10 replies to this topic

[gt40mkii]

I'm thinking its probably all three.

First off, the equipment:

• Celestron 8" SCT (2032mm NOT REDUCED!)
• Celestron AVX Mount
• ASIAir
• ASI585MC Pro
• UV/IR Filter
• ZWO Electronic Auto Focuser
• Askar FMA180 guide scope (220mm without the flattener/reducer.)
• ASI224MC guide camera (values are generally under 1.0)

 

The issues:

1. Auto-focus gets me stars around 8.  Seems pretty big to me.
2. ASIAir is having a very hard time doing a polar alignment.  Pointed at Polaris, I usually can "see" around 25-30 stars.  Often Polar Alignment fails in the ASIAir, using both PA algorithms.  Can't plate-solve.  I generally expose from 1s to 10s.  Neither make much difference except that I see more stars with 10s exposures.
3. Photos are pretty underwhelming.  So much so that I haven't processed any of the lights.

 

Causes?

1. I'm wondering of the seeing is just terrible this time of year.  Ground temp was around 80F and humidity was 75%, (North Texas,)  at about 22:30.  Stars were twinkeling a lot and I figure twinkling will blow out the stars in a 30-sec exposure.
2. Am I expecting too much from this non-reduced SCT?  Its not the EdgeHd model.  I originally bought it for visual use otherwise I would have bought something else.
3. Am I overlooking something?  I'm still learning and Its likely there's something going on that I don't know about.

[NoClouds]

With your scope and camera combo the field of view is .31x.18 degrees. The ASIAir will have trouble plate solving with very small field of view so that may be problem with polar alignment. There is also a rule of thumb about the ratio of main scope focal length to guide scope, 3 or 4:1 I think. You are at 9:1. Seeing will definitely have an effect, I have the Edge version and I know on certain nights there is just no getting tight stars. This is when a nice short refractor will salvage your night!

[Notdarkenough]

That kind of focal length on a AVX is just not reasonable. That mount isn't great with that kind of payload, either. Additionally, a guidescope just doesn't work over 1000mm of focal length; you need a OAG. Lastly, the AA really doesn't perform well over 1500mm.

 

AP at long focal lengths needs a much better mount, for starters. Even if you added a quality Starizona reducer, you are still over weight for the AVX. An OAG is pretty much required over 1000mm.

 

My recommendation is simple, determine what it is you want to do. Option #1: If it is AP you want, start with an 80ED refractor. Your AVX can handle that much easier, you can still use a guidescope and the AA will work well at that focal length. It will also make learning AP much easier. Or #2: get the Starizona reducer, use short exposures and EAA techniques. There are many folks using a reduced 8" SCT on an AVX for EAA. If trying AA, I would start without any guide gear. The AA can polar align and platesolve at 1400mm. The AA can also do Live View for short exposure stacking, which is what EAA is, essentially. 

[Phil HD9.25]

If you haven't already, throw on a bahtinov mask to see if you can manually focus, then check the HFR's.  If that gives you better stars, work on your AF settings.  

[ram812]

...And if all else fails and your bent on sticking with your OTA/setup, get a higher capacity mount. AVX at that focal length is tough outside of B1 skies (And all else being perfect). Reducing is good, but the mount....

Edited by ram812, 18 June 2025 - 09:24 PM.

[michael8554]

1.  "Photos are pretty underwhelming."

 

That's not specific enough.

 

Is the problem in RA, or in Dec, or both ?

 

2. Imaging pixel scale is 0.29arcsec/pixel, which many will call "Over Sampling".

 

That means your Peak guide errors have to be below 0.29arcsecs to get round stars.

 

Either Bin 3x if you can, to a more reasonable 0.97arcsecs, which will also make guiding 3 times easier.

 

Or resample the elongated final stacked image, if the subs will stack.

 

Ideally a large pixel imaging camera is required, not a 2.9um camera.

 

Your problems (whatever they are) may well be solved by Off Axis Guiding with a mono large pixel guide camera.

 

3. The guiding pixel scale is 3.52arcsec/pixel, which is fine.

 

Because if you manage to guide the mount to say 1arcsec RMS, then the imaging camera will also be guided to 1 arcsec RMS.

 

So imaging-to-guide ratios are largely irrelevant.

 

But back to answer 1, OAG may be required.

 

4. Auto-focus gets me stars around 8

 

     8 whats ?

 

Perhaps the autofocus is stymied by the extreme pixel scale ?

 

5. As NoClouds suggests, the FOV is too narrow to PA with ASIAir, try Drift Aligment pointing south etc.

Edited by michael8554, 19 June 2025 - 03:48 AM.

[Scott Badger]

2. Imaging pixel scale is 0.29arcsec/pixel, which many will call "Over Sampling".

 

That means your Peak guide errors have to be below 0.29arcsecs to get round stars.

Doesn't that assume not seeing limited? My image scale is 0.33", but if my seeing is 3" (about average for my location), guiding only has to be below 0.75-1" to preserve star size. Also, I find that even if seeing or guiding is generally poor, I still get round stars, but if RA and DEC RMS differ significantly, then stars won't be round even if total RMS is low.

 

Cheers,

Scott

[smiller]

I’ll second most of the feedback so far: You have several factors that compound that make things difficult:

 

1) Very long focal length

2) Relatively small camera

3) Mount and guiding that, in all likelyhood, aren’t as precise as #1 and #2 demands

 

I started with a long focal length unguided with the same size camera, so I had all your challenges and more.  My focal length was a bit shorter at 1500 mm but being unguided, my star quality was a bit worse and I was forced to take very short exposures, so I would say I was in a similar boat with respect to platesolving: Too few stars.   The combination of a long focal length in a small camera means that, in general, there are just not a lot of stars in your field of view and this can make it challenging, especially if there are other factors that play: poor seeing, haze, wind, light pollution, use of any filter, etc…

 

Things that will improve your situation:

 

1) Test your setup pointing to a star rich field near the Milky Way with lots of brighter stars (ex and open cluster in the band of the Milky Way).  There is a huge difference in the number of stars in different areas of the sky and if you are just getting going, start with a star rich field if you are having trouble finding enough stars.  This at least allows you to debug your system and find out how sensitive you are to the number of stars in your field of view.   If you are successful in a star rich field, then you know you may be close to being “good enough to go”.

 

2) I know you don’t wanna throw a lot of money at this, but in my telescope when I got a reducer, it made a huge difference in finding stars for platesolving for me. It was 0.75X reducer and that increase the amount of sky my camera saw by 80%.   This not only captures more stars, but concentrates the light of the stars onto fewer pixels so they are brighter and sharper as platesolvers can have more issues with very small fields of view combined with small pixel scales. It just makes it more difficult even when you have the large platesolver databases installed.   If you get a good quality ~0.63x reducer, you’ll get 2.5x more sky… that’s huge.  And the stars will be brighter and sharper… double huge.

 

3) I’m curious if a program like NINA with two platesolvers (PS3 annd ASTAP) installed (one can failover to the other) would be more robust.  I know that would be a huge change, I’m just thinking out of the box here…

 

 

And the final thing is you mentioned that the images just aren’t that sharp and that will be the case when you’re shooting at a really fine pixel scale because your atmospheric seeing is going to limit your quality as you have a small camera pointing at a little tiny portion of the sky and atmospheric seeing is going mean the overall scene that you are capturing is just fairly low resolution.  In this case, you will simply get a much sharper image as you expand your field of view with either a reducer or a larger size camera (or both) or a telescope with a much shorter focal length.

Edited by smiller, 19 June 2025 - 11:04 AM.

[Tapio]

Start again with the Askar FMA180 - polar alignment, plate solving and imaging.

And use the bigger scope only for moon and planets.

Much easier road to success.

[BucketDave]

I have an 8 inch ACF, guided by an FMA180 bolted on to the saddle of my GEM45. As others have said, do your PA and plate solving and guiding with the FMA180, then hook up your ASIair to the main imaging camera.

This route is simple but does mean that the two scopes are properly aligned. I leave my FMA180 on the saddle for a few months, once I've spent the time to align it (it sits in finderscope rings). It guides to below 1" which is enough for my ACF.

My stars are also quite large: 3" FWHM - that's the reality of average SCT optics / seeing.

[Spaceman 56]

Celestron 8" SCT (2032mm NOT REDUCED!)

 

 I'm still learning and Its likely there's something going on that I don't know about.

I will take that to mean that you are a beginner, so I say all this with good intentions.   

we are all still learning, but the scope you are using is a very long focal length device, which makes learning harder.

 

you have a few options that I can see.

 

1. use the listed equipment for Planetary or Lunar, and take very short exposures. it will work for this.

 

2. upgrade the mount, and add a reducer to bring down the focal length (which makes things easier)

 

3. get a smaller refractor and learn the basics at 500mm focal length or Less, and come back to the SCT in a couple of years.

 

if you persevere with the equipment that you have, as a first scope, you will be in for a hard time. I'm not saying its impossible for a learner to come to terms with a long focal length SCT, but its not the easy road.

 

just for reference I am 3 years into this hobby and currently image at 1484mm, and thats still challenging. 

 

Good Luck.