To get those really flat images requires 3 things. Good collimation, good backfocus, and good tilt! We can do those things in this order, but there’s no rules! This process can hold true regardless of your overall equipment. SCT’s or Refractors or RASA’s all require the same good flat images, but achieve it slightly different.
In my case, I utilize the RASA fast optics system and it becomes even more important to achieve these three items. Refractors for example will still require good backfocus and tilt, while an SCT or RASA require more attention to collimation and scope speed can affect sensitivity to these items.
In order to collimate, your scope will have collimation screws built in. SCT’s for example provide collimation screws in the front secondary assembly, as do the RASA. For tilt and backfocus, you need a special tilt device. While some cameras come with tilt plates, they are often impossible to reach. We designed our own ASG Photon Cage Here to deal with filter sliding, tilting, and backfocus all in one device.
Step 1: Collimation
Most RASA’s have good factory collimation, but sometimes you need to tweak it, and they suggest to in their own documentation if it’s been shipped or travelled much. Collimation simply centers all of your mechanical components in your scope so they are centered when exiting your scope. This is easily done with a defocused star in the ‘center’ of your image. Many guides exist out there, and on SCT and RASA’s it’s the 3 screws on the front of your scope. Get your optics and light nice and parallel to your optical train this way.
Step 2: Backfocus
RASA 8 backfocus is 29mm, RASA 11 is 72.8, but these vary depending on filters in your optics, even your camera manufacturer may state that backfocus is +- up to .5mm in some cases. “IT CAN VARY”. For this reason, try to get your RASAtron under 1 or 2mm so you have adjustability. Simply use washers in your optical train to extend your backfocus, or with the camera holder ring, loosen it’s clamping screw and slide your camera body out slightly. We even use small piece of tape on the side so we can easily measure how much we slide our camera outward to increase backfocus. Using software such as NINA, ASTAP, or CCD inspector is a great way to see the curvature of your image.
Step 3: Tilt
Now that you have a nice collimated image with good backfocus, you can tilt your image. You should have your tilt adjustment neutral at this point. Either flatten the tilt screws all the way down, or use small shims to have equal gaps in your tilt plate. NINA is the preferred software now to adjust tilt and with the HocusFocus extension, you can see actual measurements for each of the 4 corners to adjust tilt, including measurements. ASTAP and CCD Inspector are great tools to verify single imagery, but do not provide near the analysis give from NINA. The tilt screws are easily accessed now from the front of the scope and in 4 corners. Tilt under 10% is pretty good. 5%-0% is great.
Get those optics straight and true.
Collimation is nothing more than getting all the optics, mirrors, and other mechanical features in your telescope aligned so you get the optimal focus of an image. We can use a RASA scope for example or an SCT as an example, where you have a primary mirror re-pointing light to a front window or secondary mirror, then to an astro camera. All of this requires things to be aligned properly to get optimal focus. Faster optics like a RASA aren’t necessarily worse or better collimating, they are just gathering mistakes faster and are more readily visible.
Collimation is the first and most critical step. It is a base for all other adjustments below. It takes 5 minutes to check and verify if your scope is nicely collimated. I have had several RASA scopes and one was completely out of collimation, while a brand new RASA 11 was in perfect collimation.
Checking collimation is as easy as defocusing on a star. Make sure the star is in the ‘Middle’ of your image. This way you eliminate tilting and backfocus issues for the most part and are focused solely on having a good central collimation into the middle of your imaging plane.
Below are 5 images from a brand new RASA 11 that is collimated at the factory. This is very good collimation with the central obstruction very centered when defocused. Slowly bring your scope into focus and continue checking for a good central obstruction. You may see it start to wander as you get smaller and smaller, and then it can be adjusted very slightly as needed. My RASA 8 was way out of alignment, but used, and with this same process, collimation can easily be achieved in about 20 minutes.
Get the sensor the optimal distance for flat fields.
After collimation, I like to work on good backfocus. You have a perfect focus on your central star, so with a flat field, all your stars should be in good shape across your sensor. The problem is if backfocus is too close or too far, meaning your image sensor does not match the best placement with your given optical train and telescope field flatness.
Regardless of what anyone says, backfocus is unique to everyone’s setup. Even when a scope manufacturer provides a suggested backfocus, such as a RASA 11 backfocus at 72.8mm or a RASA 8 being 29mm, it will most likely be slightly different for everyone. Every filter you add will change backfocus, even camera manufacturers recommend they have +-.5mm of backfocus accuracy in sensor placement. .5mm +- backfocus can make a huge difference with a fast optic system such as a RASA.
It’s actually pretty easy to spot backfocus problems because it introduces ‘curvature’ to your system. NINA is the preferred method to adjust tilt and backfocus at once with precise data and analysis of your sensor. You can also spot backfocus visually, and even statically with software such as ASTAP or CCD inspector. Here are a few examples of what you see visually with star behavior. It’s very common to see stars ‘egging’ out in the corners. Which way they egg out can show you a lot about your backfocus ‘curvature’.
This can be difficult to really nail down. The best solution is start at the manufacturer recommended backfocus and view your stars visually. If they look good, move on. On fast optic scopes like RASA’s, it’s common to have to use small shims and washers to adjust. Shims down to even .1mm thick will have an influence here. My RASA 8 should have 29mm, but I found that 28.5mm was best for my scope and filters. I experienced the circular egg shaped stars heavily. On a RASA 8 there is very few options in some circumstances to adjust backfocus, so get creative.
My RASA 11 with filters were very close to manufacturer suggested 72.8mm, so you can vary greatly depending on scope, extensions, filters, even camera sensor locations.
Square that sensor to the light.
Camera tilt is the last aspect I like to tackle. I know I have good collimation here, and backfocus is set up good now at this point as well. I may have to go back and adjust backfocus because every time I tilt the camera, I am moving the sensor closer or farther away… very small amounts, but it may affect backfocus. So I sometimes have to go back to backfocus and adjust it slightly and then tilt again. In order to properly adjust tilt and backfocus, we use the ASG Photon Cage Here in order to manipulate. Without a proper piece of hardware to tilt or adjust backfocus, you are fairly stuck with using spacers and wishing good luck.
Tilting is the process of taking your camera sensor, which is flat, and aligning it to the light coming in so it’s as consistent as possible across the sensor. It will never be perfectly the same, scope manufacturers always say they offer good flat images, but perfect is another story. You will find curvature coming in from nearly all telescopes. The larger the sensor (full frame cameras), the more this is an issue due to much wider sensor area. So tilting is a matter of actually moving your camera around side to side and top to bottom to get it oriented and tilted to square up as best you can with incoming light.
The key here is remember you have a good collimation in the center of your image, and that backfocus is adjusted as well. So a tilt should not be a consistent swirl or streak of stars like backfocus… but rather a corner or edge of bad focused stars. It’s very hard to determine tilt and backfocus issues at once, that is why I do backfocus first, then tilt.
One of the newest software tools out there that allows you to spot and measure tilt and backfocus is NINA and the Hocus Focus extension. This unbelievable tool allows you to run autofocus curves and it measures all 4 corners to determine true curvature and tilt in your image with statistical measurements. Other methods such as ASTAP or CCD inspector rely on a single image for measurements. With NINA, you can now see how much your adjustments need to be and make them accordingly.
Software is of great help here. You could visually inspect images here for various HFR’s in your corners, edges and plot out a diagram of your focus in various parts of an image, or take 5 seconds and run it through these 2 programs. ASTAP is free to use and really very good software. Just drop an image into ASTAP, hit F4, and you can view tilt, even aberration inspector can help spot backfocus swirls and eggs. CCD Inspector is also a nice application of showing image tilt & collimation, but is a bit pricey when you use it sporadically. Below are some good examples that best explain how I use them.
Here we simply cover all our Photon Cage designs and modules to give you a better understanding of the tools we make to help make tilting and backfocus adjustment possible.
This video is designed for users of the Photon Cage and how to properly setup and install your camera as well as zero the device and prepare it for mounting.
The lengthiest video in the series shows how to use NINA, Hocus Focus, and ASTAP to adjust tilt. We do several real runs from the previous night to give you an actual scenario and how we would fix the issue in 15-30 minutes.
In my experience, backfocus always presence itself as a the following in ASTAP or CCD inspector. The central part of your image begins to defocus.
On ASTAP, it doesn’t show itself in the tilt, but notice the bottom curvature number of .60 which is very high. I try to get that curvature below .20. This is where ASTAP is a little harder to find the value, but curvature is presented here in the lower corner.
In CCD Inspector, it’s easier with their curvature or 3D view. Notice the central hump in this 3D image and curvature at 27.4%. This shows good collimation, good tilt, but just backfocus short, meaning camera is too close and I need to increase or move my sensor away using shims. This is on a RASA 11 and the same image processed in both ASTAP and CCD Inspector.
This is an image from my RASA 11 with backfocus too far. This begins to curve the image more and more the farther you get away.
This image has a lot of tilt, but you can see on ASTAP it shows curvature of .94 which is very high along with the tilt values on the right side of the image. ASTAP again is a little harder to interpret, but the figures are given with tilt in the visual portion and the curvature presented in the bottom row of figures.
CCD inspector shows the same values in 3D and curvature with the right side being very tilted as well as the curvature getting extreme at 38.2%. In this case, bring the camera closer or decrease your backfocus to get the image flatter and reduce your curvature. Both tools work good for backfocus adjustment, but having both works good to back up the results of each other.
After fixing my RASA 11 backfocus in example, you can see the image go flat so we know backfocus and curvature are getting better.
This is shown in ASTAP with the curvature value of .12 compared to our above backfocus up at .9, but you can see the tilt in the image on the right side. 6% tilt isn’t really that bad, anything below 10% is going to be hard to spot. But on the RASA’s, it’s pretty easy to zoom in on corners and see elongated stars in one corner vs another.
CCD inspector curvature shows 18.3%, again, pretty good solid deep blue pattern with slight tilt on the right side, made even more obvious using the 3D graph feature. Good collimation here, good backfocus, just a slight amount of tilt. Both ASTAP and CCD inspector are picking up pretty similar results.
It’s important to remember now as you do tilt that sensor, you will be adjusting backfocus as well. Imagine bringing up the right side of this sensor, or pushing away the left side of the sensor and the affects that will have on the backfocus. It can get frustrating here!
In order to fix tilt, you must use a tilting device. We use our own ASG Photon Cage Here and if you are looking for better images then you need to get one.
Notice ASTAP shows .08 curvature and tilt is less than 1% (Anything under 10% tilt is really pretty good). For my purposes, this is very close to good as it will get for me. CCD inspector shows 3% tilt, so fairly close to ASTAP. The curvature 3D map in CCD inspector should look flat, little curve, and deep blue consistently. If you get a good flat deep blue image, it’s good and consistent.
Every telescope is going to have a varying degree of focal plane curvature, some more than others. This can help of course and is why flatteners are used so much.
Using new software such as NINA and Hocus Focus to spot and adjust tilt can take as little as 15-20 minutes and 4 or 5 autofocus runs. ASTAP and CCD Inspector can be time consuming and often take 30-60 minutes using a bit of trial and error in my opinion. They are still good tools to do final verifications though and worth testing, especially ASTAP.
The RASA telescopes are not any more or less easy to fix the tilt & backfocus than other telescopes, but they tend to make issues more apparent quicker. With such high light gathering capability at F/2, these scopes will show more stars and saturate quicker and thus show aberrations in your images quicker, so it can seem to be more noticeable. They also present a faster light cone, which simply means that a specific amount of tilt is more noticeable than a slower F ratio scope light curve. You can easily envision a fat light cone and tilt it, compare to a skinny light cone of a slower telescope and it requires more tilt to show the same effect… all due to the fast focal ratio.
RASA scopes require your backfocus within + or – 1mm, but I find it’s even closer than that to get good backfocus. RASA scopes also require the optical axis be perpendicular to be within .01 degree or better, and that is pretty sensitive. Critical focus at this speed can be down to around 6 microns, so it’s no joke to work with the fast optics like this.
RASA light cones are especially steep as well, which is why special F/2 or high speed filters are used to deal with that light angle. But keeping an unobstructed light cone to the full frame larger sensors. This is where our special M54 and M68 RASA adapters come in and our 50×50 square frame PRO sliders. Check out our full RASA special considerations page for more info.
By Chris White – updated April 29th, 2023
For a great comprehensive in-depth read on tilting and backfocus, Chris has constructed a nice 29 page paper on the topic that is worth reading and provides a nice overview of using several tools from ASTAP to CCD Inspector and Hocus Focus to really pull the best data you can at this time on the topic. It really is a marriage between software and hardware here to achieve good results so be sure to give this a read if looking for detailed information on the topic.
What is good enough?
These are just my examples above with my RASA systems… they are not perfect, and you can always do your own workflow to figure out how you want to achieve good results. There is a point of no return here, I found these results in about 30 minutes of effort on my RASA 11. My RASA 8 took a little longer, maybe an hour because it was so far out of collimation. In the end though it was about getting my curvature fairly low and my tilt down below 10% and then I began to find stars not elongated or egged out and within acceptable personal tolerance.
I tend to follow the same pattern, 1. collimation, 2. backfocus, and then 3. polish it up with slight tilt adjustment. With no tilt system, it’s going to still be great imaging if you have good collimation and get backfocus dialed in. You may need to use smaller sensor cameras or you may need to crop a full frame image if you have corner issues and no way to adjust tilt and backfocus. With the ASG Photon Cage, the premise is to have a tilt/backfocus system on your camera so you can move from the RASA 8, RASA 11, or other SCT, Refractor, or Newtonian with ease all using the same device.
Hopefully this helps you in your quest, this is just my two cents worth here, so feel free follow on facebook or join our ASG RASA group.