Anyone tried eyepiece projection for EAA?

[vlaiv]

I was replying in another thread about imaging with small sensor and long focal length scope on alt-az, which is pretty much same type of imaging as EAA - short exposures, and came up with idea and suggestion for OP to try eyepiece projection.

It dawned to me that large reduction in focal length can be obtained with combination of eyepiece and careful positioning of sensor. For small sensor, aberrations might be tolerable. Perfect setup for EAA.

Anyone tried this?

Also, it occurred to me that eyepiece can act as very high power focal reducer. In "standard configuration" sensor is placed behind focal point of eyepiece (greater distance than focal length of eyepiece) like this:

But what if we put it in this configuration:

Eyepiece can be seen as positive lens element (compound lens) - same as regular 2 element simple focal reducer. One just needs enough in focus travel, and close placement of sensor to eye lens of eyepiece (eyepiece with T2 thread and camera directly screwed to that without extension tubes). This could probably only work on SCT / RC scopes and perhaps some refractors - ones with enough back focus.

If anyone fancies having a go, here is calculator for focal length and resulting F/ratio:

http://www.wilmslowastro.com/software/formulae.htm#EPP

For example, in regular configuration with my 8" RC - F/8 scope by using 32 mm plossl and sensor placed at 40mm (problem is of course determining exact center of the lens, but I suspect that it can be found with some experimentation) would give F/2 system with 400mm FL! Very fine for small sensor with small pixels like ASI178.

[Thalestris24]

Hi Vlaiv

I've not tried EP but have been thinking about how to focus most of the light from the objective/flattener on to an area the size of a smallish sensor chip. A normal focal reducer/flattener still spreads the light out in order to fill an aps-c area. I thought maybe a good quality 2" ed eyepiece could do what I had in mind. I have a kit 2" 28mm lens but there doesn't seem any simple way of attaching it to the scope optical train or of attaching a camera to it. Another idea was maybe a Barlow  if it could be spaced not to increase the focal length. I'm sure what I'd like to do must be possible! I just don't know exactly how!

Any thoughts?

Louise

[vlaiv]

This is what I've reach so far in thinking about this issue and reading around:

- Barlow won't be much of a use in this scenario - it is negative lens and negative lens spreads light over surface making things bigger on sensor (we use it to get magnification). It can be used to collimate incoming beam of light (make it parallel) - which can be useful for some applications like transmission diffraction grating  (Star analyzer) - which works best in collimated beam, or perhaps solar Ha applications where filters work best again in collimated or close to collimated beams. You of course need another positive lens to focus parallel beams back sensor.

- Eyepiece and focal reducers, or even simple binocular lens - any positive lens element can work as focal reducer. It concentrates converging beam to converge even more. Difference of course will be on how fast it converges beam - reduction factor, and what sort of aberrations it adds to wavefront - sometimes these aberrations are beneficial because they cancel already aberrated wavefront from objective - these are correctors (SCT, MCT) or field flatteners, coma correctors (ones with focal length reduction).

- For best application with small sensor you need to take into consideration aberration free field of instrument / scope. Let's say that your instrument has aberration free (or another name for it is corrected) field with diameter of 30mm and your sensor has diameter (diagonal) of 10mm. If you use perfect reducer, maximum reduction that you can have without introducing field aberrations from instrument is 1/3. You can go even further than this but only if your reducer is correcting for outer field aberrations of your instrument or you accept edge of field deterioration.

- I have no idea if eyepiece will be better or worse in handling wavefront aberrations than a simple reducer (something tells me it might since eyepieces are carefully designed for best performance). It probably depends on type of eyepiece used. For starters I would go with simplest plossl design - being simple and affordable option. Next step up - I would look at well corrected eyepieces with flat field. I don't think eyepiece needs to be wide field one, but it will certainly help in regards to vignetting in very aggressive reduction.

- Mounting of eyepiece projection system is really straight forward - if you can use eyepiece on your scope to observe visually, you probably can do eyepiece projection with it - focus position will be inward from regular observing position of eyepiece. Most scopes are able to accommodate this I believe. This way eyepiece will work in "regular configuration" or first configuration (see post 1.).

- Simplest mounting mechanism for eyepiece projection that I've found is something like this:

https://www.teleskop-express.de/shop/product_info.php/info/p143_TS-Optics-direct-Adaptor-to-T2-for-1-25--eyepieces-for-eyepiece-project.html

Works by removing rubber eye guard and exposing groove that you can fit this adapter on, or alternatively something like this:

https://www.teleskop-express.de/shop/product_info.php/info/p8656_TS-Optics-eyepiece-projection-adapter-for-eyepieces-30-42-mm-O-D-.html

They are available in number of diameters so many eyepiece designs can be fitted with appropriate one.

I've seen couple of other designs also.

Important thing with eyepiece projection is understanding reduction (or magnification - you can use it as magnifier as well) relation. It depends on focal length of eyepiece. If you place sensor at distance equal to two times focal length of eyepiece you will effectively get no reduction nor magnification - this is "relay" configuration - eyepiece just relays image at scope focal plane to its own focal plane on the other side. If you increase sensor distance you will get magnification, if you decrease sensor distance you will get reduction. In most cases for EAA you want second option - reduction (faster system), and this is why you want to mount sensor as close to eyepiece as possible. If reduction is to aggressive you can always add T2 spacer to bring it down.

[Thalestris24]

13 minutes ago, vlaiv said:

This is what I've reach so far in thinking about this issue and reading around:

- Barlow won't be much of a use in this scenario - it is negative lens and negative lens spreads light over surface making things bigger on sensor (we use it to get magnification). It can be used to collimate incoming beam of light (make it parallel) - which can be useful for some applications like transmission diffraction grating  (Star analyzer) - which works best in collimated beam, or perhaps solar Ha applications where filters work best again in collimated or close to collimated beams. You of course need another positive lens to focus parallel beams back sensor.

- Eyepiece and focal reducers, or even simple binocular lens - any positive lens element can work as focal reducer. It concentrates converging beam to converge even more. Difference of course will be on how fast it converges beam - reduction factor, and what sort of aberrations it adds to wavefront - sometimes these aberrations are beneficial because they cancel already aberrated wavefront from objective - these are correctors (SCT, MCT) or field flatteners, coma correctors (ones with focal length reduction).

- For best application with small sensor you need to take into consideration aberration free field of instrument / scope. Let's say that your instrument has aberration free (or another name for it is corrected) field with diameter of 30mm and your sensor has diameter (diagonal) of 10mm. If you use perfect reducer, maximum reduction that you can have without introducing field aberrations from instrument is 1/3. You can go even further than this but only if your reducer is correcting for outer field aberrations of your instrument or you accept edge of field deterioration.

- I have no idea if eyepiece will be better or worse in handling wavefront aberrations than a simple reducer (something tells me it might since eyepieces are carefully designed for best performance). It probably depends on type of eyepiece used. For starters I would go with simplest plossl design - being simple and affordable option. Next step up - I would look at well corrected eyepieces with flat field. I don't think eyepiece needs to be wide field one, but it will certainly help in regards to vignetting in very aggressive reduction.

- Mounting of eyepiece projection system is really straight forward - if you can use eyepiece on your scope to observe visually, you probably can do eyepiece projection with it - focus position will be inward from regular observing position of eyepiece. Most scopes are able to accommodate this I believe. This way eyepiece will work in "regular configuration" or first configuration (see post 1.).

- Simplest mounting mechanism for eyepiece projection that I've found is something like this:

https://www.teleskop-express.de/shop/product_info.php/info/p143_TS-Optics-direct-Adaptor-to-T2-for-1-25--eyepieces-for-eyepiece-project.html

Works by removing rubber eye guard and exposing groove that you can fit this adapter on, or alternatively something like this:

https://www.teleskop-express.de/shop/product_info.php/info/p8656_TS-Optics-eyepiece-projection-adapter-for-eyepieces-30-42-mm-O-D-.html

They are available in number of diameters so many eyepiece designs can be fitted with appropriate one.

I've seen couple of other designs also.

Important thing with eyepiece projection is understanding reduction (or magnification - you can use it as magnifier as well) relation. It depends on focal length of eyepiece. If you place sensor at distance equal to two times focal length of eyepiece you will effectively get no reduction nor magnification - this is "relay" configuration - eyepiece just relays image at scope focal plane to its own focal plane on the other side. If you increase sensor distance you will get magnification, if you decrease sensor distance you will get reduction. In most cases for EAA you want second option - reduction (faster system), and this is why you want to mount sensor as close to eyepiece as possible. If reduction is to aggressive you can always add T2 spacer to bring it down.

Yeah - it's complicated, ha ha! I did just try with the 2" 28mm eyepiece. I don't normally use eyepieces at all so this was it's first time out of the box. I should have realised that the output from the eyepiece is intended for eyes... Duh! So, although the diameter of the glass is quite large, the output image is small - maybe only 5mm. Fine for an eye! I'd thought a 1.25" eyepiece would be too small and shut off too much light - defeating the object (I was just fiddling but I guess I need to move the eyepiece close up to the field flattener so it would be like your second diagram above.) I'll have another fiddle again tomorrow in the light. The thing is, will the image size being projected still be so small? If it seems to actually work as desired I still don't have a means of attaching a camera to test it. It's times like this I wish I had a 3d printer... Maybe I'll be able to cobble something together just to try it in the day.

Louise

[vlaiv]

11 minutes ago, Thalestris24 said:

Yeah - it's complicated, ha ha! I did just try with the 2" 28mm eyepiece. I don't normally use eyepieces at all so this was it's first time out of the box. I should have realised that the output from the eyepiece is intended for eyes... Duh! So, although the diameter of the glass is quite large, the output image is small - maybe only 5mm. Fine for an eye! I'd thought a 1.25" eyepiece would be too small and shut off too much light - defeating the object (I was just fiddling but I guess I need to move the eyepiece close up to the field flattener so it would be like your second diagram above.) I'll have another fiddle again tomorrow in the light. The thing is, will the image size being projected still be so small? If it seems to actually work as desired I still don't have a means of attaching a camera to test it. It's times like this I wish I had a 3d printer... Maybe I'll be able to cobble something together just to try it in the day.

Louise

How did you measure output image?

Eyepieces in combination with scope give certain exit pupil, but diagram for eyepiece / eye combination is different than that of eyepiece / camera combination.

Eye expects collimated (or almost collimated beams) - like when looking at infinity (for relaxed observation), here is diagram what eye sees:

So you see, red, green (? maybe grey, I can't really tell) and blue beams are parallel when exiting eyepiece and are concentrated in exit pupil that is rather small (just a few mm in diameter - to fit human iris).

With eyepiece projection situation is a bit different:

Eyepiece is "moved forward" than when looking with eye, and beams are no longer parallel, but converge again. Depending on "speed" of convergence - we have couple of cases - magnified image, 1:1 image and reduced image.

This means that we cant judge illuminated field from exit pupil of eyepiece. Field stop and "speed" of telescope might provide us with some data needed to calculate fully illuminated field, but I don't think that we need to do that.

We just use long focal length eyepiece - it already has large field stop, and we just place sensor closer than twice focal length of eyepiece.

[Thalestris24]

7 minutes ago, vlaiv said:

How did you measure output image?

We just use long focal length eyepiece - it already has large field stop, and we just place sensor closer than twice focal length of eyepiece.

I just projected the 'real' image on to a piece of paper. I'll try moving the eyepiece closer? in tomorrow. I was doing all these optical calculations a year ago in physics at uni but I've forgotten it all now!

Have you tried the eyepiece projection yourself?

Louise

[vlaiv]

32 minutes ago, Thalestris24 said:

I just projected the 'real' image on to a piece of paper. I'll try moving the eyepiece closer? in tomorrow. I was doing all these optical calculations a year ago in physics at uni but I've forgotten it all now!

Have you tried the eyepiece projection yourself?

Louise

No unfortunately not. I have so many ideas to test out, but so little time

Fact that it's winter (or cold if not calendar winter) does not make things easier. A lot of neighborhood is burning fuel for heating (wood, coal) so air quality is poor as well - lot of haze and smog.

[Thalestris24]

I have bad atmosphere and city lp all year round  Weather forecast = cloudy for next 5 days (though forecasts are really only valid for the next 3 hours  

Louise

[Thalestris24]

Hi Vlaiv

I had a play around with my ST80 and 5Mp microscope camera. I found I could get a real focus point with a 20mm EP and the focus tube racked in as far as it would go. The ST80 normally has a focal length of 400mm and aperture 80mm i.e. f-ratio = 5. With the EP in place the focal length was about 343mm, so f-ratio = about 4.3. I only had the camera (loosely!) connected via a cardboard tube so getting (and keeping) focus wasn't easy. I managed to take a couple of snapshots using SharpCap. The problem is that the focal point seems to be at what is just the eye relief. That was too close to be able to use a qhy5l-ii or gpcam. The way around that, I'm thinking, is to just use a simple convex lens rather than an eyepiece but I'd need some way of holding it in place and holding a camera in place. If I had a 3D printer I could probably make one. I have been thinking about getting one - an Ender3 Pro, maybe.

I have to say I'm a bit confused as to how an actual reducer works. I have a x0.79 one on my 115mm apo which has a focal length of 800mm (f6.95). So the reducer should effectively reduce that to 632mm i.e. f5.5. Yet the physical focal length is still the same? I assume that all the x0.79 reducer actually does is change the way the light cone converges? Anyway, it seems to work!

Louise

[vlaiv]

51 minutes ago, Thalestris24 said:

Hi Vlaiv

I had a play around with my ST80 and 5Mp microscope camera. I found I could get a real focus point with a 20mm EP and the focus tube racked in as far as it would go. The ST80 normally has a focal length of 400mm and aperture 80mm i.e. f-ratio = 5. With the EP in place the focal length was about 343mm, so f-ratio = about 4.3. I only had the camera (loosely!) connected via a cardboard tube so getting (and keeping) focus wasn't easy. I managed to take a couple of snapshots using SharpCap. The problem is that the focal point seems to be at what is just the eye relief. That was too close to be able to use a qhy5l-ii or gpcam. The way around that, I'm thinking, is to just use a simple convex lens rather than an eyepiece but I'd need some way of holding it in place and holding a camera in place. If I had a 3D printer I could probably make one. I have been thinking about getting one - an Ender3 Pro, maybe.

I have to say I'm a bit confused as to how an actual reducer works. I have a x0.79 one on my 115mm apo which has a focal length of 800mm (f6.95). So the reducer should effectively reduce that to 632mm i.e. f5.5. Yet the physical focal length is still the same? I assume that all the x0.79 reducer actually does is change the way the light cone converges? Anyway, it seems to work!

Louise

That is very interesting, so you tried configuration number 2? One that uses prime focus of scope? I suspected that spacing might be an issue in such configuration - one needs to put sensor fairly close to eye lens to make it work. How about first configuration - I suspect that in this configuration you will have more room to adjust sensor distance, but you also probably need to rack focuser quite a bit out, maybe even use extension, or diagonal mirror to get enough backward spacing?

Reducers work in similar (yet opposite) way to barlow. Barlow diverge incoming rays, while focal reducers converge incoming ways. Spacing between sensor and focal reducer determines reduction factor. It is effective focal length of scope + reducer that changes. Best to view scope + reducer (or barlow) as a single unit, or compound lens. Such compound lens will have different focal length, and in case of reducer it will be shorter than original scope focal length - this gives wider field of view and "speed" of system since there is same effective aperture but shorter focal length.

Here is diagram of focal reducer and light rays that can help understand what is going on:

From this diagram you see that focal point moves "inward" with focal reducer (outward with barlow) and space between two stars (on-axis and off-axis, or black ray and red ray) is less with focal reducer (reduction of image) and more with barlow (amplification of image). You can also see that using eyepiece in front of focus (or camera at prime focus) as reducer will require very close positioning between eyepiece and sensor.

First configuration allows more "room" for adjustment, because 1:1 distance is twice the focal length of eyepiece, so if you use 20mm eyepiece, you will need sensor at 40mm distance to get no reduction or amplification, and at 20mm distance you will get x0.5 reduction - making ST80 very fast F/2.5 system.

[Thalestris24]

I'm still a bit confused! My 800mm scope is still 800mm between the objective and the camera sensor even though I have a reducer in between. Obviously, the distance between the reducer and sensor is fixed at the 'standard' 55mm. Am I right in thinking the change in physical focal length is small, which is why it's not obvious to me?

Back to the EP experiments... :

The eyepiece projection I did was from inside the normal focal length of the ST80. As I say, that is 400mm but I focused at ~343mm with the EP in. I'm not sure which configuration that is? I suppose your original post, second configuration has the EP further away than the normal focal point? And that gives the longer distance to the sensor?

Louise

[vlaiv]

2 minutes ago, Thalestris24 said:

I'm still a bit confused! My 800mm scope is still 800mm between the objective and the camera sensor even though I have a reducer in between. Obviously, the distance between the reducer and sensor is fixed at the 'standard' 55mm. Am I right in thinking the change in physical focal length is small, which is why it's not obvious to me?

Back to the EP experiments... :

The eyepiece projection I did was from inside the normal focal length of the ST80. As I say, that is 400mm but I focused at ~343mm with the EP in. I'm not sure which configuration that is? I suppose your original post, second configuration has the EP further away than the normal focal point? And that gives the longer distance to the sensor?

Louise

Physical distance between objective lens and sensor will be a bit smaller - simple focal reducers usually require a bit of inward focus travel, but it is very small amount - as per diagram above, so 800mm scope, focus position will be at let's say 785mm or something like that. So physical distance changes by small amount, but focal length of instrument changes more. Don't mix physical position of focus with focal length of instrument - just think of SCT/MCT type telescopes - physical distance between front corrector objective and focal position is very small, but focal length of instrument can be large. It is not even due to folded light path - take for example 102 mak - it is about 30cm long, but has focal length of around 1300mm. Even if you account for light traversing length of tube 3 times, it is still not enough - this is because secondary is amplifying with such scopes (similar effect of barlow lens).

In case of 55mm distance between reducer and sensor - if you look above diagram, shortening of physical distance will be fraction of this last part, so maybe one fifth or sixth of 55mm - about 10mm. This is because light rays travel "regularly" until last 55mm when they are bent.

As for EP configuration, we need to get nomenclature straight . Let us give "proper" names to configurations (instead calling them 1 and 2 to avoid confusion).

Proper EP configuration - in this one, eyepiece stands behind prime focus of telescope in the same way it does for observing, here is diagram:

Prime focus EP configuration - in this one eyepiece stands in front of prime focus of telescope and sensor is placed in prime focus of telescope, there is no projection focus:

So if you had to move focuser all the way in, I would say that you used Prime focus EP configuration. With Prime focus EP configuration, eyepiece is behaving like "regular simple focal reducer" with given focal length. Formula for reduction factor of simple focal reducer is:

http://www.wilmslowastro.com/software/formulae.htm#FR

So if you used ST80 and 20mm eyepiece, and you placed sensor roughly at 5mm distance, this formula gives reduction of: F/3.8

(you need to bring sensor very close to get what you got - F/4.7, around 1.2mm, but we don't know exact point from which this distance is measured - we assume it is somewhere around eye lens, but can't be sure where - it is up to eyepiece design).

If you increase distance to sensor in this configuration, you will get larger reduction, but you will need more in focus travel - something that you don't have on ST80 since you already pushed it in as much as it'll go.

In Proper EP configuration calculation is different - and formula for it is:

http://www.wilmslowastro.com/software/formulae.htm#EPP

Here by moving sensor closer to eyepiece you increase reduction factor (opposite from Prime focus EP configuration where moving sensor closer to eyepiece reduces reduction factor). In this case you will certainly be able to reach focus for higher reduction factor because all you need to do is add extension, and there is no obstacle to do that. You can even use diagonal - as it will act as extension. Good eyepiece/sensor distance will be around 20mm - this will give you fast F/2.5 system.

 

 

 

[Thalestris24]

OK - that's a lot clearer now - on both accounts  . So I was doing the prime focus configuration  Since the interest is in using the EP as a reducer, there doesn't seem much point in trying the 'proper' EP configuration, unless there's something exciting about it?? I think using a simple convex lens in place of the EP might give a better result with the prime focus config but might introduce some chromatic aberration. I'll have a go tomorrow if I can find the time and can rig something up - sticky tape and cardboard tubes, probably ha ha.

Louise

[vlaiv]

Actually I think Proper EP configuration is better - you can achieve greater reduction with ease - no in focus problems. Sometimes eyepiece projection (proper EP configuration) is recommended for newtonians if one can't reach focus with DSLR because there is not enough in focus travel (focus plane is close to tube) - like in visual configurations when using standard height focuser (as opposed to low profile photographic one).

Difference (yet to be determined, that was one of reasons for starting this thread) to simple two element reducer (GSO x0.5 for example) could be in quality of the field. CA is not only aberration that we should worry about. Field curvature and astigmatism can be amplified with simple lens. Most of photographic reducers are limited to x0.6 or above for this reason - they tend to preserve (or flatten) field. They are designed as either matching particular scope or with certain limitations - like need for relatively flat field instrument. Most eyepieces are well corrected optically and introduce minimal aberrations - so I figured that they will behave better at aggressive reduction ratios than focal reducers.

Proper EP configuration could also prove better than Prime focus EP configuration because eyepieces are designed to "receive" diverging light rays - coming from focus of telescope rather than converging light beams coming into focus.

[Grogfish]

+1 to both of you! Thank you for spending the time thrashing this one out - I too have often wondered about the theory and practice of using an EP for focal reduction.

Eyepiece projection has often appeared to me to be a dark art that is only discussed in hushed corners, and you get funny looks for talking about it out loud...

I think you’ve handsomely demonstrated why this is the case; it’s fiddly and the results don’t merit the effort compared to the alternatives. Ah well, good job I spent the twenty quid years ago on a simple x0.5 reducer......

[vlaiv]

A little update.

While I wait for EP T2 adapter, and recuperating from flu (and guess what - it's finally clear sky outside, but I'm still not quite well to do astronomy), I decided to test out EP projection parameters.

My idea is to use EP projection as focal reducer and "squeeze" 20mm imaging circle onto ~8.8mm diagonal sensor. Initial trial will be with 8" RC and x0.67 focal reducer - that alone should "squeeze" 30mm onto 20mm imaging circle. Further reduction from 20mm to 8.8 should bring whole system to F/2.35 - that should be really fast.

This image was obtained with 32mm Plossl and ASI185 (8.56mm sensor size), although I plan to use ASI178mcc for EAA (maybe 1-2 mm larger diagonal).

Image is blurry because of "hand held" focus - I was holding both camera and EP and trying to focus on ruler by moving whole (shakey) assembly forth and back. I managed even a bit more sharpness - but trying to press "print screen" with other hand did not help the sharpness

Quick measurement shows that this configuration is giving 2.6cm diagonal, so I can move sensor further (that will help since back focus is estimated to be about 10cm in this configuration) to get magnification down to 2cm on diagonal

It looks like this approach has good potential for EAA - looking forward to test it in the field.

[SteveNickolls]

Thanks for your update, I also help you recover from the 'flu as quickly as possible. ? 

Cheers,
Steve

[Deflavio]

Very interesting topic! Any new update or more tests? I was thinking to combine a Televue ploss 32mm with an asi224 but not sure about the distance...

 

(also we should probably move this topic out of the reports section and in discussions)

Edited April 23, 2019 by Deflavio

[Deflavio]

Here the results of a quick test I did this afternoon...  

Setup:

• SW Evostar 72ED
• ASI224 MC
• Televue Plossl 40mm
• TS eyepiece projection for eyepiece 30-42mm as suggested by @vlaiv
• several M42 extenders 

and here some results...

Left: Exposures=0.000168s gain=234(auto), Right: Exposures=0.000168s gain=0

Ok, I'm not entirely sure how I shall calculate the reduction but just looking at the scaling between the different tree branches, I think I'm getting something close to 0.294 ?. So if the scope is f5.8, does it mean I'm running at ...ahem... f1.7 !? ?   Is this right or I'm missing something here?  

More details about the test:

Eyepiece projection was in "prime focus" mode as discussed above in this discussion. Unfortunately, right image is taken with eyepiece/camera handheld because I can only get focus removing all 1.25'' adapters. More thinking is required here as focus is going very deep inside. If I "ignore" the blurriness (out of focus) and considering also that I was clearly off-axis when I took the picture, I think distortions are not too bad...

The camera/sensor was as close as possible at the eyepiece as allowed by the EP adaptor and the sensor position. I think ~10/12mm but need to check. I tried to increase the distance just adding a T-mount UV-IR filter or other small spacers but I could not get focus anymore. I guess it went further inside the tube.

Unfortunately, I didn't manage to get a bright image of the "proper EP" configuration. Working on it but it seems there is a minimal distance between eyepiece and camera below which I can't get any focus. I need to look more into the theory of EP projection and where/how the focus is moving...

 

Anyway, regardless of the final reduction I'm getting, I think EP reduction is definitely something worth experimenting more...

[vlaiv]

Here are some pointers for proper EP projection configuration.

If your EP is 40mm than I think twice that you'll get 1:1 "relay" configuration - this means that EP needs to be at 80mm from scope focal plane and sensor needs to be 80mm from EP.

To get reduction you need to move sensor closer to EP, but that also means that focal plane distance to EP needs to increase.

For that configuration to work, you'll probably need quite a bit of extenders - either straight one, or maybe using diagonal mirror or something that adds optical length.

Here is calculator for EP distance / magnification factor:

http://www.wilmslowastro.com/software/formulae.htm#EPP

Note one more thing - in proper EP projection configuration, you can't mount sensor closer to EP than focal length of EP. If you mount sensor at exact distance as focal length of eyepiece - you will get small "scope" - it will act as lens focused at infinity (this is why it won't work as it will expect parallel rays but it is getting diverging rays from scope).

So for 40mm EP, your "operating range" is >40mm - 80mm. Best to try somewhere at the middle at around 60mm. If that is further than you can mount your camera - try shorter FL EP - like 32 or 25 mm one.

 

 

[Deflavio]

Thanks @vlaiv, that's really helpful. Now I understand the "proper" EP much better.

I don't think I have enough extensions to reach that distances on the 72ED but yes I can try with a shorter eyepiece and maybe using my 130p instead. Since it has already a deep focus that would make a better use of the extensions. In the worst case... I'll get more  ?

 

[Deflavio]

Ok, more testing this morning and I manage to get the proper EP working with quite strong reductions with different eyepiece. Interesting results but image quality is not there yet... Here a quick summary:

 

@vlaiv as you mentioned by going with eyepieces with shorter focal lengths the extension are more manageable. However it seems I'm getting quite a bit of distortions (coma?) even at 0.68 reduction. I got similar distortion with a SW plossl 25mm but I'm not sure I good is this eyepiece. Optically my TV 15mm seems really good...

Going back again to the long TV 40mm I can get EP by adding more spaces between the camara and the EP projection adapter. I have to admit the total extension is quite embarrassing and I'm pretty sure I may have some bending going on. Still, with the 40mm I think image quality seems more uniform although more fuzzy/blurrier. I  tried but can't get a better focus...either because is very short or image quality is poor. Unfortunately I don't have a 32mm to test... I may try next the 40mm with less reduction and see if image quality improves.

 

 

 

[vlaiv]

59 minutes ago, Deflavio said:

Ok, more testing this morning and I manage to get the proper EP working with quite strong reductions with different eyepiece. Interesting results but image quality is not there yet... Here a quick summary:

 

@vlaiv as you mentioned by going with eyepieces with shorter focal lengths the extension are more manageable. However it seems I'm getting quite a bit of distortions (coma?) even at 0.68 reduction. I got similar distortion with a SW plossl 25mm but I'm not sure I good is this eyepiece. Optically my TV 15mm seems really good...

Going back again to the long TV 40mm I can get EP by adding more spaces between the camara and the EP projection adapter. I have to admit the total extension is quite embarrassing and I'm pretty sure I may have some bending going on. Still, with the 40mm I think image quality seems more uniform although more fuzzy/blurrier. I  tried but can't get a better focus...either because is very short or image quality is poor. Unfortunately I don't have a 32mm to test... I may try next the 40mm with less reduction and see if image quality improves.

 

 

 

At this point it would be good to consider few things that are important for image quality.

- To avoid excessive vignetting, field stop of EP needs to be larger than size of field projected on sensor. In case of 15mm Plossl, field stop is about 12.6mm. With reduction factor of 0.68 and ASI224 having ~6.1mm diagonal, projected field is 6.1/0.68 = ~9mm in diameter - so this is good, you should not have much vignetting.

- Similar thing applies to field correction - you need field to be corrected to projected size to avoid any aberrations inherent in optical design. With newtonian without coma corrector, coma free field as function of distance from optical axis is given as F/ratio^3 / 90.

You have F/5 scope, so radius of coma free field will in this case be 5^3/ 90 = 125/90 = ~1.4mm or diameter of only 2.8mm. We are placing diameter of 9mm on sensor (when projected). This means that there will be significant coma away from the center.

Do you have coma corrector for that newtonian to try with that?

[vlaiv]

@Deflavio

I'm also a bit concerned with TV 15mm image - it looks blurry. I wonder if it is due to spherical aberration that EP projection introduces or maybe just poor focus. I'm not implying that you did poor job of focusing your scope - rather what type of focuser do you have on that SW 130PS? At fast speeds, critical focus zone becomes very small, and dual speed focuser is going to be essential for sharp image. I know that 130PDS has dual speed crayford, but I'm guessing that 130PS has regular single speed focuser?

[Deflavio]

Ok, so the 15mm is ok in terms of vignette at reduction 0.68 but it may be limited as soon as I go up in reductions. For reference I put here the field stop of few plossl so people can compare with their own sensors. From the TV website we have:

• 15 mm = 12.6
• 20 mm = 17.1
• 25 mm = 21.2
• 32 mm = 27
• 40 mm = 27

I guess, the 15mm is out with reductions higher than 0.5 with a 6mm sensor. Also there is no obvious benefit in 40 vs 32mm... I had the 40 for an old Mak, now I just realised on both 130ps and 72ed its exit pupil is really off and I’m getting the secondary in the view on the reflector. Does the exit pupil value also affect the EP projection somehow or just the field stop? 

I should probably move then to the SW 25mm...although now I’m tempted to get a TV 25mm, eheh.

About the blurring on the 15mm, well, that’s probably me. I was more interest to show the coma and with a windy day yesterday focus was moving a bit. About focusing, the 130ps has a very simple focuser not dual speed. I do have a baader T2/1.25 elliptical focuser that may help a bit. No coma corrector sorry. One question, in your calculations for the coma you say coma free circle is only 2.8mm but on my usual EAA sessions I don’t see obvious distortions with the 224 while now on 0.65 reduction they are very obvious from already 1/4 of the size the sensor and will be probably more with stars...is this just coma from the reflector? On the 40 mm, the image is all bad but I don’t think I see this increasing from the centre, no?

Anyway, now that I know how to play with extensions, going back to the 72ED may actually avoid both coma and focuser issues. Let’s see if I can do something between today and tomorrow, after that I will be traveling for work for a couple of weeks...