31 replies to this topic

[Scott Mitchell]

Hi everyone, I've recently acquired an 8" LX200 ACF. I am attempting to use it in conjunction with a ZWO OAG and my Canon T5i. The good news is I've gotten focus worked out, and guiding is going well. The bad news is that it seems that I pretty much have to position the pick-off prism down so far that it partially obscures the view of the DSLR's chip.

 

Related, I believe, is that I'm also getting weird reflections that are not correcting out with flats. Here's an example of a raw sub that shows the weird vignetting/scattering of light.

 

 

And here's the corresponding flat:

 

 

Additionally, I just noticed last night that the prism itself has a chip. I don't know if it came to me that way from the previous owner, or if I have done something that chipped it.

 

 

So... I need some suggestions. The stalk is obviously not flat black, would it help if I paint it? Will the chipped prism cause issues? Is it inherently a problem to have the stalk infringing on the light path to the imaging chip (I'm OK with it conceptually as long as I can correct it with flats).

All thoughts are welcome!

[Midnight Dan]

Looks to me like that is shadowing from the pickoff prism.  It is so far in that it's blocking some light from the edge of the image.

 

If it has to be that far in before your guide camera can see stars, it says that the image circle is not large enough.  This might be due to something in front of the OAG that's restricting the image circle, or it may be the optics.  Are you using a focal reducer?  That would reduce the size of the image circle.

 

-Dan

[Scott Mitchell]

Looks to me like that is shadowing from the pickoff prism.  It is so far in that it's blocking some light from the edge of the image.

 

If it has to be that far in before your guide camera can see stars, it says that the image circle is not large enough.  This might be due to something in front of the OAG that's restricting the image circle, or it may be the optics.  Are you using a focal reducer?  That would reduce the size of the image circle.

 

-Dan

Sorry Dan, that is a detail I left out. I am, as you speculated, using a focal reducer to avoid imaging at f/10.

[ELDavis]

Scott,

Your problems are the reason I switched from an OAG to a guide scope on my Celestron C8. The Orion SSAG gives me good results when I image at F 6.3. I never tried it at F 10.

[rgsalinger]

I'm not sure why you have to push the prism so far down. That makes no sense to me as the only possibility. If the problem is that the Canon needs to be a particular distance away from the back of the OTA, then you need to get the right spacers so that you can get the prism further up. That will eliminate any shadow. Can you provide some details?

 

I'm also wondering about the guide camera. You don't mention what you are using. If you really cannot get stars then my guess is that it's got a small chip in it (not a good match for an SCT), is pretty insensitive (have you tried binning it?) or you need to adjust the gain up.  Again, details might be useful. 

 

You've show the telescope side of the OAG and it looks as is someone mishandled it before you got it because of the chip and the fact that the anodizing has been removed. Honestly, I'd get a different OAG that has a larger prism rather than trying to save this one. 

 

Rgrds-Ross

[Scott Mitchell]

I'm not sure why you have to push the prism so far down. That makes no sense to me as the only possibility. If the problem is that the Canon needs to be a particular distance away from the back of the OTA, then you need to get the right spacers so that you can get the prism further up. That will eliminate any shadow. Can you provide some details?

 

I'm also wondering about the guide camera. You don't mention what you are using. If you really cannot get stars then my guess is that it's got a small chip in it (not a good match for an SCT), is pretty insensitive (have you tried binning it?) or you need to adjust the gain up.  Again, details might be useful. 

 

You've show the telescope side of the OAG and it looks as is someone mishandled it before you got it because of the chip and the fact that the anodizing has been removed. Honestly, I'd get a different OAG that has a larger prism rather than trying to save this one. 

 

Rgrds-Ross

Hi Ross, you're right, I forgot to mention details about the guiding camera. It is an ASI120MM. The reason I have the stalk so far down is that with the focal reducer I only get half of the guiding sensor illuminated even with the stalk that far down. I'm making some assumptions about the focal reducer there which I will test out tomorrow during the day. Check another assumption of mine, if I can't even get light on the full 120 chip, I assume getting a larger chip like the 290 wouldn't help, right?

 

I'd hate to have to acquire another OAG. I'd love to figure out a way to make this work if possible.

[rgsalinger]

I always thought that the closer the prism was to the guide camera chip then the greater the illumination would be.

 

You're right ----  a larger chip won't help you if you can't illuminate a smaller chip but a more sensitive chip might help.

 

Now I'm wondering about the gain setting that you are using. Have you experimented with higher gain settings? 

 

Also, can you take a picture of the whole system. I keep thinking that you need to get the OAG closer to the back of the OTA - mine screw in directly to the focusers - so that the light cone is still pretty large when the light hits the guide prism.

 

Rgrds-Ross

[Scott Mitchell]

I always thought that the closer the prism was to the guide camera chip then the greater the illumination would be.

You're right ---- a larger chip won't help you if you can't illuminate a smaller chip but a more sensitive chip might help.

Now I'm wondering about the gain setting that you are using. Have you experimented with higher gain settings?

Also, can you take a picture of the whole system. I keep thinking that you need to get the OAG closer to the back of the OTA - mine screw in directly to the focusers - so that the light cone is still pretty large when the light hits the guide prism.

Rgrds-Ross

Ross, I'll post more pictures of the setup tomorrow. Unfortunately, if anything, I will need to move the guide camera farther away from the OTA as I don't actually have the right spacing from the focal reducer to my imaging camera yet.

I'll also try to grab an image of what the guide camera is seeing to try to explain what I'm saying about the inability to light up the full chip.

Stay tuned!


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[Midnight Dan]

The reason I have the stalk so far down is that with the focal reducer I only get half of the guiding sensor illuminated even with the stalk that far down. I'm making some assumptions about the focal reducer there which I will test out tomorrow during the day. Check another assumption of mine, if I can't even get light on the full 120 chip, I assume getting a larger chip like the 290 wouldn't help, right?

 

I'd hate to have to acquire another OAG. I'd love to figure out a way to make this work if possible.

That's correct - a larger sensor won't help.  If you only see half an image on the guide camera when the stalk is farther out then you've definitely got too small an image circle.  

 

The view you posted of the front of the OAG shows that there is nothing there restricting the edge of the circle. So it's either the focal reducer, or some other component that's in front of the OAG.  Can you provide a photo of your complete setup from the side?  It would be helpful to know what exactly is between your OAG body and the back of the scope.

 

-Dan

[ChrisWhite]

Try imaging as it is and see if flats correct it out. 

 

I've dealt with some pretty serious prism shading in the past when I was imaging with a newt reduced to f2.8.  Flats always took care of it.

[Scott Mitchell]

Can you provide a photo of your complete setup from the side?  It would be helpful to know what exactly is between your OAG body and the back of the scope.

 

Also, can you take a picture of the whole system. I keep thinking that you need to get the OAG closer to the back of the OTA - mine screw in directly to the focusers - so that the light cone is still pretty large when the light hits the guide prism.

Dan/Ross, first up, as requested, here's the full imaging train:

LX200 -> f/6.3 reducer -> OAG -> 2 x 5mm spacers -> Canon t5i
                                                  -> 1.25" adapter -> ASI 120MM

 

 

I did some testing this morning with and without the focal reducer and can confirm that it is the source of the guide camera FOV issue. Here's with the focal reducer:

 

And without:

 

I also confirmed that without the focal reducer I can withdraw the stalk far enough to not impinge on the view of the DSLR.
 

Try imaging as it is and see if flats correct it out. 
 
I've dealt with some pretty serious prism shading in the past when I was imaging with a newt reduced to f2.8.  Flats always took care of it.

Chris, that's actually my main issue. The flats aren't correcting the image properly. That's what made me wonder if the shadow from the OAG isn't the issue so much as possible reflections coming from that stalk that isn't blacked out very well. Thoughts on that?

Edited by Scott Mitchell, 05 April 2020 - 10:27 AM.

[ChrisWhite]

 

Dan/Ross, first up, as requested, here's the full imaging train:

LX200 -> f/6.3 reducer -> OAG -> 2 x 5mm spacers -> Canon t5i
                                                  -> 1.25" adapter -> ASI 120MM

 

 

I did some testing this morning with and without the focal reducer and can confirm that it is the source of the guide camera FOV issue. Here's with the focal reducer:

 

And without:

 

I also confirmed that without the focal reducer I can withdraw the stalk far enough to not impinge on the view of the DSLR.
 

Chris, that's actually my main issue. The flats aren't correcting the image properly. That's what made me wonder if the shadow from the OAG isn't the issue so much as possible reflections coming from that stalk that isn't blacked out very well. Thoughts on that?

 

Can you share a single flat frame and a single light frame with Dropbox? 

[Midnight Dan]

Yep, definitely the focal reducer.  

 

As to why the flats don't correct for it, it's hard to say.  Usually flats have a fairly narrow peak in the histogram, and only have to compensate for a small range of darkening due to dust motes or mild vignetting.  It could be that the shadowing from the stalk is widening the peak too much for it to be able to completely deal with it.

 

Not sure how much you've stretched the flat shown above, but the corners show some pretty severe vignetting as well.  That will also be due to the focal reducer.  If you remove the focal reducer, you'll find that those corners brighten up as well.

 

When I had my "plain jane" C8, I had a lot of trouble trying to use a focal reducer for imaging.  It reduces the size of the image circle which has several negative effects.  Vignetting is just one.  In addition, because the field is not flat in a standard SCT, the stars near the edge of the image circle where significantly distorted and out of focus.  Shrinking the image circle brought those ugly stars into the frame and into the OAG field of view which made guiding more challenging.

 

The Edge8 that I now own is designed to have a sharp, flat field across an APSC sized sensor, even when used with the Celestron Edge Focal Reducer.  Makes a huge difference when trying to use an OAG.  I place my stalk as far to the edge of the field as I can with the focal reducer in place.  I still get a small amount of shadowing, but it is mild and can be handled by my flats.

 

-Dan

[Midnight Dan]

And another thing ... :-)

 

That focal reducer you're using is one of the common, inexpensive SCT reducers that are designed for visual use, not so much for imaging.  If you have a small sensor camera, you can get away with using it because you're using such a small part of the image circle.  But it really isn't designed for astrophotography, especially with larger sensors.

 

Here's a CN thread addressing this exact issue for Meade ACF SCTs.

 

https://www.cloudyni...lescope-optics/

 

-Dan

[Scott Mitchell]

Can you share a single flat frame and a single light frame with Dropbox? 

Folder created here with 1 light, 1 flat, and master flat. I'm running and experiment in APP right now of saving off the normalized frames prior to the integration phase to make sure I know what they look like. I want to make sure the flat application is the source of the strange result I'm seeing. In case I haven't shared that yet, it looks like the following image. Note that you can still vaguely make out the shadow of the OAG at the top, pick up a very dark area at the bottom, and a faint horizontal banding that becomes more apparent during processing.

 

[Scott Mitchell]

And another thing ... :-)

 

That focal reducer you're using is one of the common, inexpensive SCT reducers that are designed for visual use, not so much for imaging.  If you have a small sensor camera, you can get away with using it because you're using such a small part of the image circle.  But it really isn't designed for astrophotography, especially with larger sensors.

 

Here's a CN thread addressing this exact issue for Meade ACF SCTs.

 

https://www.cloudyni...lescope-optics/

 

-Dan

Thanks Dan, I know I'll eventually have to acquire a more appropriate focal reducer. I started with this one as I had it from an old LX-90 I used to own. Do you think either the Antares reducer or the CCDT67 would actually help with the size of the image circle, not just the star shapes?

[Scott Mitchell]

Can you share a single flat frame and a single light frame with Dropbox? 

Chris, I uploaded a second light frame. My test of saving the normalized frames had an interesting result. The first frame I uploaded for you was the reference frame. It seems to correct well with the flat, but of the dozen or so normalized frames I reviewed, it was the only only that looked well corrected. The second one I uploaded is an example of one that didn't seem to fully correct. Maddening to say the least.

[Midnight Dan]

Thanks Dan, I know I'll eventually have to acquire a more appropriate focal reducer. I started with this one as I had it from an old LX-90 I used to own. Do you think either the Antares reducer or the CCDT67 would actually help with the size of the image circle, not just the star shapes?

Because of the special optics in the ACF, it's like the Edge8 and would require a specialized reducer.  I couldn't really say if those particular reducers would work for your or not.  The thread I linked in my previous note has comments from various people on various reducers.  That's probably the best place to look for info.

 

One thing I can say is that when I was looking at reducers for my Edge8, the only other one besides the Celestron that was purpose built for my kind of scope was the Optec Lepus.  Unfortunately, these are expensive, as is the Celestron Edge Reducer.  Since Meade does NOT make a reducer designed to work with the ACF, if I owned one I would probably not be looking at any other besides the Optec.  It claims to work with the Edge8 and the Meade ACF and it provides for 28mm diagonal sensor coverage, which should work with any ASPC sensor just like my Edge reducer.

 

Here's a link:

https://optecinc.com...lepus/index.htm

 

Just be sure to note the back focus requirements and be sure you can put your sensor at the specified location with the OAG in place.

-Dan

[Midnight Dan]

One more thing to consider before spending a lot on another reducer.  In your initial photo of the camera and OAG, there are two horizontal bars, one above and one below the sensor.  What are those?  I wonder if they aren't adding to the shadowing, especially on the opposite side from the stalk as seen in your shadowed image above. When I look at a photo of the Canon T5i body online, I don't see those bars.  The sensor is much more exposed than what I see in your image.

 

-Dan

[Scott Mitchell]

Because of the special optics in the ACF, it's like the Edge8 and would require a specialized reducer.  I couldn't really say if those particular reducers would work for your or not.  The thread I linked in my previous note has comments from various people on various reducers.  That's probably the best place to look for info.

 

One thing I can say is that when I was looking at reducers for my Edge8, the only other one besides the Celestron that was purpose built for my kind of scope was the Optec Lepus.  Unfortunately, these are expensive, as is the Celestron Edge Reducer.  Since Meade does NOT make a reducer designed to work with the ACF, if I owned one I would probably not be looking at any other besides the Optec.  It claims to work with the Edge8 and the Meade ACF and it provides for 28mm diagonal sensor coverage, which should work with any ASPC sensor just like my Edge reducer.

 

Here's a link:

https://optecinc.com...lepus/index.htm

 

Just be sure to note the back focus requirements and be sure you can put your sensor at the specified location with the OAG in place.

-Dan

Thanks Dan, I wasn't really asking you to endorse one as I know that's unfair. I know that these will likely all produce flatter field than my current reducer, but I was surprised by the idea that maybe they would yield a larger image circle and therefore help with theOAG issue. Do you think that's actually likely? I can also do other research as well.

[Scott Mitchell]

One more thing to consider before spending a lot on another reducer.  In your initial photo of the camera and OAG, there are two horizontal bars, one above and one below the sensor.  What are those?  I wonder if they aren't adding to the shadowing, especially on the opposite side from the stalk as seen in your shadowed image above. When I look at a photo of the Canon T5i body online, I don't see those bars.  The sensor is much more exposed than what I see in your image.

 

-Dan

Do you mean the areas I've highlighted here? If so I can't tell you, that's just the way the camera is. I wonder if it looks different that what you see online due to my camera flash lighting them up. I can tell you that I have taken many, many successful images with this camera with smaller refractors.

Edited by Scott Mitchell, 05 April 2020 - 01:35 PM.

[Midnight Dan]

OK, the definitive way to determine if you need a different focal reducer (like an Optec) is to measure the image circle on your current FR.

 

Install the FR and nothing else on the scope.  Aim the scope at a bright outdoor scene with the rear of the scope heavily shaded.  Easiest way to do this is aim it outdoors through a window at a bright scene.  Turn off the room lights and pull the drapes around the scope to keep light from coming in.  If that's not possible, you might have to use a blanket over the rear of the scope instead of a darkened room.

 

Now get a piece of white paper or cardboard and hold it up behind the focal reducer.  You should see a circular patch of light.  Move it back and forth until the scene is in focus.   Put a couple marks on the paper with a pencil on opposite sides of the circle, then measure that to get the diameter.

 

If it is 28mm or more, then another focal reducer won't help with your stalk positioning.

 

-Dan

[ChrisWhite]

I looked at the flat and light you supplied, and it is certainly not a perfect match. 

 

Something worth considering, is how have you taken your flats?  It's possible that your flats are just not optimal.  It might be worth taking flats again and/or trying a new method for acquiring to see if you can correct for the shading. 

[rgsalinger]

That focal reducer is NOT for the ACF scopes. "NOTE: Not for use with Meade Advanced Coma-Free optics." from the Meade website. So ditch that thing for sure for better results.

 

Any/all reducers reduce not only the focal length but also the imaging circle as well.

 

The 1120 is another issue with this setup (IMHO of course).

 

Get a better guide camera and see how well you can do at F10. Then add stuff into the mix as necessary once you are getting good results at F10. 

 

You may not need a reducer - the ACF is supposed to give you a large enough imaging circle for use with an APS-C sized chip.  

 

Rgrds-Ross

[ELDavis]

Scot,

Before spending money on a new focal reducer or OAG, I would consider going to a guide scope. I quit using my OAG when I went from film to digital.The time I save focusing and acquiring a suitable guide star is significant and better spent improving polar alignment.You already have a guide camera, so all you need is a guide scope which can be had for less than $100. By liminitng the length of your lights an taking more of them you can minimize the vignetting from the focal reducer. The concerns of differential flexure, much discussed i've never foumd to be a problem.

 

 

 

This is an image obtained with Celestron Ultima 8 and Orion SSAG. Lights were 240 seconds at iso 800 (using Celestron focal reducer,) You can judge for yourself the quality of the guiding and Vignetting.