Guide camera filter

[RyanJones]

Hi all,

A quick question for those in the know.

I image though some pretty average skies it’s fair to say. Now I know that the more poor seeing the harder it is to achieve good guiding. I generally get decent guiding numbers after dialing things in but I always notice how much bloat there is around my guide stars.

So the question is, if I add a UV/It filter to my guide camera to tighten up the star, is it likely to make a difference?

Thank you in advance,

Ryan

[Merlin66]

I use the older DMK cameras for guiding/ eFinder and found adding a UV-IR filter significantly reduced the star bloat and gave better results.
I’d recommend a UV-IR on mono cameras.

[RyanJones]

Thank you for your response Ken, I’ve ordered one.

Cheers

Ryan

[PRejto]

I would have suggested a filter that blocks visible and passes IR....sort of what an ONAG does. Yes, guide stars are dimmer for sure but stars are less affected by atmospheric turbulence. See innovations Foresight for more about this.

https://www.innovationsforesight.com/education/guiding-with-near-infrared-nir/

Peter

[Startrek]

Ryan,
Just like to share some of my personal guiding observations whilst imaging
I use the same mount ( EQ6-R ) and guide scope and guide camera at both Sydney ( Bortle 8 ) using a 6” f6 newt 900mm focal length and my weekender south coast NSW ( Bortle 3 ) using my 8” f5 newt 1000mm focal length and achieve “similar guiding error values” which are both dependent on seeing and transparency ( ie upper atmospheric conditions) and local conditions like wind , dew , humidity etc...
Excellent seeing 0.70 to 0.80 max arc sec error
Average to good seeing 0.80 to 1.00 max arc sec error
Poor seeing 1.00 to 1.30 max arc sec error

However after many imaging hours with both set ups I can honestly say that my Bortle 3 dark location has produced some of my best and most consistent guiding ( PHD2 ) after a 6 month comparison and using a heavier payload (the 8” f5 newt).Whether this is due to “lucky seeing conditions” down south compared to Sydney, I really don’t know or the EQ6-R performs better under a slightly heavier load or my polar alignment was slightly better , lots of variables !!!!
I balance both rigs “slightly east heavy” too

Also one observation I have experienced at both locations , under a +50% waning or waxing moon my guiding has been marginally worse than under a “No Moon” Bortle 8 and Bortle 3 sky.

Just thought I would share some of my guiding experiences over the past 6 months or so with a similar rig at 2 different locations

Cheers
Martin

[RyanJones]

A very interesting post there Peter. It’s probably towards the top end of my understanding but not beyond my interest and I can work on the former. From what I gather though, the I/R spectrum is used because it is less effected by atmospheric conditions as you said but the downside is a significant reduction in luminance. This is probably where my issue would lie given I can probably only go for 3sec or so before my mounts accuracy would diminish the result. My initial thought with the use of an I/R filter was to reduce the glow around the body of the star to tighten it up and as a result make the response to movement better. It seems that although in visual theory this might work, the atmospheric star dance will still occur across the rest of the visible spectrum.

Thank you for taking the time to post this reply, it was very informative.

Cheers

Ryan

[RyanJones]

Thank you for your input Martin. I have a question for you regarding the variability of your observations based on mine. I find that the further away from SCP the object is, the worse my guiding is. This stands to reason as the sky effectively follows a much longer curved path and as such there is greater room for error. I wonder if you have also found the same?

My mount is heavily over weighted for imaging and although the 6 and the 5 are similar mounts, the payload is quite different. My total payload with the 200mm F/4 , guide setup and camera tops in at just over 11.5kg. Yes I do realize that that is half the problem. All that said, in my Bortle 8 suburban backyard with FWHM of rarely less than 4, I generally achieve average errors between .5 and .8 arc seconds. I am pretty happy with those numbers given what I’m dealing with but my practice with the newt guiding and subsequent tuning is more for my SCTs benifit. The filter was just one of the thoughts that went through my head and to be fair not the most expensive thing to try.

Thanks again for your input Martin

Cheers

Ryan

[Camelopardalis]

Ryan, I use a Baader R610 filter to try and smooth it out a little but some nights the seeing is so bad you can see the star jumping around. On those nights it isn't worth it.

Crap seeing means crap images :shrug:

[The_bluester]

I am interested to try a filter on mine at some point to see if it makes a measurable difference. I reckon you would need to use a night when you don't actually mean to image anything, just get it guiding and log the results for an hour or so, then fit the filter and log again, maybe back and forwards a couple of times with/without to prove the point.

[RyanJones]

So the consensus is that by minimizing the bandwidth through which we guide we should in theory get better guiding. After reading that article and from others experience it’s seems that towards the I/R band is the way to go. I feel I might have jumped the gun in ordering the I/R cut. Not an expensive experiment though. We’ll see how it goes and maybe order an I/R bandpass and compare the two.

Agreed Paul, it seems that to get a good test we’ll have to sadly dedicate some clear nights to the project but hopefully we get some results to share with others.

[Merlin66]

Ryan,
I can't comment on the possible benefits of an IR pass filter for guiding, but I do know, for a mono guider that a UV/IR filter gives a "cleaner" stellar image for guiding.
I use an ol' DMK with a 60mm finder as an eFinder (plate solving etc.) and the UV/IR filter has made a significant difference.
My 2c

[RyanJones]

Ken,

Please don’t think that I’ve been dismissive of your original post. Quite the contrary, hence why I ordered the filter I initially ordered. In light of the alternate filter suggestions I feel it would be worth giving both a go and making a comparison. It may well turn out that the extra exposure time required for the significantly tighter bandpass of the I/R pass may negate the advantages suggested. My initial thoughts were aligned with your experience that an I/R cut would suffice my needs and that may well still prove to be right. Thank you again for your input.

[Startrek]

Ryan,
The most popular guiding algorithm Hysterisis in PHD2 should and does allow variances across the celestial sphere but one observation is clear to me and that is guiding definitely marginally improves as you ascend in Altitude past the 35 to 40 degree mark
To answer your earlier question about guiding near the SCP , I haven’t found a significant improvement near the pole.My rigs guide just as well on one side of the meridian near the pole or south east , due east or north east . It’s only when the rig ascends in altitude that I see some gradual improvement

Quite frankly conditions change so much from night to night and guiding has so many variables it’s hard to nail a definitive trend each time you go out but I try to keep a standard approach and not get to hung up on the graph. At the end of the day round stars are round stars and that’s what counts

Cheers

[Startrek]

Ryan
The HEQ5 and EQ6-R are similar in “looks” but very different on the inside.Yes the EQ6-R is the bigger brother but performance wise out of the box and payload capability are superior
The best guiding error I could ever get out of my HEQ5 over 2 years was around 1.15 arc sec error with 9kg on board and good balancing ( I could never get it under 1 arc sec error )
The EQ6-R with same rig but at 10kg on board ( added larger losmandy dovetail bar ) is 0.75 arc sec error
When dithering with he HEQ5 it knocked the mount around to much and took minutes to recover, especially in DEC.With the EQ6-R I can send a medium to high dither and it recovers beautifully in 10 seconds
The HEQ5 when slewing sounds like bucket of bolts rattling and the EQ6-R is so silent like a hummingbird

So in my experiences there’s no comparison between the HEQ5 and the EQ6-R out of the box , it’s chalk and cheese , the EQ6-R is a much more robust and superior performing mount

My 2 cents......

[Camelopardalis]

Just remember that while guiding using an IR filter may damp the seeing a little (it's not a huge difference in my experience), your main camera is still going to be subject to whatever bandpass filter you have in play.

[The_bluester]

There is that, but it would be interesting to see if it makes a difference, you should in theory be able to have your guide program behave a little more aggressively in chasing the errors of the mount by reducing the amount of seeing induced "error" in the system input.

I suspect that the guiding assistant in PHD might show any difference more objectively than anything else. If the long wavelength pass reduces the influence of seeing then the measured high frequency star motion should change with or without the filter.

[RyanJones]

You’re 100% right martin and I don’t doubt for a second that the 6 is better than the 5 as the 8 would be better than the 6 but the 5 is what I have. I’m accordingly happy with the fact I’ve got my 5 guiding at under .7 arc seconds with 11.5 kg sitting on it. I’m not nessesarly looking into this on a purely better image level. At the end of the day I’m currently imaging through a 15 year old entry level camera so you can’t make a silk purse out of a sows ear as they say. This is more an experiment of greater understanding. Part of my personal enjoyment in this hobby is really learning ( as much as my brain can handle ) how things work and what I can do to make things better. The filter may make no difference at all but it’s not an expensive experiment to try.

[RyanJones]

Paul and Dunk,

It will be interesting to see the results. I think I will get the I/R bandpass filter also and see what the differences are ( if any ). In theory the addition of the filter would make marginal difference the better the seeing is already. I rarely get FWHMs under 4 so it’s probably a pretty good test ground for the experiment.

[RyanJones]

So the results are in. I finally received my IR cut filter. I tested it backwards and forwards with and without filter. Does it improve guiding ? Not much in my case. Maybe marginally. What it does do though is make the guide image significantly cleaner meaning the guide star is sharper. I think if my mount was more capable of more accurate guiding, it would have made more of a difference. The areas where the mount was stable did seem to be cleaner lines. Not what I would consider a waste of time but not a “ result “ either.

Ryan

[Merlin66]

Ryan,Thanksk for the feedback....
Did you test it against a UV-IR filter??

[RyanJones]

Hi Ken,

Not as yet. I’ve been meaning to order it but I just haven’t gotten around to it yet. I ordered an OAG at the same time and I’ve had secondary dew issues to contend with which have drawn my attention to building a dew heater system. I might jump online now and see if they’re in stock and order it now. The post has been pretty slow of late so it should give me some time to deal with the other issues before it arrives.

Cheers

Ryan

Edit: it’s on back order

[gregbradley]

Autoguiding programs calculate the centroid of the star so whether they are sharp or not may not be so important. I do find though that if I start an autoguiding sequence the first thing I do is select another star. Especially one that is a bit less bright and tighter. It usually works and I get a significant improvement.

I have used a 720nm IR filter on my guide cam before. I haven't used it in a while.

As I recall it did help somewhat but it also dropped the luminance of the guide stars a lot so it was really only practical when you had a bright guide star which you can't always get. M104 for example has hardly any guide stars near it.

So unless you are prepared to remove it and put it back on depending on the guide star availability its probably not worth it.

You'll get a bigger gain by doing a more perfect polar alignment with T-Point.

Also I believe you raise aggressiveness of guiding when the seeing is good not when its poor. If seeing is poor you end up chasing the seeing and overcorrect. When the seeing is good you can correct more aggressively without chasing the seeing and overcorrecting.

On my high quality mounts (Software Bisque PME and AP1600) I use 6 second guide exposures. When I had a Tak NJP mount I used 1 second. 6 seconds works better as the corrections don't overcorrect as the mounts periodic error is low. Of course that will vary with the mount but I think its most likely the weaker the mount the shorter the guide camera exposure time.

Greg.

[RyanJones]

Hi Greg,

Thank you for your input. Very interesting. Can you explain what PA with a T point is ? I use the PA sequence in my ASIair. It tends to get me within a couple of dozen arc seconds. I find that the Dec is pretty good at under 0.5 RMS which I would imagine suggests that the PA is “ good “ relatively speaking. I am interested is what the T point is though. To be honest I’m not expecting miracles from testing the different filters, I just find it interesting. I image in Bortle 8 skies and my seeing is never “ good “ so I’m working with a pretty bad canvas to start with.

Cheers

Ryan