole thread on it earlier this year. 

It you do your collimation in mono or with a filter, and rule out any other mirror related issue, I suspect you will still see this problem on a cmos colour camera. Easy to correct in processing, so no biggie. 

[ollypenrice]

Just use it for visual! I have a 14 inch Meade and it's great for that... I took a firm decision regarding imaging which was, NO!

lly

[gorann]

22 minutes ago, ollypenrice said:

Just use it for visual! I have a 14 inch Meade and it's great for that... I took a firm decision regarding imaging which was, NO!

lly

He he, then I have a challenge Olly - should I take it? Yours is not an ACF (is it?) so maybe I may succeed.... In any case it will be something to play around with in the second obsy while my Esprits will be collecting data in the other one, so no stress😎

[Chris]

2 hours ago, ollypenrice said:

Just use it for visual! I have a 14 inch Meade and it's great for that... I took a firm decision regarding imaging which was, NO!

lly

Hey! What about planetary and Lunar imaging I know it's not DSO's but still  

[kirkster501]

47 minutes ago, Lockie said:

Hey! What about planetary and Lunar imaging I know it's not DSO's but still  

That is what mine is for.  Collimation even more critical for that.

[gorann]

I had a go at collimating it. It came with Bob's nobs so at least I did not have to fiddle around with metal tools in the dark next to the corrector plate.  I did it by looking at the DSLR screen at 10X live view. The defocused star looked very centered to me at the end but it was not too bad to start with. Right now it is collecting data without reducer so at a FL of 3.5 meters - never tried any FL more than 2 meters before so a bit overwhelming. PHD2 shows 0.4"/pixel RMS which is better than I ever had before but then I invested in a Lodestar X2 and an OAG. Still, I am quite impressed with the old EQ8 managing to do this while carrying about 45 kg. I am aiming at a galaxy - NGC 7331. I will report on this thread tomorrow.

[gorann]

Now 02.00 and I have to get up early tomorrow🙄 but the guide curve looks great (RMS around 0.4"/pix) and I am very impressed how well the old EQ8 is handling 45 kg. I am shooting away on a small galaxy far far away at FL 3.5 meters: NGC 7331. Should close the roof and go to bed.....

[gorann]

Friends,

here is the result from last night. First a single raw sub, then a raw stack of the 7 x 15 min I managed to grab (only done Canon banding reduction and SCNRgreen in PI), and finally a processed more presentable version. Maybe @vlaiv want to have a go at dissecting the images to extract some useful ideas for future improvement?

The image was done at native FL of 3556 mm (f/10), so without reducer and with the CANON60Da as imaging camera. Guided with Lodestar X2 on ZWO OAG. Reomoving the reducer may be the mail reason why there is now much less chromatic aberration, but I also collimated it (was slightly out of collimation). I still have some problems with varying star shapes. My guess that there is some tilt but since I am not going to use it with DSLR in the future, I will try to deal with that if I also get these star shapes when using one of my cooled cameras.

The plan tonight is to collect Lum with the ASI1600MMpro that I can add to this RGB image.

So Olly @ollypenrice, does it still look like a rather hopeless endeavour? At least I am surprised about how well the EQ8 appear to handle 45 kg scope with over 3.5 meter FL. Bear in mind that this is still only 105 minutes of DSLR data collected at 15 °C. Cooled CCD & CMOS will so be attached to the scope.

Edited August 27, 2019 by gorann

[vlaiv]

Oh, I like it!

Yes, there is tilt, and it now it clearly shows as there is "gradient" of defocused stars - worst in top left corner. This is one of my favorite targets, and it looks like it is quite challenging one. I'm yet to see sharp rendition of it (well, at least sharp to my liking ).

I like the final version, and love the fact that you brought it down in resolution since your shots at 1:1 are very over sampled. How did you do it? Did you bin your data?

Thing with ASI1600 is that you are going to have crazy sampling rate, and you will need to bin it quite a lot to get to decent sampling rate.

[gorann]

Thanks a lot Vlaiv, much appreciated!

I never imagined that I could use this scope for imaging at its native FL, but had a little hope to do it with the 0.62x reducer, so I am a happy bunny now!

Regarding the resolution, I just brought it down for the jpg version in PS, changing image size from 4444 x 3042 pixels (after crop) to 2000 x 1369, so nothing fancy. Not sure how I could bin a DSLR image, can it be done and is that a better way to do it? I may bin the Lum I get from the ASI1600mono tonight. I probably run 2 or 3 min exposures with the ASI1600 (gain 139, offset 50) so it will be quite a number of subs, but it is only really dark here for 2-3 hours right now so it will be managible.

[alan potts]

OK there may well be a few faults here and there but you have got to say for imaging 3.5M of F/L which is beyond many, it is a very good result, this the first effort too.

Alan

[vlaiv]

18 minutes ago, gorann said:

Thanks a lot Vlaiv, much appreciated!

I never imagined that I could use this scope for imaging at its native FL, but had a little hope to do it with the 0.62x reducer, so I am a happy bunny now!

Regarding the resolution, I just brought it down for the jpg version in PS, changing image size from 4444 x 3042 pixels (after crop) to 2000 x 1369, so nothing fancy. Not sure how I could bin a DSLR image, can it be done and is that a better way to do it? I may bin the Lum I get from the ASI1600mono tonight. I probably run 2 or 3 min exposures with the ASI1600 (gain 139, offset 50) so it will be quite a number of subs, but it is only really dark here for 2-3 hours right now so it will be managible.

Ok, let's run some calculations to see what we can get.

As far as I can tell 60da has 4.3um pixels, so you were sampling at (fl 3556mm) - 0.25"/px, or in reality 0.5"/px given that it is OSC sensor. There is "space" to bin x2 additionally to get to about 1"/px, which would be probably good sampling rate given aperture and guiding.

Easiest way to do it is to first do split of bayer matrix and then do additional split bin. Not sure if you can do it in your software, but if you like to do a test, maybe simplest thing would be to upload calibrated subs as 32bit fits, and I can prepare all the files for you to stack again? If you have something like 7 subs, this will result in 28 subs for red and blue channel and 56 subs for green (yes, I know, "magic" ).

With ASI1600 you will be sampling at about 0.22"/px - which is really "crazy" that should be binned at least x4 or maybe even x5 in some cases (aim is to get to about 1"/px - a bit less if seeing and guiding are exceptional and a bit more if atmosphere is not so good).

Btw, since you will be over sampling by this much - use longer exposures. Short exposures on CMOS sensor work because read noise per pixel is relatively small compared to other noise sources, namely LP noise. Darker the skies - you need longer exposures. When you over sample like this - you spread signal over more pixels and each pixel gets less signal. This is also true for LP, so your background is going to be really dark in single sub if you use short exposure. If you can guide for 15 minutes - yes, use that sort of exposure on this resolution as well with ASI1600, because at that resolution 16 pixels divide signal of a single pixel at 0.88"/px, and in order to get same level of signal as 1minute exposure at 0.88"/px you will need 16 times that amount of time.

[Chris]

15min subs at that focal length and weight, your EQ8 is a beast! Fab first image too. Obviously there is a bit of waste with the oversampling but I'm sure there are ways around this. I would be delighted with this, worth the very late night  

[gorann]

21 minutes ago, vlaiv said:

Ok, let's run some calculations to see what we can get.

As far as I can tell 60da has 4.3um pixels, so you were sampling at (fl 3556mm) - 0.25"/px, or in reality 0.5"/px given that it is OSC sensor. There is "space" to bin x2 additionally to get to about 1"/px, which would be probably good sampling rate given aperture and guiding.

Easiest way to do it is to first do split of bayer matrix and then do additional split bin. Not sure if you can do it in your software, but if you like to do a test, maybe simplest thing would be to upload calibrated subs as 32bit fits, and I can prepare all the files for you to stack again? If you have something like 7 subs, this will result in 28 subs for red and blue channel and 56 subs for green (yes, I know, "magic" ).

With ASI1600 you will be sampling at about 0.22"/px - which is really "crazy" that should be binned at least x4 or maybe even x5 in some cases (aim is to get to about 1"/px - a bit less if seeing and guiding are exceptional and a bit more if atmosphere is not so good).

Btw, since you will be over sampling by this much - use longer exposures. Short exposures on CMOS sensor work because read noise per pixel is relatively small compared to other noise sources, namely LP noise. Darker the skies - you need longer exposures. When you over sample like this - you spread signal over more pixels and each pixel gets less signal. This is also true for LP, so your background is going to be really dark in single sub if you use short exposure. If you can guide for 15 minutes - yes, use that sort of exposure on this resolution as well with ASI1600, because at that resolution 16 pixels divide signal of a single pixel at 0.88"/px, and in order to get same level of signal as 1minute exposure at 0.88"/px you will need 16 times that amount of time.

Thanks again for the thoughtful hints! So what exposure time and gain/offset do you suggest. At least last night 15 min was OK (I could probably do 30 min) but that seems a bit long for the ASI unless I use a really low gain. However, tonight I thought of using the 0.62 reducer since chromatic aberration will not be a problem for Lum and the chip in the ASI1600 is a bit small compared to the APSC in the Canon and it would be nice to cover about the same FOW.

I will of course examine the first image and adjust accordingly (exposure or gain?) to make sure I am not blewing out the brightest parts.

[vlaiv]

14 minutes ago, gorann said:

Thanks again for the thoughtful hints! So what exposure time and gain/offset do you suggest. At least last night 15 min was OK (I could probably do 30 min) but that seems a bit long for the ASI unless I use a really low gain. However, tonight I thought of using the 0.62 reducer since chromatic aberration will not be a problem for Lum and the chip in the ASI1600 is a bit small compared to the APSC in the Canon and it would be nice to cover about the same FOW.

I will of course examine the first image and adjust accordingly (exposure or gain?) to make sure I am not blewing out the brightest parts.

Well, my view on the matter is that you should keep about 15 minutes exposures. If you use reducer you'll need to bin x3 to match x4 bin of color data (btw, if you don't want to upload subs so we try processing it the way I described, maybe you would be willing to do it yourself? In that case let me know, and I can post plugin for ImageJ that will do this for you and explain how to use it).

No need to use lower than unity gain, and offset of 50. These should work well with longer exposure. Don't worry if you blow out a few stars in luminance. Once you stretch your data, those will saturate anyway. With luminance it is not critical to have full star profiles like it is with color where you need proper balance of R, G and B - and you loose proper ratios if star cores are clipped.

[gorann]

But I am moving from f/10 to f/6.2 so maybe 15 min is a bit over the top. I could also take a few shorter exposures to at least save the galaxy core if needed. As I understand it CMOS could equally well be binned later but maybe to save disk space and processing time I should at least bin 2x2 on the camera?