Getting into auto-guiding

[Toddylamo]

Hi all,

I’ve been doing AP for about a year now, and after a few months photographing some of the brighter deep sky targets, I’ve decided it’s time to get an auto-guiding setup to help me increase my exposure times. I’ve been reading through forums for a while now but I’ve seen no clear answers on what route I should take. 
My setup is a NexStar Evo. 8” SCT (2032mm FL at F/10) on a wedge, typically using an F/6.3 reducer, and I currently image with a micro 4/3 DSLR with all the appropriate spacing (haven’t made the upgrade to a dedicated AP camera yet). I run this through 1.25 inch tubes. I recently started controlling my mount through CPWI on a windows 10 laptop. My guiding understandably drifts slowly over time, but is barely noticeable in sub-minute exposures. 
 

With this setup in mind, should I get an OAG or a Guide Scope, and with those options, is it necessary that I fork out a little bit more cash for a better guide camera (like an ASI290mm mini) or will a 120 mini suffice? I would ideally like to hear from someone who uses a 120 mini with a long focal length telescope. I am also a bit worried about my balance, as I can only slide my OTA so far down the mount rail to compensate for all the weight on the back. 

 

[ollypenrice]

The standard advice with an SCT would be to use an off axis guider. This is because 1) it 'sees' any movement of the mirror within the main tube when a separate guidescope, mounted on the main tube, does not. 2) You are imaging at a very long focal length and at very high (arguably excessively high) resolution in arcsconds per pixel so a short FL guidescope might not guide at the resolution you need.

If using the reducer you need to bear in mind that an OAG will be fitted between the reducer and the camera, so adding to the distance between reducer and chip. This distance must be respected (check the spec of your reducer) and bear in mind that an OAG will add maybe 20mm or so to the kit between the two.  If your chip distance is currently correct you'll need to lose the thickness of the OAG. In general the SCT reducers are not ultra-critical of chip distance, unlike most reducers, and when the chip distance is a little out the only result is that the true focal length will not be exactly what the reducer's reduction value says it is.

Some people do manage to get wedge/fork mounted SCTs to perform well under autoguiding but the combination is not popular because it is very tricky to get right. A German equatorial is much easier, hence the overwhelming popularity of the design in imaging circles.

A detail to bear in mind is that an autoduider cannot guide out polar misalignment. It can lock onto a guide star but a misaligned imaging rig will show field rotation over time around that star.

You probably know the best news already: the best guiding software, PHD2, is free and straightforward to install and apply.

Best of luck. Guiding is the life blood of deep sky imaging.

Olly

Edited May 7, 2021 by ollypenrice
Typo

[mackiedlm]

Hi, I am guiding my 8" LX200 GPS forked on a wedge, with a cheap 50mm  guidescope and a 120mm mini and it works OK. My stars are not perfect and that could be down to flexure but they are acceptable for me at the moment (Note I am now using a SW 0.85 reducer as my Meade 0.63 was giving me awful coma). If you look at my profile and follow the links to my recent galaxy images you can see what I have achieved with that set up with both reducers- .

OAG's are, of course, the "Gold Standard" for guiding SCTs and I will likely go there eventually but for the moment there are other AP items with higher funding priority . 

I find the 120mm to be just about perfect for me - small, light, does the job. If you already have a small scope or even a finder that could be converted to use a a guide scope I'd say give it a go with that before spending any more cash on it.

Good luck

David.

Edited May 7, 2021 by mackiedlm

[ollypenrice]

PS It's the norm to need to add a counterweight to the front of an SCT OTA to obtain balance. You can buy the bits or just improvise something.

Olly

[Toddylamo]

Thanks guys,

At the moment my polar alignment is surprisingly good (I am in the Southern Hemisphere which makes things trickier), but I’ve had excellent results using ASPA. It will be interesting to see, once I get my autoguiding set up, if drift align through PHD2 is any better. 

I’ll probably go with OAG as I’ve got enough spacers of different sizes to get the appropriate backfocus. I guess that leaves me with the question of which camera should I buy. I understand the 120 has a smaller sensor than the 290, but that the 120 has larger pixels and is therefore more “sensitive” to light. ZWO gives a figure of “30% better tracking” accuracy on the 290 compared to the 120 as well. OAG on my slow telescope would mean that not much light is going to the guide camera sensor, so I need a more sensitive sensor, which points towards using the 120. On the other hand, the higher focal length means I need better tracking accuracy, pointing towards the 290. I’m not prepared to spend more than the price of the 290 (I’m also saving for a dedicated AP camera), so how should I negotiate this trade off?

 

[Clarkey]

I am looking into OAG for my RC8 which I currently guide with an ST80. It's OK, but at full FL it would be better with OAG. I was doing my research and the general responses seem to suggest the 120 has too small a sensor and is likely  to limit the stars available for guiding. I don't think the pixel size is too much of an issue. I think the QE on the 290 might be better which will offset any pixel size benefits.

There is a discussion about it on Cloudy Nights; https://www.cloudynights.com/topic/664415-zwo-asi120mm-mini-vs-asi290mm-mini-for-oag-sensitivity/

I will keep an eye on the post to see if  some one with first hand experience gives their opinion..

[Toddylamo]

Thanks Clarkey,

Looks like I'll be going ahead with the 290, as I see that the 120 has problems finding a guide-star through high focal lengths. This seems to be the deal breaker issue that makes the more expensive 290 worth it.

[michael8554]

20 hours ago, mackiedlm said:

my Meade 0.63 was giving me awful coma

That's very unusual for a Meade 0.63 Reducer, did you have the correct spacing to the FR ?

Was it an ACF LX ? 

The 4/3rds cameras only have about 20mm flange to sensor spacing, so there's plenty of room to put any OAG  in the remaining 85mm or so to the FR.

The ASI 120MM was definitely not sensitive enough for my similar SCT OAG setup, I went for a large pixel Lodestar.

On 07/05/2021 at 09:15, ollypenrice said:

Some people do manage to get wedge/fork mounted SCTs to perform well under autoguiding but the combination is not popular because it is very tricky to get right.

In my experience, imaging is only viable for a permanent Polar Aligned rig, as setting up and aligning is so time-consuming.

Michael

[mackiedlm]

 

5 hours ago, michael8554 said:

That's very unusual for a Meade 0.63 Reducer, did you have the correct spacing to the FR ?

I think 10 minutes searching on here and other forums will show its not unusual at all. Indeed it was on here that I found the recommendation that some modern refractor reducers would outperform both the celesteron and meade 0.63 in terms of coma. yes my spacing is correct and confirmed by many hours of adjustment and tweaking.

5 hours ago, michael8554 said:

Was it an ACF LX ?

No its an old LX200 GPS UHC. If I was getting this level of coma with a ACF I'd be sending it back!

5 hours ago, michael8554 said:

In my experience, imaging is only viable for a permanent Polar Aligned rig, as setting up and aligning is so time-consuming.

In my experience it is perfectly viable and no more time consuming than setting up my small frac on its NEQ6. 15 minutes to haul it all out, place it on the markers and connect cables then 10 minutes to PA with sharpcap, align using platesolving and line up on target. In truth, I find Sharpcap PA to be easier and quicker on the wedge because the adjustment screws are much more effective than on the NEQ6. Of course having a permanent set up would be nice but unfortunately that is not an option for many of us. But that certainly does not make imaging less viable for us.