48 replies to this topic

[Lumenarian]

Hello,

I got a Celestron Edge HD 8 late last year. I had planned on using the QHY medium OAG with a ZWO 290 mini as a guider, but I have had no luck after many hours of messing with it. I can't find a single star using it. I have the backspacing correct per QHY and Celestron. I have the extension tube on the 290 mini in order to achieve focus. I have tried exposures up to 15 seconds in phd2. I have messed with the gain. And still nothing in phd2 except static. I can clearly see the moon even if it is blurry (I think it's just way over exposed since I can't reduce the exposure time low enough), but I can never see any stars no matter where I have the scope pointed.

So my question is, has anyone successfully used this or the ZWO equivalent with an edge hd 8 at native focal length and the 290mini or equivalent? Mostly, I'm seeing people use the Celestron OAG with a asi174. The asi174 doesn't seem to be a useful planetary camera so I would rather not purchase that to use as only a guide camera. If I were to purchase one based on the IMX174 I would get the qhy version since it has approximately 4-5 times the framerate. Would it be more beneficial to get a camera with a bigger sensor or an OAG with a bigger prism?

[rgsalinger]

First of all, of course it would be better to get a bigger chipped guide camera and an OAG with a larger prism, but that not what I suspect your underlying problem actualy is. The moon is only useful because it's so easy to find. With a C8, you'd need an HA filter and about a .001 exposure to actually see anything, in my experience. Do some measurements and research so that the guide camera is roughly par focal with the main camera.

 

Then ------

 

First of all, is the OAG prism placed just above and parallel to the main camera chip? If not lower it. SCT's don't have terrific off axis performance so getting the right position is key. Don't worry if there's a shadow for now, just get it as close to the center as you can while not obscuring the main camera chip. 

 

Second,  if you are having trouble finding stars, point the camera at the Milky Way. It's galaxy season and if you're trying to image (say) M81, there aren't many stars out there in the first place.

 

Third use this workflow. Use the focuser on the OTA to go back and forth while taking 3 second exposures binned 3x3. That's equivalent to a 27 second exposure binned 1x1. Since you're then able to keep the  exposures fast, as you slowly move the focuser, you are much more likely to stars come into focus. Now you know that the system works and in which direction you need to move the OAG. If stars appeared as you moved out then the OAG needs to move in relastive to the main camera chip. If stars appeared when you moved in then you need to move the OAG out relative to the main camera chip.  

 

Rgrds-Ross

[Scdouglas]

I'll tell you from my experience with a C8, I hate OAGs. I know a lot of people really like them and I know those people have no problems with them, but I don't want to ever think about not having a guide star and  I don't want to spend hundreds and hundreds to do it. I'm sure there was a lot of user error involved on my part, but I'll tell you what I'm at now. This is on a CGEM DX and C8 btw. I got an 80mm refractor to use as a guide scope and I'm using an asi178mm with a plugin for NINA called "Flexure Correction". So far my guiding sits at between .6-.75" total RMS and my stars have been pretty much pinpoint outside of my backfocus issues which are now resolved. Flexure Correction basically figures out the drift caused by flexure between your guide scope and OTA and corrects PHD2 to fix the issue. It works amazingly well, it's free, and it saved me a lot of time and money because I didn't have to go buy a lodestar/174 or celestron OAG which would work great, but both are much more expensive than my current setup. I'm using the 178, but because I'm guiding with a guide scope, your choice of camera can be expanded hugely vs an OAG.

 

Just my 2 cents though, a lot of people are really happy with their OAG and spending on a nice OAG and good camera like the 174 would allow you to use things like the ASI Air because my setup limits me to NINA only. 

[Lumenarian]

First of all, of course it would be better to get a bigger chipped guide camera and an OAG with a larger prism, but that not what I suspect your underlying problem actualy is. The moon is only useful because it's so easy to find. With a C8, you'd need an HA filter and about a .001 exposure to actually see anything, in my experience. Do some measurements and research so that the guide camera is roughly par focal with the main camera.

 

Then ------

 

First of all, is the OAG prism placed just above and parallel to the main camera chip? If not lower it. SCT's don't have terrific off axis performance so getting the right position is key. Don't worry if there's a shadow for now, just get it as close to the center as you can while not obscuring the main camera chip. 

 

Second,  if you are having trouble finding stars, point the camera at the Milky Way. It's galaxy season and if you're trying to image (say) M81, there aren't many stars out there in the first place.

 

Third use this workflow. Use the focuser on the OTA to go back and forth while taking 3 second exposures binned 3x3. That's equivalent to a 27 second exposure binned 1x1. Since you're then able to keep the  exposures fast, as you slowly move the focuser, you are much more likely to stars come into focus. Now you know that the system works and in which direction you need to move the OAG. If stars appeared as you moved out then the OAG needs to move in relastive to the main camera chip. If stars appeared when you moved in then you need to move the OAG out relative to the main camera chip.  

 

Rgrds-Ross

Thanks, Ross. I've tried some additional troubleshooting today, but I'm no closer to figuring out what is going on. I have the OAG installed according to this document, except I don't have the slim CFW3M, mine is 20.5 mm so I have 3 mm less spacers between the OAG and the filter wheel than what this diagram says. I have exactly 133 mm from the Edge HD 8 flange and the camera sensor, per Celestron's documentation. I focused the OTA on a a distant mountain. I pushed in the prism far enough that I see a shadow, more or less covering the top third of the image (I know this isn't what I want, but I wanted to ensure I was getting light into the OAG). I then attempted to focus the 290mm mini, but nothing I do brings it into focus. All I see is a blob of light, no details no matter what I do. So clearly focus is an issue. It was my understanding that the OAG prism should be equidistance from the guide camera sensor and the main camera sensor. I measured this to be about 40 mm, and tried that, but still no focus. I don't know what to do to get this to work. I agree with scdouglas that this OAG is insanely frustrating, and a refractor would probably be easier. but I feel that it wouldn't give me the guiding that I want for these long exposures.

 

 

[HubSky]

Well just a couple of things.  Make sure the OAG or prism isn't backwards.  And when measuring your distance from the prism to the guide camera, remember the mini's sensor is located at the bottom of the 1.25" tube and not at the top.   With my Celestron OAG I had to add a spacer to get the mini far enough out to bring it into focus.  

[rgsalinger]

If the OAG is not backwards (been there) per the previous post, then try the technique I outlined. The OAG HAS to focus somewhere, but with an SCT it can be frustrating. Don't worry about being par focal or the backfocus too much at this point, just find some stars. Use the milky way to make sure that you are going to have them. Make sure that you play with the gamma a bit and see if you can focus it during the day. Again, I use a setting to take continuous images binned 3x3 and just move the main focus know in/out until I see something. Once that's done then I can worry about par focality and backfocus.  

 

Note also that if you want to, you can just avoid using PHD entirely and use NINA or whatever - connect the guide camera as the main camera. It might be easier to see the stars that way as you can play with the stretch. 

 

Rgrds-Ross

[Lumenarian]

So I'm using an EAF in NINA for focusing. Does anyone know which direction the mirror is moving on an SCT when the counts are increasing? Also if I can get focus on the OAG while increasing the counts from the focus point of the OTA, what is that telling me with respect to the distance between the OAG and the primary camera? Refractors I get, but I'm new to reflectors. I'm going to try to do some daytime testing again tomorrow on the mountain.

[nemo129]

If I understand what you are asking about focus direction in an SCT, you watch the EAF as it turns and determine clockwise versus counterclockwise when the EAF position counter is increasing:

 

Counterclockwise = uphill - i.e. primary is moving closet to secondary
Clockwise = downhill - i.e.  primary is moving away from secondary

 

I am a bit unclear as to what you second question is asking. I always focus my main camera to get well focused stars (Bahtinov mask helps), then fiddle with the focus on the OAG/guide camera so stars are in focus. Some folks suggest doing this in the daytime using a distant terrestrial target to get the main camera and the guide camera in the OAG to focus on the same point.

[lumos]

When Ross says it has to focus somewhere I once had it where that somewhere was 15mm outside the focus tube, i.e. I needed to add an extension tube, and one other time I tried to use my camera on my ZWO OAG-L and found that the focus point was too far in so I had to use my mini version (to be fair I did finally find that the instructions for the L version state that you need the mini version to get focus). I suspect as soon as you get the OAG in focus in daytime you will leap forward. My technique is to designate the guide camera as the main camera and set it in focus mode so I get a 'live image' so that should allow you to see what is going on much faster.

[Lumenarian]

Ross was suggesting that the OAG may be in the wrong location and focusing using the OTA focuser would show that. I've measured everything out and the distance from the prism to the main camera and the 290 mini sensor are near identical. I have positioned the 290 mini a bit closer so that I can move it outwards to focus. But according to everything I am reading, it shouldn't need to move more than a couple of mm. I do have an extension tube, and have tried that before but still couldn't get it to focus. I'm going to try it again on this mountain today.

[rgsalinger]

Just to be redundant..... If you are focusing during the day, connect the guide camera as the main camera and take very short exposures. I use NINA and the image display is much clearer than PHD.

 

Also, at some point whether you will eventually need an spacer of some kind, the system must come into focus. Lots of people are using an SCT with an OAG (I have an Edge 11),  so it's just that your workflow is a bit off.

 

So, think about any kind of situation where your display might not be set up so that you can easily see the image on the screen. It can be hard to focus a telescope in broad daylight using a laptop screen, for example. They just aren't bright enough.

 

Rgrds-Ross

[Lumenarian]

Thank you all for your help so far. I did some more troubleshooting this morning. Still no luck so far. In the image below, the left shows what the Edge HD 8 shows focused on a distant mountain. This isn't the best focus, but it is rather windy in Northern New Mexico so it's the best I can do at the moment. You can see a faint shadow at the top where the prism is. The right image shows what I have through the OAG using the 290 mm mini. This is the best I can ever get. This is how it looks no matter what I am pointed at. I have tried moving the camera from fully in to fully extracted using the extension and the helical focuser. I have moved the OTA focus through 12000 steps on either side of the OTA focus point. Nothing improves this blob of light that I see on the guide cam. It gets bigger and it can vanish if I move the camera too far out, but nothing ever comes into even a relative focus. I have confirmed that the prism is aligned with the flat edge towards the OTA, but I don't know what else to do at this point.

 

Any thoughts?

 

[lumos]

surely mean the angled face of the prism facing the OTA?

[lumos]

Could you just do a quick sanity check, could you take a rough measurement of the distance of your main camera's sensor to the OTA's rear element and compare that to the length of the light path from the guide camera's sensor to the OTA's rear element. The light paths should be more or less the same to get focus. I just wonder if there is too much space in the guider's light path so the camera can never go far in enough. To not focus it has to be either too far away or too close. With your OTA back in good focus, you could try and pull the 290 mini right out of the OAG, using your hand or a makeshift tube and see if it pops into focus. If it doesn't then we need to shorten the light path, in effect moving it further in.

 

I am really sorry that this is so frustrating for you.

[rgsalinger]

No the flat side of the prism must be facing the backplane of the telescope. Here's a nice tutorial on how to set up an OAG. 

 

Rgrds-Ross

[lumos]

Of course you are right. Mine is in France or I would have checked.

[Lumenarian]

I did a quick check last night when I was adjusting the prism and the distance from the rear flange of the Edge HD 8 to the camera sensor is 133 mm. The distance from the flat face of the prism to the main camera sensor is 50 mm, and the distance from the prism to the guide camera sensor when the camera is fully inserted is about 48 mm. I suppose I can try inserting another spacer between the OAG and the filter wheel, increasing the distance to the main camera, which should give me a couple of more mm to play with. I'll have to see if I have the right combination of spacers to make that work.

[rgsalinger]

If you are at 48 and the main camera is at 50 then all that you have to do is to move the guide camera out 2mm and you're in focus. If things were reversed and the guide camera fully inserted is too far out then you need to add a spacer between the OAG and the wheel or (less optimal) the wheel and the main camera. 

 

Rgds-Ross

[Lumenarian]

Yeah, that's what I figured too, but no matter where I put the guide cam, all I have is that blob of light. It never gets into focus.

[HubSky]

Even though you said you measured it from the flat side of the prism to the imaging camera, you could do that in either orientation, I haven't heard you confirm that the angled side of the prism is pointed to the telescope side especially since you said "I have confirmed that the prism is aligned with the flat edge towards the OTA..."  

[lumos]

Even though you said you measured it from the flat side of the prism to the imaging camera, you could do that in either orientation, I haven't heard you confirm that the angled side of the prism is pointed to the telescope side especially since you said "I have confirmed that the prism is aligned with the flat edge towards the OTA..."  

Actually I introduced that tomfoolery; it has to be the flat side, as Ross corrected me, facing the OTA as the light enters through the flat side, continues until it hits the internal facet that turns the light up towards the OAG.  It was obvious which way to put it when assembling mine, but just thinking about it, and I did for some time before posting, I imagined the light hitting the front angled face but prisms are all about internal reflection. I am so sorry to you and the OP for confusing things. 

[lumos]

By any chance do you have access to an eyepiece. The mini cameras focus in exactly the same position so you can focus with an eyepiece and then swap it over for the camera.

 

Can I just ask a stupid question, please don't be offended, but do you definitely have the 290mini and not the 290? Clearly you know what you are doing but I just want to eliminate every aspect that could cause this. 

 

My 290mini came with an extender that screws on to the front of the camera. Have you tried it both with and without. When you measured the 48mm where was the sensor in relation to the OAG; was it all the way in with no front extender, all the way in without it, all the way out or somewhere in between. 

[HubSky]

Yep, confused myself.  

[Lumenarian]

Yes I most definitely have the 290 mm mini. When I measured the 48 mm it was fully inserted with no extension tube. I also accounted for the bit of recess from the edge of the tube to the sensor which is about a mm or so. By any chance do OAG's have any sort of protective film over the prism? I don't think thats the case here since I can look through the prism when it's not connected to the OTA and see clear reflections, but I thought I would check. I do not have access to any eyepieces as I do only photography.

[lumos]

My ZWO OAG-L did not have anything on the prism. If the 48mm is when the camera sensor is fully in then moving it out is easily covered in the focus range of the tube. 

 

At this point, personally, I would run with the hypothesis that you need the guide camera sensor to be closer in rather than further out. Can you rearrange your existing spacers to give you more space between the OAG and the filter wheel (essentially moving the main camera out to meet your guide camera's focus position?).

 

Mind you my experience level is practically zero compared to Ross's so you may want to wait and see what he suggests.

 

Finally, I hope that I did not insult you by confirming your 290 mini.