STF CCD, FW8 and OAG 8300 Backfocus Validation

So, is my focus problem related, or is it something different entirely, or... is my particular setup doomed. At this point I just want to get something/anything working. It's my fault of course for letting this equipment sit for so long before using it, I envisioned having more time available when I bought it, but needless to say, that didn't happen. I recently retired so I finally have time to spend on this hobby... but given that, is there a trade-in option or a 'good guy' discount of some kind I can leverage to get a setup that is known to work? (i.e. current CCD, OAG, etc... with optimal back-focus, compatibility with my EdgeHD, etc...), would appreciate your recommendations/options?

 

Just saw your last post... looks like I uploaded the wrong FIT, I saved both the STi and the STF images... stand by, i'll get the right ones uploaded shortly...

 

Oh darn, when I saved my STF images first, then switched to the STi camera, and saved those over top of the STF (thus the file name)... so I lost those... I'll capture them again tonight if the sky is clear... sorry about that

 

Just one last round to see if I can figure out my focus problem. This evening I tested focusing with both an eyepiece and then the STF on the same objects, and I can get a perfect focus using the eyepiece, but when I focus with the SFT image train, I just get a blob. I've manually moved the focus in and our for 30 minutes or so on each object (using the Celestron focus interface on Windows). With the SFT image train setup I've tried two scenarios controlling the focus motor, the first is with the Celestron focus windows app connected, (i.e. in Maxum the focuser is not connected) and I can move the focus motor in and out, moving from the donut to the fuzzy blob, but at no point does the image ever get crisp, at the middle where it should get clear is remains a blob where it passes through the point in which (when using the eyepiece) the image should get crisp. The second method is with the Celestron focus windows app unloaded (disconnected) and the Maxim auto focus connected, the autofocus function sends two adjustments then hangs (or waits, the software isn't hung because I can press abort twice and it ends ok). Also, manually pressing In and Out a similar thing happens, it will accept the first In or Out command but then will hang, (and again, pressing abort twice resets it). I've attached two image files as well as the screen shots for both (one with Jupiter and the other with Altair). Let me know if I'm doing something wrong relative to operating Maxim DL and the autofocus routine. Also, given I can't get a crisp focus with the SFT, does this mean that shadow problem is causing an issue, or something else?

 

Right, when I move the focus knob manually from all-the-way out to all-the-way in, I go from donut to donut and the middle is hard to get exactly because the best it get's is a blob between donuts, but a large span so it's hard to tell where the right focus is. My original premise was I may have a backfocus problem, I'm uncertain about what the right backfocus is... my original question in the first post of this thread was:

Background: My understanding is the STF-8300 CCD with FW8 and the OAG-8300 with STi camera combo is 57.5mm of backfocus. My telescope is the Celestron EdgeHD 1400, which requires 146mm of backfocus and the Celestron T-Adapter gives 88.5 of backfocus, which leaves the standard T2 length of 55mm for camera/etc. This means (i think) the STF/FW8/OAG combo is 2.5mm longer than the standard T2 length of 55mm, and I'm struggling with finding the right permutation of spacers (instead of the T-Adapter) to give me 57.5mm for this STF/FW8/OAG setup.​

Question: is the STF/FW8/OAG combo really 57.5mm of backfocus, or is it 55mm by chance, or something else? (i think you indicated it is 57.5mm, which i guess means I'm 2mm off, but is that really a big deal, i.e. shouldn't the focus move in/out beyond 2mm... ​

So, I guess I'm either in backfocus hell, or my CCD chip's shadow is causing my focus to be unachievable. Any other ideas?

 

Yes, indicated in my post yesterday, with my eyepiece in it focuses perfectly, super crisp.

 

Darren.

If it is of any help....

I had a hard time looking at the posted picture of your FW deciphering exactly which filter is in which slot, couldn't really see a clear luminance filter at all but it is always difficult to determine from a photo.

Can you confirm that when focusing with the camera using the luminance filter that the selected luminance filter really is in position and not an empty slot or one of the other filters?

You can verify that in the daytime with the camera mounted on the OTA by selecting the various filters with a long exposure time and looking into the front of the OTA you should be able to see the selected filter reflected in the primary, you may need to shine a flashlight or angle poise lamp etc into the OTA to brighten the reflection.
Alternatively, take the camera off the OTA and look straight into the camera port while exposing on the various filters and confirm that the filter you select in the acquisition program really is the filter active in the camera.

Have you always tried to focus only with a luminance filter selected? is there is a difference if you choose green or red filters?

Your posted .fits of Altair shows a very fuzzy broad star but with curiously sharp tendrils around the star periphery, when you begin the session are you allowing sufficient cool-down time?
Even in an observatory the 14" Edge HD might take an hour or more to acclimatise after the dome is opened and trapped air currents in the OTA to diminish.

I don't know if the Edge 14" has OTA fans fitted, but if so do you use them?

 

Thank you William for he pointers, I hadn't thought of looking down through the OTA to the STF/FW8/OAG (duh), I just did that and noticed the filter wheel was not moving. Regardless of where I positioned the filter option in the drop-down box in Maxim DL (in the camera dialog box) the filter wheel didn't move. If I power the STF off then on again the filter wheel moves, but then doesn't move thereafter regardless of what I select in Maxim. I took apart the FW8 to inspect the inside, the wheel looks fine, the filters are in the positions I have them named in the filter setup dialog box in Maxim, etc... when I've been imaging, there is an audible clicking sound coming from the STF/FW8/OAG, which I interpreted as the filter wheel moving, but with the filter wheel apart, but plugged in still, i loaded Maxim and ran some exposures and the filter position it's stuck on is the clear one (i.e. no filter), so that's what I've been imaging with. I fiddled with a few settings in Maxim and think I found the reason the filter wheel wasn't moving... in the setup dialog box there's a field called 'Model', and I had selected FW8 (because that's what I have). I tried a few other settings and found that "Standard" actually moves the wheel correctly. I tested with the FW8 apart and validated all the filters move now according to what I designate in Maxim DL, so I'll try focusing tonight. Hopefully better results. Thanks again for the tip.

 

@Darren Wesemann

Hi Darren.

I hope you were able to achieve focus when the luminance filter (with IR blocking) was in the optical path instead of the empty slot.

Regarding your opening question about back focus lengths and the Edge HD14.
One of our observatory syndicate members in Germany operates a 12" Edge HD and I asked him to look at your post, this is his translated (and paraphrased) reply:

"Your current back focus arrangement as described is perfect, you do not need any additional spacers or reduction in BF length.

The 55mm standard T-mount back focus distance that you mention only applies in regard to DSLR cameras using a T-mount adaptor, in such a configuration this allows a 55mm known distance between the DSLR sensor and the front of the T-mount adaptor when it is attached to the camera. The 55mm distance is then used when calculating any necessary additional spacers needed when the DSLR camera is used with a flattener or reducer.

With your SBIG camera the T-mount 55mm distance has no meaning and is irrelevant because you are not using a DSLR adaptor.

What is important is that your Celestron Edge SCT-T adaptor, #93622, has a back focus measured from the shoulder of the T-thread to the face of the SCT visual back adaptor, you state that this is 88.5mm.
You give the SBIG STF/FW8/OAG total back focus of 57.5mm and added together this gives 146mm which is the exact distance as specified in the EDGE HD optical manual, 146.0mm +/-0.5mm, (page 13):

https://celestron-site-support-file...XLT_EdgeHD_OpticalTube_Manual_English_web.pdf

Provided that the 88.5mm BF you quote for the #93622 Edge SCT-T adaptor, measured between the shoulder of the T thread and the face of the SCT visual back adaptor is accurate, then your system is perfectly spaced."

Even if it was out by quite a large margin my colleague suggested that this would only be expected to have an effect on the star shapes at the edges of the field where curvature would be over or under corrected, it should not have any effect on the focus of a star in the centre of the field.

I hope this answers your question regarding correct back focus for your system.

William.

 

Many thanks William, that was super helpful information from your friend. That validation has allowed me to have confidence in looking at other areas for my focus problem. I think I'm close. I did some capturing last night and tried a number of things, I'm still not able to get great focus but here are a couple of things I've run into, if anyone has any ideas:

1. No matter what I try, I can't get the MaximDL auto focus to work. I've attached three examples (screen shots of the autofocus tab after it fails, and the FIT file for each for context), these are Andromeda, Deneb and Orion. I'm likely doing something wrong, let me know if it's obvious where I'm messing up.

2. I haven't been able to get tracking to work with my OAG and STi either... the field of view captured from the STi is different than the STF, and i've played with the STi's orientation and focus by moving it around in how it sits in the OAG housing. The attached three files of Betelgeuse are: one capture from the STF with the clear filter and two files captured from the STi, at perpendicular orientations. In terms of moving the STi in and out to adjust it's focus, this is the best I could get it, but still a bit fuzzy.

I have FITs of the above examples, and others, with all 8 filters and different exposure lengths if any of those are useful.

Any insights would be greatly appreciated.

Thanks!
Darren