Orion RC 8” with 0.67 ccd47 reducer and ASI1600 mono

[Orion Carl]

Hello everyone, long time reader first time posting in this wonderful forum.

I recently purchased the Zwo1600 asi mono with filter wheel and a 0.67 reducer to use on my RC.

I’m at 86mm back focus in the image below. Recommendation is 85mm. Image taken through the Ha filter.

I’m getting these WiFi symbol shaped stars and the only way I’ve managed round ones is to remove the reducer.

Do you have any hints or suggestions that may work?

I’ve read that the filters could have an impact on back focus around 1mm but I’m at 86 anyway.

The whirlpool image was taken minus the reducer , I’ve also added an out of focus star with the reducer in case this helps.

Any help or advice greatly appreciated.

Many thanks,

Carl.

[Gerry Casa Christiana]

Looks like to me that your collimation is quite a way off. What are you using to collimate it? 

Gerry

[vlaiv]

Although recommended distance is 85mm, I think you will find that your combination of equipment is not going to work well on "default settings".

I have same gear (TS RC8", x0.67 reducer - ccd47 variety from TS, and ASI1600).

Main problem with this setup is FOV. 8" RC has fully corrected field less than the size of ASI1600. If you have well collimated RC and use ASI1600 at native - you should get round stars up to the edge with slight field curvature showing - stars close to edge will be a bit larger / slightly out of focus. Best to focus about 3/4 from optical axis - if you use Bahtinov mask or measure FWHM when focusing - place star somewhere around 2/3 to 3/4 between frame center and one of the corners. This way you will "spread curvature" over sensor (stars at the edges will be closer to focus, and stars in center will be slightly out of focus, but almost impossible to tell). This is of course in case you will be keeping whole FOV. If cropping out - focus closer to center of FOV.

Now, back to star shapes and reducer - With RC8" it is advertised as being able to do 30mm without corrector. I would say that is probably pushing it and corner stars will suffer. ASI1600 has about 22mm diagonal. When you apply x0.67 reduction, you are squeezing larger field on smaller surface. Equivalent to this is using larger sensor. How much larger? 22mm / 0.67 = ~32.8mm. This way you are imaging out side of corrected circle. Edge stars will surely suffer because of this.

I would say that going up to 28/29mm imaging circle would be appropriate (even then, prepare for very specific focusing - finding focus place that gives best focus over FOV). This reducer acts as slight field flattener - but not proper one, so field will be a bit flatter but you will still have to deal with field curvature. Now let's see what sort of magnification one should get to cover 28/29mm "imaging circle". Again some math 22/x = 29 -> x = 22/29 = x0.75.

If you do a search on web about this combination - RC8 / x0.67 you will find that people often say - best results are not at x0.67 but x0.72 - x0.75 (above is the explanation why it is so, some will even say it works good with "native" x0.67 - but that is simply because they are using small sensor - 11-17mm, for APS-C that is larger than ASI1600 they probably need to go as low as x0.75 or lower).

What would be proper distance for x0.75? Go for distance of 55-58mm (correct distance will not be of big importance, just use spacers that you have) . You can actually experiment and start with 55mm and then move reducer further and further until you reach point where things start to fall apart.

So to reiterate:

1. Well collimated scope.

2. Check that there is absolutely no tilt in imaging train and focuser is well collimated as well. I had to switch to threaded connection because of slight tilt.

3. Reduce distance to 55mm and work your way up from there - try to find sweet spot (best reduction with the least star distortion) - experiment with focus position for best results.

4. Understand that you introduced another optical element in optical train and that it is not perfect - so star shapes will suffer (just how much depends on optical quality of item).

HTH

[Gerry Casa Christiana]

21 hours ago, vlaiv said:

Although recommended distance is 85mm, I think you will find that your combination of equipment is not going to work well on "default settings".

I have same gear (TS RC8", x0.67 reducer - ccd47 variety from TS, and ASI1600).

Main problem with this setup is FOV. 8" RC has fully corrected field less than the size of ASI1600. If you have well collimated RC and use ASI1600 at native - you should get round stars up to the edge with slight field curvature showing - stars close to edge will be a bit larger / slightly out of focus. Best to focus about 3/4 from optical axis - if you use Bahtinov mask or measure FWHM when focusing - place star somewhere around 2/3 to 3/4 between frame center and one of the corners. This way you will "spread curvature" over sensor (stars at the edges will be closer to focus, and stars in center will be slightly out of focus, but almost impossible to tell). This is of course in case you will be keeping whole FOV. If cropping out - focus closer to center of FOV.

Now, back to star shapes and reducer - With RC8" it is advertised as being able to do 30mm without corrector. I would say that is probably pushing it and corner stars will suffer. ASI1600 has about 22mm diagonal. When you apply x0.67 reduction, you are squeezing larger field on smaller surface. Equivalent to this is using larger sensor. How much larger? 22mm / 0.67 = ~32.8mm. This way you are imaging out side of corrected circle. Edge stars will surely suffer because of this.

I would say that going up to 28/29mm imaging circle would be appropriate (even then, prepare for very specific focusing - finding focus place that gives best focus over FOV). This reducer acts as slight field flattener - but not proper one, so field will be a bit flatter but you will still have to deal with field curvature. Now let's see what sort of magnification one should get to cover 28/29mm "imaging circle". Again some math 22/x = 29 -> x = 22/29 = x0.75.

If you do a search on web about this combination - RC8 / x0.67 you will find that people often say - best results are not at x0.67 but x0.72 - x0.75 (above is the explanation why it is so, some will even say it works good with "native" x0.67 - but that is simply because they are using small sensor - 11-17mm, for APS-C that is larger than ASI1600 they probably need to go as low as x0.75 or lower).

What would be proper distance for x0.75? Go for distance of 55-58mm (correct distance will not be of big importance, just use spacers that you have) . You can actually experiment and start with 55mm and then move reducer further and further until you reach point where things start to fall apart.

So to reiterate:

1. Well collimated scope.

2. Check that there is absolutely no tilt in imaging train and focuser is well collimated as well. I had to switch to threaded connection because of slight tilt.

3. Reduce distance to 55mm and work your way up from there - try to find sweet spot (best reduction with the least star distortion) - experiment with focus position for best results.

4. Understand that you introduced another optical element in optical train and that it is not perfect - so star shapes will suffer (just how much depends on optical quality of item).

HTH

Where did you learn this stuff! Is there a book somewhere I can buy and study this? 

Gerry

[vlaiv]

14 minutes ago, Gerry Casa Christiana said:

Where did you learn this stuff! Is there a book somewhere I can buy and study this? 

Gerry

Honestly, I don't know

I did not learn it all in one place, it is a product of reading multiple sources on the web and then simply applying basic logic to it to draw conclusions. Some of it was from personal experience since I've got the same gear.

First thing that I've learned related to above is importance of collimation on RC scopes - you don't collimate it well you will end up with stars that don't look nice.

You can find formula for reducer magnification (or minification? ) here - http://www.wilmslowastro.com/software/formulae.htm#FR but I've seen it elsewhere on web. It works for barlow calculations as well (very similar formula - barlows have negative focus).

On AP website there is a document for CCDT67 technical data: http://www.astro-physics.com/tech_support/accessories/photo/Telecompresssor-techdata.pdf

There you can see that FL of this reducer is 305mm, and that housing is such that you need 85mm from thread to be at 101mm for x0.67.

TS RC specs can be found on TS website.

I've also read on multiple places online that reducers (similar to barlows, but unlike field flatteners) work over the range of distances.

Problems with tilt I experienced first hand on different scopes, so that I something I pay attention to.

[Orion Carl]

22 hours ago, vlaiv said:

Although recommended distance is 85mm, I think you will find that your combination of equipment is not going to work well on "default settings".

I have same gear (TS RC8", x0.67 reducer - ccd47 variety from TS, and ASI1600).

Main problem with this setup is FOV. 8" RC has fully corrected field less than the size of ASI1600. If you have well collimated RC and use ASI1600 at native - you should get round stars up to the edge with slight field curvature showing - stars close to edge will be a bit larger / slightly out of focus. Best to focus about 3/4 from optical axis - if you use Bahtinov mask or measure FWHM when focusing - place star somewhere around 2/3 to 3/4 between frame center and one of the corners. This way you will "spread curvature" over sensor (stars at the edges will be closer to focus, and stars in center will be slightly out of focus, but almost impossible to tell). This is of course in case you will be keeping whole FOV. If cropping out - focus closer to center of FOV.

Now, back to star shapes and reducer - With RC8" it is advertised as being able to do 30mm without corrector. I would say that is probably pushing it and corner stars will suffer. ASI1600 has about 22mm diagonal. When you apply x0.67 reduction, you are squeezing larger field on smaller surface. Equivalent to this is using larger sensor. How much larger? 22mm / 0.67 = ~32.8mm. This way you are imaging out side of corrected circle. Edge stars will surely suffer because of this.

I would say that going up to 28/29mm imaging circle would be appropriate (even then, prepare for very specific focusing - finding focus place that gives best focus over FOV). This reducer acts as slight field flattener - but not proper one, so field will be a bit flatter but you will still have to deal with field curvature. Now let's see what sort of magnification one should get to cover 28/29mm "imaging circle". Again some math 22/x = 29 -> x = 22/29 = x0.75.

If you do a search on web about this combination - RC8 / x0.67 you will find that people often say - best results are not at x0.67 but x0.72 - x0.75 (above is the explanation why it is so, some will even say it works good with "native" x0.67 - but that is simply because they are using small sensor - 11-17mm, for APS-C that is larger than ASI1600 they probably need to go as low as x0.75 or lower).

What would be proper distance for x0.75? Go for distance of 55-58mm (correct distance will not be of big importance, just use spacers that you have) . You can actually experiment and start with 55mm and then move reducer further and further until you reach point where things start to fall apart.

So to reiterate:

1. Well collimated scope.

2. Check that there is absolutely no tilt in imaging train and focuser is well collimated as well. I had to switch to threaded connection because of slight tilt.

3. Reduce distance to 55mm and work your way up from there - try to find sweet spot (best reduction with the least star distortion) - experiment with focus position for best results.

4. Understand that you introduced another optical element in optical train and that it is not perfect - so star shapes will suffer (just how much depends on optical quality of item).

HTH

The advice and suggestions contained in your answer is simply amazing. I couldn’t of hoped for a better answer.

very much appreciated.

many thanks

[Gerry Casa Christiana]

3 hours ago, vlaiv said:

First thing that I've learned related to above is importance of collimation on RC scopes - you don't collimate it well you will end up with stars that don't look nice.

Do you use a Howie Glatter collimator? Or the fail safe star test. I think it's always put me off a RC because they seem pretty difficult to get them right. 

Thanks. 

Gerry

[vlaiv]

45 minutes ago, Gerry Casa Christiana said:

Do you use a Howie Glatter collimator? Or the fail safe star test. I think it's always put me off a RC because they seem pretty difficult to get them right. 

Thanks. 

Gerry

No, I did not use collimator at all. I found this very informative blog post (let me see if I can find a link) that gave interesting instructions for collimating RC.

https://deepspaceplace.com/gso8rccollimate.php

I adopted it a bit for my gear. I did it once with Bahtinov mask, but found that it is not as precise as I would like it to be, so I switched to SharpCap and measuring of FWHM for defocus measurement. I did all as recommended, but when I focus on star in center field then I inspect all corners for FWHM and pick the one that has the biggest value to adjust. It actually took me two rather short sessions (of about 15-20 minutes) to get it quite good over ASI1600 chip. First one was with Bahtinov mask, I did I think 2 iterations and proceeded with imaging, results were not that good, so on next night I repeated with FWHM and additional 2-3 rounds of tweaking.

Here are results:

Top left corner prior to any collimation :

After first round of collimation:

(improvement but still some elongation)

After second collimation:

  or this one:

 

[Gerry Casa Christiana]

I'm not sure if you have encouraged me to stay away from RCs  still sounds quite complicated. I guess it isn't. I think I have enough problems with my F5 newtonian although its getting better. Have you heard of the gold star collimator? Works with its own software and looks great. I think that's what it's called if I find the link I'll let you have a look. You probably already know it. I was tempted to get one for my scope. Comes with a special type of Bahtinov mask depending on what type of telescope you have and the software with it. 

Do you post any pictures anywhere from your RC? I'd be interested to see. 

Gerry

[vlaiv]

1 hour ago, Gerry Casa Christiana said:

I'm not sure if you have encouraged me to stay away from RCs  still sounds quite complicated. I guess it isn't. I think I have enough problems with my F5 newtonian although its getting better. Have you heard of the gold star collimator? Works with its own software and looks great. I think that's what it's called if I find the link I'll let you have a look. You probably already know it. I was tempted to get one for my scope. Comes with a special type of Bahtinov mask depending on what type of telescope you have and the software with it. 

Do you post any pictures anywhere from your RC? I'd be interested to see. 

Gerry

No, I have not heard about that one, but I'll have a look.

I don't have a particular place for images (yet, I'm sort of working on it), but to tell you the truth I don't have much to show off. I started using RC last summer, and managed to get just a couple of images with it - most L channel (no NB or color taken with RC yet - I did combine color data from other scope on one of images but it did not turn out that great). Weather is just not cooperating for at least past 6 months (only one session, and that was short one).

I post most of my images here in DSO imaging section, but to save you trouble, I'll link in ones taken with RC:

http://serve.trimacka.net/astro/2017-07-18/

http://serve.trimacka.net/astro/2017-07-21/

http://serve.trimacka.net/astro/2017-08-27/

http://serve.trimacka.net/astro/2018-05-07/

Just open links and click on png. One of ngc7331 has a lum only (that is pure RC) and color one - mixed with data from another scope and another camera (TS80 F/6 apo and ASI178mcc - so not much color data and it is not that good).

You can use search here on SGL for my posts in DSO imaging to see capture details for above images.

 

[Gerry Casa Christiana]

5 minutes ago, vlaiv said:

No, I have not heard about that one, but I'll have a look.

I don't have a particular place for images (yet, I'm sort of working on it), but to tell you the truth I don't have much to show off. I started using RC last summer, and managed to get just a couple of images with it - most L channel (no NB or color taken with RC yet - I did combine color data from other scope on one of images but it did not turn out that great). Weather is just not cooperating for at least past 6 months (only one session, and that was short one).

I post most of my images here in DSO imaging section, but to save you trouble, I'll link in ones taken with RC:

http://serve.trimacka.net/astro/2017-07-18/

http://serve.trimacka.net/astro/2017-07-21/

http://serve.trimacka.net/astro/2017-08-27/

http://serve.trimacka.net/astro/2018-05-07/

Just open links and click on png. One of ngc7331 has a lum only (that is pure RC) and color one - mixed with data from another scope and another camera (TS80 F/6 apo and ASI178mcc - so not much color data and it is not that good).

You can use search here on SGL for my posts in DSO imaging to see capture details for above images.

 

Your way ahead of me I'm still learning dslr so luminance etc is not something I have tried. I imagine that at the resolution of those pictures it's going to be tough although it depends on the mount I guess. Great pictures. 

Here is the link

http://www.goldastro.com/

Gerry

[ggkids]

On 21/06/2018 at 16:32, vlaiv said:

No, I did not use collimator at all. I found this very informative blog post (let me see if I can find a link) that gave interesting instructions for collimating RC.

https://deepspaceplace.com/gso8rccollimate.php

 

 

Hello,

 I ran into this topic and I checked the link you suggested: very useful. I still don't understand what means: "Use the 3 sets of push/pull screws on the back of the scope to move the star from the bad corner toward the center of the field of view."

Is it meant to move the star from the corner to the center of the image using the collimation screw on the back of the focus? Why does this step work? 

I just want to be sure before acting and destroy what I have gotten so far 

Thanks in advance for your help!

Clear skies.

[vlaiv]

3 hours ago, ggkids said:

Is it meant to move the star from the corner to the center of the image using the collimation screw on the back of the focus? Why does this step work?

It works for any mirror - if you tilt it, reflected rays will change direction.

In above image - incident rays 1, 2 and 3 are all the same - they don't change - but depending on tilt of the mirror - reflected rays go different ways (they all obey same rule - incident angle is the same as reflected angle).

Only difference between concave and flat mirror is that concave mirror converges rays - in case of telescope to a single point - but position of that point will depend on tilt of primary mirror.

When you change tilt of primary mirror - star position in the image changes. Simple as that.

Thing is - if you have one corner that star is particularly defocused at - this is due to tilt of primary and correct way to tilt back primary is such that star image moves towards the center.

If you've ever collimated reflector of any kind - you might have noticed that adjusting tilt of any component causes star image to shift at the eyepiece (or sensor). Same is here - star moving from bad corner to center is just indicator of good direction of tilt adjustment.

[vlaiv]

and @ggkids

Welcome to SGL

 

[ggkids]

11 hours ago, vlaiv said:

It works for any mirror - if you tilt it, reflected rays will change direction.

In above image - incident rays 1, 2 and 3 are all the same - they don't change - but depending on tilt of the mirror - reflected rays go different ways (they all obey same rule - incident angle is the same as reflected angle).

Only difference between concave and flat mirror is that concave mirror converges rays - in case of telescope to a single point - but position of that point will depend on tilt of primary mirror.

When you change tilt of primary mirror - star position in the image changes. Simple as that.

Thing is - if you have one corner that star is particularly defocused at - this is due to tilt of primary and correct way to tilt back primary is such that star image moves towards the center.

If you've ever collimated reflector of any kind - you might have noticed that adjusting tilt of any component causes star image to shift at the eyepiece (or sensor). Same is here - star moving from bad corner to center is just indicator of good direction of tilt adjustment.

Thanks for your complete explanation, I misunderstood an important detail: the movement from the corner to the center is just an indication of the right correcting movement. 

When I read first I thought the star had to be placed there, right in the center of the fov.

Clear skies,

 Giovanni.