Guiding on an 8" SCT for long exposures. - Forums

Topic starter

May 22, 2021

I have been trying to use an auto-guiding scope (400mm FL) with ASI120mm mini camera on my NexStar 8se. I get good polar alignment and my guiding error is consistently less than 0.5. In my imaging sessions, I use a 0.63 focal reducer and so my effective main scope focal length is 1350mm. I do my imaging with ASIAIR pro and use ASI1600mm pro.

The issue is that I am unable to get exposures of more than 4 mins. In fact, even at 4 mins, I see some smaller stars trailing (more predominant in Ha). I was wondering if other folks have any experience with a dedicated auto-guiding setup at those focal lengths, and are able to push exposures in the 7-10 min range. Or if off axis guider is the only way to get into longer exposure time. Even then, what is a typical range of exposures one can get with Off axis guider?

ps: With my smaller (420mm) refractor, the same auto-guiding (120mm FL) to main scope focal length relationship (~3.5x) works great and I can push 12+mins easily.

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Tim Hutchison

May 22, 2021

I'm not sure if your problem is related to differential flexture, but when I was using my edge hd 8 and a similar guide scope I had lots of trouble with flexture. I ended up using an OAG from ZWO with my 1600 and my problems all disappeared. I was consistently able to shoot exposures of up to 20 minutes without any trouble and without any star trailing issues. That was all while using a CGX mount. When I switched to a 10 micron mount I was able to get 30 minute exposures without difficulty using the OAG.

With the edge HD, the native focal length is 2032 mm. I either shot at 2032, or used the 0.7 reducer to shoot at 1422mm.

If you choose to go to an OAG, the only thing to watch out for is the back focus. You want to make sure that the back focus closely matches what Celestron recommends for your telescope.
I hope that is helpful. Feel free to reach out if you have any questions.

Tim.

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Michael Broyles

May 22, 2021

Do you have your NexStar 8se on a wedge? You would start to get rotation if just using Alt-Alt mount.

Ryan Kinnett

May 22, 2021

If you’re using the stock 8SE one-arm az/el mount then the effect you’re seeing is field rotation. The effect will be more pronounced at longer focal lengths, as you observed, and it will limit how long you can expose.

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Anthony Quintile

May 22, 2021 · Edited: May 22, 2021

If you are using the Nexstar mount, you will really not be able to get consistent exposures with round stars over 20 seconds, at least that was my experience.

If you are using the OTA on a better quality GEM, then you are probably experiencing a bit of mirror shift and an OAG will solve that.

I actually had excessive mirror shift with my 8SE to where it would not hold collimation across the sky. I ended up returning it to Celestron and they reattached the mirror to the carrier. It apparently was defective. This would give you even more star trailing than a typical SCT.

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Hartmuth Kintzel

IOTD Staff

May 22, 2021 · Edited: May 22, 2021

Hi Kapil,

I have comparable equipment. (Celestron C8 with Antares F6.3 reducer on EQ6 mount).
I have done finderscope-guiding / off-axis guiding and a 60/240mm mini-guiding telescope with it.
With off-axis guiding I even had the most problems, because the stars at the edge of the C8 image field are too bad.
With the mini guiding tube + ASI290Mini I manage relatively well to take pictures up to 7min.
However, I sort out about 10% of the images on average because I am not satisfied with the tracking accuracy.
PHD shows me an average tracking error of about 1".
My EQ6 can't do any better even at good seeing conditions.

CS
Hartmuth

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Kapil K.

Topic starter

May 22, 2021

I should have clarified. I am using an EQ6 R pro mount.

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Kapil K.

Topic starter

May 22, 2021

Tim Hutchison:
I'm not sure if your problem is related to differential flexture, but when I was using my edge hd 8 and a similar guide scope I had lots of trouble with flexture. I ended up using an OAG from ZWO with my 1600 and my problems all disappeared. I was consistently able to shoot exposures of up to 20 minutes without any trouble and without any star trailing issues. That was all while using a CGX mount. When I switched to a 10 micron mount I was able to get 30 minute exposures without difficulty using the OAG.

With the edge HD, the native focal length is 2032 mm. I either shot at 2032, or used the 0.7 reducer to shoot at 1422mm.

If you choose to go to an OAG, the only thing to watch out for is the back focus. You want to make sure that the back focus closely matches what Celestron recommends for your telescope.
I hope that is helpful. Feel free to reach out if you have any questions.

Tim.

Hi Tim, I think this might be the issue. NexStar is not so great at the outer edges. So I am wondering if OAG will have other problems due to sad edge optics of Nexstar? I could give this a try, although it is quite possible I may have to upgrade to edge HD or something else 🙂

Tim Hutchison

May 23, 2021 · Edited: May 23, 2021

Kapil K.:
I may have to upgrade to edge HD or something else

I was always happy with the edge hd. I used it for quite some time. If there is anything I can answer for you don't hesitate to reach out.

Anthony Quintile

May 23, 2021 · Edited: May 23, 2021

Kapil K.

Topic starter

May 23, 2021

Tim Hutchison:
Kapil K.:
I may have to upgrade to edge HD or something else

I was always happy with the edge hd. I used it for quite some time. If there is anything I can answer for you don't hesitate to reach out.

Thanks Tim. I will :-)

Kapil K.

Topic starter

May 23, 2021

Hartmuth Kintzel:
Hi Kapil,

I have comparable equipment. (Celestron C8 with Antares F6.3 reducer on EQ6 mount).
I have done finderscope-guiding / off-axis guiding and a 60/240mm mini-guiding telescope with it.
With off-axis guiding I even had the most problems, because the stars at the edge of the C8 image field are too bad.
With the mini guiding tube + ASI290Mini I manage relatively well to take pictures up to 7min.
However, I sort out about 10% of the images on average because I am not satisfied with the tracking accuracy.
PHD shows me an average tracking error of about 1".
My EQ6 can't do any better even at good seeing conditions.

CS
Hartmuth

Michael Broyles:
Do you have your NexStar 8se on a wedge? You would start to get rotation if just using Alt-Alt mount.

No, I use EQ6 r pro mount.

Kapil K.

Topic starter

May 23, 2021

Hartmuth Kintzel:
Hi Kapil,

I have comparable equipment. (Celestron C8 with Antares F6.3 reducer on EQ6 mount).
I have done finderscope-guiding / off-axis guiding and a 60/240mm mini-guiding telescope with it.
With off-axis guiding I even had the most problems, because the stars at the edge of the C8 image field are too bad.
With the mini guiding tube + ASI290Mini I manage relatively well to take pictures up to 7min.
However, I sort out about 10% of the images on average because I am not satisfied with the tracking accuracy.
PHD shows me an average tracking error of about 1".
My EQ6 can't do any better even at good seeing conditions.

CS
Hartmuth

Hi Hartmuth, it is possible I have some other issue with my scope. I am getting guiding error less than 0.5". So it is probably not anything I can do better in guiding. It may just be out of my scope's abilities (or seeing conditions); or as Tim pointed out, it maybe mirror shift or something. I will probably try OAG and see how it goes.

Björn Arnold

May 23, 2021

Hi Kapil,

From all the descriptions, I would point to flexture as the cause of the star trails. Mirror shift usually also changes focus. You're not saying if that happens but the image should be less sharp than a shorter exposure. I also assume you're not refocusing every sub. Therefore you'd see a continuous deterioration of image quality due to mirror shift.

With 400mm, your guide scope is pretty heavy and flexture easily induced. I'm using a cheap 60mm f/4 guide scope on top of my 8" Meade SCT and haven't observed any trailing even for long exposures.

Cheers,
Björn

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Riedl Rudolf

May 23, 2021 · Edited: May 24, 2021

Hi Kapil!
I was using a C11 on EQ6 for many years. Guiding is done with the MGen2 with a 50 mm guide scope and fl 180mm. Even at native FL of the C11 at 2800mm I get with 10 - 15 min exposure sharp pictures. What you have to take care is mirror shift. +/- 15 deg from vertical position mirror starts moving! I do not loose focus, but the stars are trailed.
If you focus the mirror upwards in the final step than its a bit better, but you cannot avoid it.
Mirror fixing is the only chance. There are some companies offering modification on the backside of the mirror cell like Baader in Germany.
Maybe thats the problem. OAG helps, because if the guide star moves due to mirrot shift, the guider will correct the scope. My problem with OAG was to find a guide star in many cases!
The NEQ 6 is normally not the problem. I do have now a EQ8 still with the Mgen2 guider which can carry heavier loads, but is not more accurate!
Maybe this helps! CS Rudi

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Tim Hutchison

May 23, 2021

Kapil:

One other thing to consider…

I see a lot of others suggesting mirror shift is the culprit. That is certainly another possibility. The Edge HD scopes have a mirror lock that eliminates and mirror flop. Since you cannot focus if the mirror is locked down you typically add a crayford focuser as well. I used a Moonlite focuser. With this setup I never experienced any mirror flop. As I said above, I was able to take very long exposures with guiding and my starts were sharp from edge to edge.

This solution is a bit pricey, I'll admit. I may be able to help a bit. I recently sold my edge hd scope but I still have my Moonlite focuser. I intend on selling it as well. I just haven't got around to it yet. If you have any interest in this solution let me know. I'm in no rush so I can be patient if it will be a help to you. It would save you a bit from buying it new.

If you just want to chat about it I'm happy to help with that too.

Best.
Tim

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Kapil K.

Topic starter

May 23, 2021

Hello folks, thank you so much for your suggestions. I do also experience that my image quality does go down slightly as the session goes. I thought it was due to LP as targets go down in the night sky, and/or the inherent quality of NexStar scopes. It is not so bad as to focus every sub, but after the session (2-3 hours), I do see my scope is out of focus slightly. It does hold collimating for a few weeks though.

I have only recently started using Nexstar for long exposures and most pictures are slightly blurrier than my refractor. I thought it was just the mirror quality difference between the two scopes.

I’ll look into the mirror shift issue. I am also thinking of replacing this with edge HD, once they are in stock.

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Björn Arnold

May 23, 2021

Kapil K.:
It does hold collimating for a few weeks though.

If the scope is not exposed to frequent vibrations or shocks, collimation should actually hold for very long time and just a few weeks. Usually collimation is lost during shipment ("fragile" stickers don't help too much on packages, sometimes people need to be reminded that black is the letters and white the background) or when you have a bumpy car ride to the observation location. One thing that helps is also when collimating to try to tighten the screws. If that suffices to reach collimation that's better than loosening the screws. Of course, don't stress the collimation screws.

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rhedden

May 23, 2021

I have used an On-axis guider (ONAG) from Innovations Foresight since 2013 on my C11 Edge, which is operating at 2000 mm focal length and f/7. Once I bought it, I never had another problem with guiding. There is even a newer version available that allows for larger sensors. It gives you an enormous number of guide stars from which to choose. The drawbacks are cost, and possible limitations with back-focus. Gaston from Innovations Foresight can help you figure out the back-focus issues if you are curious. As with many pieces of equipment, you get what you pay for, and I have found the ONAG to deliver what it promises. Disclaimer: I have no stake in the company.

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Kapil K.

Topic starter

May 24, 2021

This is what I ended up with after a relatively long integration: https://www.astrobin.com/5sgeck/?nc=user.

Ha details are limited due to this exposure issue (only 4 mins). I am also capturing data at 5mins. Below is a stretched single exposure at 5mins showing trailing stars.

Tim Hutchison

May 24, 2021

All of you star trails go in the same direction. This tells me that you issue is very likely a guiding issue (flexture). You're not dealing with mirror flop out your image would have focus issues. It's not an optics issue or the star trails in the corners would go different directions, either all towards our all away from the center.

I feel pretty confident in saying that you are likely dealing with some sort of differential flexture.

Tim

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Kapil K.

Topic starter

May 24, 2021

Tim Hutchison:
All of you star trails go in the same direction. This tells me that you issue is very likely a guiding issue (flexture). You're not dealing with mirror flop out your image would have focus issues. It's not an optics issue or the star trails in the corners would go different directions, either all towards our all away from the center.

I feel pretty confident in saying that you are likely dealing with some sort of differential flexture.

Tim

Awesome. I am glad I posted that. Thanks Tim. I will look into investing in an OAG. Do you have any recommendations?

Tim Hutchison

May 24, 2021

Kapil K.:
Do you have any recommendations?

I used the two different ones over time. The ZWO OAG and the Celestron OAG. I think the Celestron guider is a much better piece of equipment. I have one here that I would be willing to sell if you are interested. Just pm me if you are.

The ZWO OAG is ok and works fine. It's just not as sturdy as the Celestron and the pick of prism is a bit smaller. But I was able to use it successfully. I just had to be a bit more careful with it, that's all. One thing to check is the back focus of your scope and your gear. The ZWO takes up much less back focus.

Tim

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Kapil K.

Topic starter

May 25, 2021

Tim Hutchison:
Kapil K.:
Do you have any recommendations?

I used the two different ones over time. The ZWO OAG and the Celestron OAG. I think the Celestron guider is a much better piece of equipment. I have one here that I would be willing to sell if you are interested. Just pm me if you are.

The ZWO OAG is ok and works fine. It's just not as sturdy as the Celestron and the pick of prism is a bit smaller. But I was able to use it successfully. I just had to be a bit more careful with it, that's all. One thing to check is the back focus of your scope and your gear. The ZWO takes up much less back focus.

Tim

Thanks Tim. I will need to understand the back focus requirements. I am not very clear on them. Especially on a nexStar 8se with focal reducer, filter wheel and mono camera. I’ll have to do some research.🙂

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Ara Jerahian

May 25, 2021

I agree with what Tim has already suggested as a possible cause and solution. I will add one more thing though: This could also very well be a vibration issue with a failing fan in your 1600, which I experienced just a couple of months ago with my ASI294MC Pro.

An easy test for this is to turn ON cooling to your 1600 and take a good length of an exposure to confirm the trailing stars, and then turn OFF your cooler completely (it needs to hit 0% before it's off) and take another exposure of the same length. If you see a difference in your stars, it's your fan.

I certainly could be wrong, but thought I would just put this out there as a quick and easy test before you go buy anything.

CS, Ara

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