Discussion in 'STC Series CMOS' started by Ian H., May 9, 2021.
That Guiding image - it doesnt look anywhere near focused.
Exactly. The focus dial is as far as it can go, and it needs a little more travel to get to focus. But that was not an issue with the refractor.
Understood. And did you try sliding the pick off mirror position ?
Not with this set up. Initially, I played with the position of the pick-off mirror, thinking there was a shadow cast on part of the sensor, which turned out not to be the case. The mirror has been fully extended ever since.
The reason I mention this:
When the pick off mirror moves inward to the centre of the field, that increases the focal distance to the CMOS APS guide sensor.
What I don't know is which side of focus you are on - any idea? eg is the guide sensor approaching focus, or has it passed focus?
If approaching focus, but the knob doesnt quite get it there, you could try to slide the pick off toward the centre of the hoop.
Alternatively, I'll talk to @Doug about a spacer.
My best guess is that it is approaching focus, because backing it off consistently makes it worse, i.e., the star image gets larger and more diffuse. I can try adjusting the pick-off mirror, but that introduces other issues in my light polluted skies; I frequently have trouble finding guide stars, which seem to be especially scarce around deep sky objects, and is made worse in this configuration because of the reduced field of view.
I was thinking about the spacer idea this morning, and it occurs to me I might be able to mock something up with some 1/4" UHMW I have lying around; the only issue for me being the length of the current fasteners (I should be able to pick some up - - but for hex sockets I might have to order them from Fastenal). But I'm limited in the choice of thickness: 1/4" is the smallest, next size I have on hand is 5/8". But I'm guessing 1/4" might just be enough (and worst case I could make duplicates and stack them together until I get there). If that fixes the issue, then my spacer accessory package idea might be something you want to consider. This is an unremarkable configuration of equipment, so this will probably come up again from time to time.
Some notes for configuring an SC-2 to the STC-7:
The SC-2 has 20.7mm of focus travel. In the CW direction the imager is closes to the take off mirror, hence shortest distance. If one needs a thicker spacer, the knob should have bottomed out in the CCW direction. Using the ACC-3 required an additional spacer of 0.300" and longer bolts. For the STC-7 + SC-2 configuration a 3.5" OD x 2.5" ID x 0.400" spacer would bring the SC2 into focus at the center of it's focus range. Having this spacer bolt to the SC-2 and the STC-7 separately would save the problem of various bolt lengths to accommodate varying spaced depths.
A distant terrestrial target was used to derive the above values and was proved out under the stars using two different APO refractors.
With the extra spacer in addition longer bolts were required and luckily Panhead Phillips bolts were available locally and fit properly.
The curvature of focus will effect the values. One can focus the main imager (at the center FoV) and then adjust the telescopes focus until the guider (SC-2) is in focus. Measuring the distance the telescope drawtube moved gives one the minimum spacer required.
For the Celestron Edge HD it appears there will be some coma in the guider/SC-2. Page 5, Figure 2 of the users manual, the spot diagram shows this at 20mm offset from the optical axis. A quick check of the offset of STC-7 center to pick off mirror center is ~20mm. In otherwords the SC-2 mirror is 20mm offset from the optical center.
Curious the TV NP101 was able to achieve focus.
Once the skies clear will try an AP R-H scope with this setup and verify the above information holds up. The AP R-H has "diffraction-limited performance over a huge three (3) degree field". With the STC7 in focus the SC-2 should be also in focus with perfect pin point stars.
That's a good point about coma; visually in the main scope, it's difficult for me to detect, so I hadn't really considered it. And I'm going to re-check the NP101 tonight; I don't think I was clear about that - - the focus issue with the SC2 didn't show up until the AO-8A was added (I tested all the equipment in sequence as I assembled it: camera, camera plus SC2, camera plus SC2 and AO-8A), which made me think there might be an issue with the curvature of the lens in the AO-8A. But if I remember correctly, the problem wasn't nearly as bad as it is with the Edge (and in the Edge, even without the AO-8A, the problem doesn't go away). The main trouble with the refractor was guiding performance being worse than unguided, per the example I showed from the Edge. But the seeing hasn't been great, and I haven't played with the aggressiveness settings much yet, so I may yet sort that out. (And the success I had using PHD2 with the other autoguider - a ZWO ASI290 - might have had more to do with the size and sensitivity of that sensor than anything else. I bought it to use with a separate guidescope on the NP101, when doing long exposure / wide field DSLR shots.)
Thanks for your input! I will try the spacer methodology you've laid out here as well!
So this thing continues to make a liar out of me; when I tried camera/SC2 combination with the NP101 again, I had the same issue with out of focus guide stars (see attached screen shot). See the "locate star" screenshot - - you'll see that the stars are perfectly focused (though in that case the AO-8A was attached, so my memory was faulty on that one; I'll screenshot everything from now on). The only thing I can think of is that I must have been using a Barlow lens last time. Either that, or something has gone wrong mechanically between now and then (which seems very unlikely - - that much damage would have impacted everything, not just pick-off mirror focus, and it's never been dropped, etc.).
Anyway, I'll fabricate the spacer before my next clear night.
Just as an FYI - the AO-8A has a flat piece of specialty glass - no curvature.
@Doug has 3d printed a spacer here and some measurements are planned.
Sounds good - - thanks!
Success! I fabricated the spacer out of UHMW, using the dimensions Tim specified above, except the thickness of the material: I used 1/4" (~6mm) material, with the same ID and OD dimensions. I also managed to assemble it using included fasteners per his recommendation, but as you can see in the assembly photo, it's a bit of an inelegant solution; the recessed holes on the ACC13 spacer could only be used by flipping it over, so that instead of the inside shoulder nesting inside the inside opening of the SC-2, it was the mating surface to the camera (adding another ~2mm extension by my measurements). This also meant the hole locations were 45 degrees off, so instead of aligning the SC-2 perpendicular to the body of the STC 7, it was as you see it here.
The only other option I could see was to tap holes into the UHMW; I've done that before and it takes a thread pretty well, but with this small a fastener with a load of this weight (and cost), I was reluctant to risk it. The right length of fastener is definitely a better way to go.
I expected that this configuration would cause severe vignetting as the pick-off mirror looked like it might cast a shadow over one corner of the sensor window; in practice, I was surprised to find it did not seem to make a difference - - even though the mirror is still fully extended. However, I'm not crazy about having to invert the ACC 13 spacer this way; I guess the assembly is unlikely to shift much during use anyway, but it makes me nervous to deviate from an engineered design for fit/function.
On the spacer I made, I tried to insert a ring of flocking tape to the inside surface to avoid introducing any additional light scatter (I wasn't crazy about having to use white, but I didn't have the right dimension in black), but I couldn't get it to adhere. As you can see from the attached screenshot, stars are now in focus! But I believe the added 2mm from the shoulder of the ACC 13 is not necessary / desirable in this configuration: the travel of the pick-off mirror focuser is almost to the limit in the opposite direction - - the wheel was starting to have more resistance as I got the stars into focus, meaning there was not a lot of extra room to work with.
This is the setup I used last night:
Televue NP101, F5.4
No focal reducer or Barlow
2" Televue extension tube to reach focus
Losmandy G11 mount with Gemini 2 ("Mini"), connected with a straight through cable.
I will try this setup adding in the AO-8A next, or will try this same setup with the Edge 11 HD (depending on my choice of targets that night). But I think it's safe to say it would be worthwhile offering this as an accessory package - - this has to be a very common imaging setup. I'll certainly buy it!
Congratulations on a job well done, the stars are very well focused!
Thanks for the update...
Thank you - - and thanks again for your help!
So some additional testing a couple of nights ago, and some additional findings:
1) The setup below yielded in focus stars for the autoguider without any problems - - i.e., the fully assembled, 3 component system - - my spacer removed, nothing exceptional required.
2) The only way I could get the SC-2 to guide properly was by using the GPUSB option through Maxim, using the GPUSP adapter by Shoestring Astronomy, but without using PHD2; once I tried it this way, the guiding was exceptional - - very low RMS values. I had always had problems with the Y axis drifting on the graph - - seemingly uncorrected. Unfortunately, I didn't use the graph until the last few sessions, so I didn't see the drift, just the impact on images. What surprised me is that the cable choice actually made no difference - - I tried both. But this is confusing to me: based on the pinout diagrams of both the SC2 and the Losmandy autoguider port, one or the other cable should have worked; the diagrams are a little ambiguous in terms of orientation, so I can see getting them in reverse order - - but in that case, one or the other cable should have worked, and neither did. This may be something peculiar to this mount model (or even an issue with the board on my mount) but you might want to pass on this solution to anyone using this particular setup.
3) I cannot get the AO-8A to work with this setup. Obviously, the GPUSB arrangement is not designed with this configuration in mind, and I couldn't get guiding to work with either cable when the AO-8A was included in the mix; I didn't fully understand this until I saw the graph and understood what was happening. I also have much more trouble identifying guide stars through the AO-8A, versus just the SC-2 (but this is not a focus issue - - stars are there and in focus, but dimmer and the AO-8A seems to have more trouble locking onto them). My guess is that I will have to use ASCOM to get this component to work for my particular setup. I was hoping to avoid using ASCOM as it seems to be particularly arduous with Gemini mounts, and clearly I have a limited skillset when it comes to networking equipment.
So I guess in conclusion, I would make the following suggestions: 1) either include an additional spacer or offer it as an option package when someone is using the STC-7 and SC-2 together, without the AO-8A; 2) maybe revisit how these three components "talk" to each other. Or maybe the second point is more of a marketing question: advanced users will likely connect everything through ASCOM anyway, but I think this package could be a 'game-changer' for you guys: an all-in-one entry level package for people wanting to move into monochrome imaging, with room for growth and advancing skill sets built in. That's exactly what I was looking for, and it mostly fits the bill admirably - - except for ultimately needing to move to a complete ASCOM setup to have all three components work together as intended. But maybe that's a consideration specific to my particular equipment, abilities, etc..
This was the setup I used:
Televue NP101, F5.4
No focal reducer or Barlow
No extension, my spacer removed.
ACC-13 Spacer (installed per design - - not flipped as in my previous setup)
Losmandy G11 mount with Gemini 2 ("Mini"), connected with a straight through cable, then the cable supplied with the SC-2, then with GPUSB hub from Shoestring Astronomy.
Hi Ian, I'm going to ask @Doug to take a look at your latest post.
I'm not sure why you're having trouble with the guider cable - shouldn't need GPUSB to guide. With the issues on the Y axis it could indicate that the cable could be reversed. Unfortunately the various guider and telescope manufacturers are not consistent on this. (We'll say we're the ones who have it right because after all it was originally called the "ST-4 Port". )
Guide commands should work the same no matter whether you're using AO or not. Maybe try guiding normally with the AO attached.
If the star is fainter with the AO attached, it may be that the guide star is being vignetted by the AO. Try moving the mirror in as close as you can without vignetting the main camera. Also if the guide chip is in fact being vignetted, it is likely that one side of the chip is brighter than the other. Try moving the guide star up/down and see if it gets brighter and dimmer as you move it; for example, it could be brighter when positioned at the bottom of the sensor.
Thanks Doug. And yes, I've noticed a lot of manufacturers are very loose in their interpretation of an "ST-4" port! I'll try your suggestions, test some specific scenarios, and grab some screenshots: guiding normally with AO attached, screenshots of the graphs with reversed and straight-through cables, and then using GPUSB for comparison. And I'll test the guide star position per your comments, and screenshot those as well. Probably Thursday night before the weather co-operates.
So I held off posting after the testing mentioned above, as I found other things to try in the Maxim DL manual (trying both "sensor 1" and "sensor 2" settings, trying the "SBIG w/AO" configuration), but those didn't work either. This is what it boils down to: STC-7 works fine, SC2 works with ASCOM (which I managed to set up) or GPUSB, but not using guider relay settings, using either the provided cable or a straight through cable. I cannot get the AO to work in any configuration. It seems to be the Y axis that never guides properly, as you can see from the graphs. (And just a reminder, if I remove the AO-8A to use just the SC-2 with the camera, I need to use the spacer I fabricated; I just got the 1.5" 4-40 screws I need to assemble it with the ACC 13 spacer in the correct orientation, which I'll try next time out.)
The AO unit behaves like it's working - - it runs through activities, but the calibration always seems to fail (both drive and AO calibration); I've tried guiding with it anyway, and the Y axis drifts until it can no longer track on the X axis, and guiding fails. Per Doug's comments, it's failing as though there is a connectivity issue in the cable, but neither cable works. And when I use ASCOM, the SC2 works great, but the AO does not behave any differently. Screenshots attached.
Is it possible there is an internal connection problem with the AO-8A? Have you guys seen anything like this before?
So... any other suggestions? For now, I've taken off the AO-8A, and reassembled the STC-7 to the SC-2 using my spacer.
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