If a sensor diagonal of 22mm is ideal for a sharp image with the RASA 8, but if you have got "only" a camera with a diagonal of 28.5mm is available - is a good image possible at all?

I tried this out and will describe here step by step my way to the finished image with the Baader [product sku="baaderfcct"] an the QHY 268M ([product sku="QHY268"]) on the RASA 8, especially the adjustment of the overall system. I suggest that you read all this before you try it out yourself!

For safety's sake, check every step – and everything at your own risk!

Why Astrophotography with a RASA 8?

Here you can find the blog post: RASA 8 - with QHY Cameras and Baader FCCT: First Tests with QHY163M

I like the RASA 8 very much. It is a very handy OTA and has very good imaging optics. I was able to test the first prototype of the FCCT filter slider and camera filter together with the QHY163M on the RASA 8 some time ago and had a lot of fun. These shots laid the foundation for my enjoyment of the current Ultra Narrowband (UNB) SHO filters.

In the past I could often show that I can "read" the RASAs quite well. What do I mean by "reading a telescope"? Well, that's what I call the "interpretation" of the system's image in focus: am I working with the right settings for the backfocus, and which adjustment are necessary so that the image is perfectly sharp in the center as well as in all corners. The latter is really a challenge with f/2-optics and large camera sensors.

The price/performance ratio of the OTA with 8" aperture is very good with respect to the field and camera, even including the obstruction. The [product sku="baaderfcct"] is a fine addition! According to the technical data of the RASA 8 f2.0, the field can be used up to 30mm chip diagonal, albeit with cutbacks or compromises in the corners – it is optimized for 22mm works up to 32mm. Previously I had "only" shown that the information about the imaging quality of the RASAs is correct if you use them correctly.

Let's go ahead and see what I did with a larger camera than recommended, and what you have to do!

Preparing the RASA 8:

The first thing I did was to remove the "clear glass filter" of the RASA 8, because the backfocus calculation showed that only then I would get exactly the right distance with the FCCT and a spacer ring as well as the filters. This is simply a MUST at f2 - because even 0.5mm is a much too big deviation here and certainly not helpful in my experiment! What plays a minor role with f4 systems or even less light intensity is a world with f2.0.

And always check: Is everything clean and dust-free?

Preparing the QHY 268M:

The  FCCT instructions give a good idea of how to install it on smaller cameras. I took everything off the QHY268M until I had the sensor glass heater in front of me and carefully kept the tiny original M2 screws. Then I was able to mount the camera base part of the FCCT. To do this, the FCCT has to be disassembled into three parts as described in the instructions.

A) Aligning the Baader FCCT to the camera:

It should be said that the sensor of the QHY 268M is installed in portrait format so that "north" is always at the top of the picture. This means that I connected the camera to the FCCT in such a way that the filter is later inserted into the FCCT from above. "Top" can be seen very well on the back of the camera.

There is another consideration behind this orientation: I wanted the power cable to lead away towards the left or right and the USB cable towards the ground. If the cables run nice and straight, a spike can be created; the effect of the spike is then always mirrored.

But at the same time, I would not change the position of the cables during an imaging session. If I pay attention (!) and don't bump into anything, nothing changes during the imaging and I can create and use flats without any problems.

If, on the other hand, I change the position of the cables between the shots, this becomes visible on the stars and leads to artefacts. Changing filters is also easier when there is no cable in the way!

Flats are an absolute must-have: without flats I can't "turn up" the depth of the image as I like to do. If the system is to deliver what I expect, then everything has to be done right during preparation.

And always check: Is everything clean and dust-free?

B) Attaching the FCCT-Tilters to the base of the FCCT with the camera:

Now take the FCCT base and turn the tilter screws until the screw tips are just visible on the inside of the FCCT. The screws must NOT protrude into the FCCT. On the camera side, the FCCT consists of two parts that are separated for assembly, namely the FCCT-QHY flange and the FCCT base with the tilter screws. We do not need the third part, the RASA flange, at the moment.

Next, place the FCCT base on the table in such a way that the pit for the filter faces you. In the next step, take the camera with the mounted FCCT-QHY flange and orient it so that the insertion of the filter also faces you.

Only when both orientations fit can you gently press the camera into the tilter base.[br]

Next, both parts are fixed in place: This is done with the three pairs of screws, which are arranged in pairs with a 120° offset. Please fix the "pull-screw" first until you feel a gentle resistance. Ideally, tighten each tension screw alternately! Then follow the "pressure screws". These must also be fixed! The same applies as for the tension screws: → Tighten until you feel pressure/contact.

Please note: All steps so far, which describe the use of the FCCT, can be done also without a camera in place, so that you can get a feeling for handling it.

Preparing the FCCT at the RASA 8:

For this purpose, I screwed the FCCT-RASA flange with a spacer and the union nut of the RASA 8

The counter ring is not screwed on too tightly, but firmly - do you know the expression "hand-tight"? Please don't "bang it tight"... Important: Tighten the retaining ring so well / tightly that it doesn't become loose later at night when it cools down.

If I had to fix the camera again during the night, the camera would certainly twist minimally or shift out of the optical axis. I wanted to avoid that at all costs. It needs some practice, but it wasn't my first time with the RASA.

Caution: If you tighten the counter ring too much, you will have to use a lot of force to unscrew it later on, which can lead to the corrector of the RASA 8 moving in the Schmidt plate after several times – then the adjustment is "gone" and the RASA 8 has to be serviced!

(And always check: clean and dust-free!)

By the way: The FCCT-RASA flange can stay on the RASA 8 so that I can still use the dust cover of the RASA 8. This way the optics can be transported. (For this, "only" the camera has to be removed again. Mark the centre of the camera towards the FCCT, this makes it easier to reattach the camera!)

The camera assembly for the FCCT has only to be done once, the FCCT remains on the camera later.

Put the RASA 8 onto the mount and connect all cables

What I do now will accompany me on all the nights I usethis setup - with every image that is downloaded. So it is important!

No "good enough" applies here - no, being really exact is the be-all and end-all here!

A) First I put the RASA 8 on my CGX mount and fixed it with a (slight) overweight to the main mirror, because with the camera attached, the centre of gravity moves forward again. I had already aligned the CGX before - so I didn't have to do that again that night.

I moved the main mirror 8-9 times by half a turn of the focuser counterclockwise - the optics are not in focus! At this moment, I am only concerned with the distribution of the weight, because if I don't do this now, I won't be able to see later whether it is pulling a little more towards the camera or the main mirror.
Shooting with an f2 system not only means that I can gain depth very quickly, no: I can also see every little tracking error - f2 shows that immediately!

AND it is much, much more important that my setup runs well! When I adjust, I need to be able to run the mount for up to 10 seconds or more, without an autoguider - but the stars have to stay round so that I can evaluate the imaging at the edges and in the field well. If the mount would let me down and not run "free and correct", I would adjust for the tracking error and not for the optics!

B) Attaching the QHY 268M at the FCCT-RASA-Flange (Very important, so work with care!)
In the next step, we leave the mount with the RASA 8 fixed in the parking position pointing to the pole star; the optics thus point exactly towards the celestial pole. Now take the camera and place it on the FCCT RASA flange that has been fixed centrally on the RASA 8. Do not let go of the camera during this process!

Turn everything so that the centre of the camera points exactly upwards or to the centre of the lens!

Hold the camera firmly and tighten the screws. I use a star pattern until the screws are tight. To ensure that everything fits, you must finally check the alignment of the camera to the lens!

Check: Are the camera and the FFC centred on each other, and both together again on the RASA? This is necessary so that later the star only moves in one direction or axis when you move one axis of the mount, and not diagonally through the field.

By the way: Perhaps you have already noticed: The screws for adjusting the camera are located on the left (S1) or right (S2) at the top of the FCCT and exactly at the bottom (S3) of the FCCT!

This means that each screw is assigned to an axis in the field! (And always check again: Clean and dust-free! This way you avoid surprises during the recording).

Installing the cables and balancing the system

When all steps have been completed (and better checked 2x), now make everything ready for use. To do this, connect the power and USB cables to the camera. In my case, I fix both cables to a heating collar and additionally with Velcro cable ties.

Look at the optics from the front: the cables should - or better: must - run straight and at right angles to each other to the edges of the optics, without bumps, knots, etc.

If the radial course is offset by more than 120° (169°), the cables become "invisible", then there will be no spike on the photo.

Auto-Guiding:

Celestron RASA 8 with StarAid Rev. B & QHY MiniGuide Scope

This time, I have attached the [product sku="1485001B" style="imgright"] with a [product sku="2457000"]. From this point on, the final balancing of the telescope and the completion of the wiring begins.

The balance is checked on both sides of the mount, on the east and the west side. Once the aperture of the RASA 8 points to the north, it is then rotated 180° so that it points to the south. In each case, the optics are aligned parallel to the ground.

In this process I check every position I can reach. The mount must be evenly balanced in all positions. This also serves to check my cable routing - no tension or pressure! Caution: With the cold, cables become "unruly" and affect the tracking!

Done!!! The setup procedure is finished, now follow the first light on the star and the adjustment of the QHY 268M with the FCCT on the RASA 8.[br]

First Light at the stars and collimating QHY 268M with the FCCT to the RASA 8

Collimating the RASA 8 + FCCT + QHY268M with the stars

First, let me give you a few notes on the steps, as well as on the use of the devices in general.

• I operated the QHY 268M at -10°C with Gain 26 and Offset 60;
• As a basis I took LRGB data in the eastern and western sky, then H-alpha, OIII and SII; and the whole thing for declination values from -6° up to high in the sky. This gave me an impression of possible deflection or a change in the position of the main mirror as well as the stability of the adjustment and the FCCT to the RASA 8;
• What I must urge everyone to do is this: Focusing must be on the optical axis, with a very small deviation (less than 150-200 pixels). This is especially true during adjustment! After that it becomes less critical, but a maximum of 1/3 of centre is a golden rule;
• Correction with bias, darks and flats. Darks and bias I took with the [product sku="2459197" style="imgright"], that worked even when the crescent moon had risen! But of course the telescope was 180° away from the moon and without direct light on the aperture of the RASA.
• The LRGB and Ultra Narrowband filters must be perfectly clean. So I could test the behaviour with the flats, because the flat only has to correct the course of the brightness, no contamination on chip or filter...
• Green flats (avg. 31000 ADU) worked well with all Ultra Narrowband filters, although a constant flat saturation seems to me to be an advantage for all channels.

Beginning of the collimation at a star:

The mount was carefully aligned in the normal way - everyone knows best how this works for their own mount. Then I looked for a bright star at an altitude of about 50° degrees to complete the adjustment at my leisure.

Note: The ASCOM driver of the QHY 268M was unchanged since the installation. As a result, every image was upside down.

When the setup is set up as described, the screws S1 / S2 / S3 on the FCCT are assigned as follows; the S1 and S2 are always effective in the diagonal.

• S1: Upper right to lower left
• S2: Upper left to lower right
• S3: Left to right over the field[br]

The field of view of the RASA 8 and the QHY 268M at 100% and the edges of the image; there is also shown in which way the adjustment screws act.

[br]

Test image before the collimation

A) The first step is finding the perfect focus for the RASA 8 (using FWHM-data and brightness!)!

Then, I could take the first shot with these settings:

• Filter: Lum
• Exposure time: 10sec
• Bin: 1x1
• Save: in a new folder called "collimation“
• File name: 0001_Start_10sec_Lum
• Following files: 0002_(corner / Screw etc…)_....

The first thing I noticed in the test shot is a left/right difference, which I will correct later with screw S3. But at first I work with S2, because the deviation was greatest there. This is the rule from now on: the axis / corner of the image where the error is greatest is optimised.

Work with great care: Take care of all cables, do not bump the telescope roughly, use a headlamp, hold the hex keys securely,...

B) After the first exposure, I enlarged a subframe of the corner of the image (via the region of interest) where the stars were blurriest/largest.

Then I exposed this image area for 2 - 4 seconds. The exposure time depends on the brightness of the stars in the field.

When adjusting the exposure time, you have to avoid oversaturation of the stars, otherwise no accurate evaluation is possible. I set the shooting mode for these focus subframes to endless loop until the adjustment of the image corner was completed! To evaluate the subframes, turn the monitor so that it is clearly visible if possible and set the zoom to 200 to 400%!

The process: Now it's easy: I say out loud the name of the screw and what I want to do. So "S2 + lower screw (on the FCCT!) + tighten"! As I do this, I always turn the screws about 1/8 of a turn. This helps to get a routine over time: When you say it out loud, it's easier to remember 😊
When I had done this, I waited for two shots, then the mount was steady again and the stars stood still again. Everyone has to find out for themselves what suits their own mount.

As a result and as a goal, the stars must become smaller. Since there is still tilt of the sensor, they do not have to be round yet - but smaller is the goal! Okay!
Now take the whole image field again and look for the next "worst image corner"!
I took the "S1" axis, chose the corner where the stars were again the "biggest", and again the FCCT is turned to make the stars smaller(r).

C) Now that two corners have been "treated", it is time to take a new picture of the whole field.
Now you should see what has happened in the corners of the picture that you have chosen, and what has happened in the opposite corners of the picture! Now you have to decide how happy you are with it!

The goal is (always): The image has become better compared to the previous images, then the direction is good. In my case it got better overall, but the left/right deviation in the image was still there. If there is a noticeable deterioration, you can use the S1 / S2 screws again to correct the image corners with the greatest deviation.

D) Start a new focus routine for the whole lens and focus again in the centre of the image field! (focus very precisely, because the errors become smaller towards the edges!). This is followed by a longer exposed shot of about 10 to 15 seconds. No tracking errors are allowed here!

Now look at all picture corners at min. 100% and compare them with the previous pictures. In my case, I now used the S3 screw, because I had similar stars on the left / right side of the image up to the image corners.
Therefore I took the side in the picture where the stars were the biggest. So again I selected a corner of the image as a focus subframe via region of interest and let it be exposed in an endless loop with 2 to 4 seconds exposure time. Then turn the S3 screw to make the stars smaller. (They may be slightly egg-shaped, but they will become round).

E) Then another shot of the whole field at 10 to 15 seconds exposure time.

Again, check all four corners of the image at 100%. You may have to make another round of adjustments using the screws S1 + S2 or even S3:

• It always makes sense to refocus after 2-3 corrections in the image corners - the errors are getting smaller!!!
• These rounds of adjustment are to be made until in the end there are only round stars left in all four corners of the image;
• The better the air quality (seeing), the better you can see the quality of the adjustment - poor seeing makes it look "good" faster, but there can be a deviation again with better seeing!
• Since you are adjusting on an f2 system and 3.7mü pixels are small, it can take a while.

I've had time to practice on my RASAs over the last few years - but it's not witchcraft!

Just in case:

You may find that the screws are no longer so easy to tighten. Remember: Too much force is not a good tool!
In this case you have to loosen the S2 + upper screw slightly (1/16 - 1/8 turn). Always do this with an active subframe so that you can see what happens!

Check: Are you turning in the right direction, so is it getting better?

If you are close to perfection, that last little "bit" may have already gone the other way, so that without noticing it you have already gone through the ideal focus! Or it may be the tilt in the field, which can then cause you to have to use the S1 and S2 screw again, because there may be a swing between the corners of the picture and left / right in the field. To eliminate this, more careful adjustment is always necessary!

Conclusion:

One thing has been confirmed over and over again in all the nights I have photographed: You should always focus on the object; drifts in focus worsen the image! Otherwise, I have not noticed any deviation.

On system optimisation: A second 3" rail on top of the RASA 8 would be helpful, so that the StarAid-B can be better balanced above the optics.
I brought the adjustment to the final state in three nights (after a total of about 2.5 hours). It was helpful that I had always recorded data. This allowed me to make a comparison with the previous night in terms of adjustment and stability. This allowed me to determine the consistency of the optics, as well as a stable position of the FCCT. After completing the adjustment, the image was the same on every object.

But don't worry: the time-consuming collimation is only necessary once. From now on, the system remains pleasingly stable, in all tube positions! Both in the eastern and western sky from -6° Dec up to high in the sky... If I had already had optimal seeing conditions on the first night, the collimation would have been done in one evening.

What I had achieves during night 1 was still the on night 2, and what I schieves on night 2 was exactly the same on night 3. Only because of the better seeing in the following nights did I have to optimize the collimation. The time required: 15-20 minutes – if you are not doing it for the first time!

I use a "partially mobile" setup: I usually dismantle the CGX and telescope, but the camera remains on the telescope. With "proper" handling, the alignment remains intact. Only the alignment of the images is a big issue: when the camera is taken down and put back on, the individual sub-frames can easily be twisted against each other.

But since I wanted to have images in narrow band and LRGB for stacking, I had to work so precisely. Pre-stacking showed whether it would work. That's why after a night I always do a run-through with the data and a mix of two nights to see if they will fit together.

The focus must be hit exactly. In case you have gone beyond the focus point, you should focus again from the beginning. The following applies: ALWAYS PRESSURE, i.e. push the mirror upwards by rotating the focuser anti-clockwise!

To achieve perfect focus: Test the FWHM values in a focus pass from intra- to extrafocal and evaluate them!

My RGB filters showed very similar FWHM values. The stars remain round to the edge of the field and show no fringes.

On recording the RGB series:
I took the blue shots when the object was at its highest altitude in the sky, then green and behind, and finally red when the object was already lower. Lum (UV/IR) is the sum of RGB and shows the FWHM averaged.
The focus position of the Ha and SII filters was the same, OIII showed a deviation in focus. After everything is adjusted, it is time to take pictures!

I exposed each subframe for 180 seconds. RASA 8, StarAid, CGX and QHY268M were now working PERFECTLY together. Even a single image gives an idea of what to expect. Here is an original luminance image after 180s - only a mild STF was made and saved in Jpeg.

A "few" shots later, all the images were "in the box", and after image processing, now the result of the effort:[br]

M65 / M66 and only 400mm focal length in LRGB!

LRGB M65 + M66 + NGC3628; Lum: 105min, R/G/B: 30min each channel (=90min); Subs: 180s!; QHY 268M @ -10°C; Celestron RASA 8 + Baader FCCT + Baader LRGB (older version of filters); Celestron CGX + StarAID-B; Size: 66%; Depth: HUGE!; 400mm focal length and APF-R; © Christoph Kaltseis

And to prove that it can be done again and again, here are two more single shots made with the large QHY268M and the RASA8 – the image sequences are stillw aiting for processing:

The spiral galaxy M51, 1x180s with green filter; © Christoph Kaltseis

IC1318, 1x180s with H-alpha 3,5nm Ultra-Narrowband-Filter © Christoph Kaltseis