Lodestar seeing zero stars through OAG!

[Gib007]

I'm having an unexpected issue with my Lodestar looking through my TS Optics 9mm OAG. This is on my Borg 77EDII F4.6 OTA. I've got the imaging camera focused perfectly and the OAG is connected directly to the imaging camera, with all other necessary adapters and spacers in front of the OAG to go into the reducer (for exact 55mm distance from imaging camera CCD sensor to reducer). This arrangement was personally recommended by Ted of Hutech Borg Telescopes. 

The Lodestar has a C-mount thread and the OAG has a T2 thread, so I've got these two adapters:

http://agenaastro.com/blue-fireball-1-25-t-t2-visual-back-e-10.html

http://agenaastro.com/blue-fireball-1-25-c-mount-video-camera-adapter.html

The first attaches to the OAG to provide a C-mount tube with a compression ring. The second attaches to the Lodestar and adds some length, that allows pulling out the Lodestar quite a bit. 

Anyway, this is the screen I saw all night on PHD Guiding:

What you see there as dots appeared even if I removed the Lodestar from the OAG and pointed it at myself or covered the telescope completely, so these are hot pixels. Subtracting darks removed them and showed a screen full of just noise (since there was no other signal). No matter what distance I pulled out the Lodestar to, or even if I set it touching the OAG T2 connection, I still saw nothing but the above (or noise if darks were subtracted). There was no way to focus any stars. I even pointed my telescope precisely to the M45 Pleiades Cluster, containing dozens and dozens of possible guide stars, and nothing - no stars came into focus. 

If I aimed the Lodestar at a bright light or a small source of light, I could see part of the image being saturated so the camera is capturing stuff. If I look through the peephole on the OAG on to the prism, I can see what the prism is pointing at very well, so the prism seems to also be ok. I've also adjusted the prism position in and out, at one point even placing it in the middle (directly in front of the imaging camera's CCD sensor), just to get a central view of the telescope's target. Nothing!

I'm disappointed because absolutely everything else was spot on - the telescope work well and images captured through it, though test exposures of varying length, looked good after focusing the imaging camera well with a Bahtinov mask. When it came to the guiding system however, I missed my 50mm guidescope!!

Any ideas of what could be the problem and how I can fix it? I know lots use the Lodestar and it's meant to be a top-end autoguiding camera so I'm keen on sorting this out ASAP. My thanks for your help!

[Gib007]

Moreover, I pointed my telescope at bright lights in the far distance to see if I could focus them but I couldn't, no matter where I placed the OAG (even in front of the reducer, which is not the intention by any means anyway!). 

[riklaunim]

Start at day. Focus the primary camera on something (like a tree), then check the OAG camera - is it in focus and is it illuminated at all. Tune the prism depth/tilt (but not to deep or the primary camera will see it) + camera distance to get a clear focused image. Then you will be able to see stars in OAG

[Gib007]

Start at day. Focus the primary camera on something (like a tree), then check the OAG camera - is it in focus and is it illuminated at all. Tune the prism depth/tilt (but not to deep or the primary camera will see it) + camera distance to get a clear focused image. Then you will be able to see stars in OAG

Thank you. May not be a bad idea to start during the day. Tomorrow I'll set up on my balcony and point out to Spain in the far distance, during daytime, and check what I capture with the imaging camera. Once focused, I'll pop on the Lodestar and have another play around with it. I'm hopeful I don't need anymore spacing. It seems to me that autoguiders on OAGs rarely need very much extension to reach focus. 

[Gib007]

Quick question actually - to achieve perfect focus for both my imaging camera and the Lodestar, do both CCD sensors need to be at the same distance along the imaging plane?

For example, for my OAG, distance from prism centre to imaging camera terminal is 4.5mm. Add my imaging camera's backfocus of 17.5mm and I get 22mm. Would the Lodestar then also require 22mm from prism centre to its own CCD sensor to reach focus? If so, I can make this process much easier to achieve via calculation. I would however probably need to use some spacers between my imaging camera and OAG to ensure these distances are equal. 

[kookoo_gr]

In order to setup your oag correctly you can do the following.

1. Achieve focus of both cameras during daytime as mentioned above. This will save you a lot of furstration.

2. Since the Lodestar is a very sensitive camera i suggest doing this adjustment before sunrise when the light is not very strong (after that even at very low capturing times the image will be saturated and it will be difficult to adjust the prism, see below) or just after sunset when the light is not so strong (after a time it will be difficult to setup the oag as it will be too dark).  A way around this (during daytime) is to place a Ha filter on the Lodestar/imaging camera to cut as much sunlight as possible and use small capturing times and be inside your house where it's darker.

3. With the camera setup on the scope, focus the main camera at a distant target, with no sky in the image if possible but with distinct features (prefrerably buildings), once focused mark the focus position on the focuser.

4. Try to focus the Lodestar camera, once done mark the focuser position. The difference in those two focus positions is the spacer you will need. At night you will only need to fine focus the Lodestar if the OAG allows fine focusing for the guide camera. Obviously you can use the fine focus of the OAG during daytime adjustments and at night you will need finner adjustments but if you use the Ha filter on the Lodestar you will need to fine focus the Lodestar at night.

Some tips on the whole proccess.

For placing the spacers it is best to place the spacers at one camera, if the focus of the guide camera is inward at the focuser tube you will need to place a spacer at the guide camera (if i remember correclty), the cameras need to be spaced so that they achieve focus at the same focuser position

For setting up the prism position. Before inserting the oag at the focuser set the prism at a middle position but not inside the FOV of the main camera's chip, you can wing this by eye. When you are doing the focusing proccess and you have the scope pointed at a ground target you can do the following. They don't have any particular order so you'll have to go back and forth on this one.

From the guide camera make sure that the image is not a ghost image from a reflection (this is how i call it and it happenned to me when i setup my OAG the first time. This led me at night not being able to know why my stars were way too enlongated, almost flattenned). Once at a ground target start tilting the prism in both extremes back and forth until you see the clearest, sharpest image, The ghost image will be either whitter than the regular image as if it is saturated or too dark (the dark/white image is caused from the prism moving at opposite positions) but you will be able to see features on the image and it can fool you, also when the prism is moving away form the scope's FOV the image will get black at one edge, when the latter happens i suggest moving the prism even further to make sure you are outside the scope's FOV. You may also have to adjust focus on this part but only for the guide camera.

Have the main camera continously imaging and at focus point. Then start adjusting the prism of the OAG until you can see it at one edge of the image, the image will either get too white or black (can't remember right now but i belive it's black) at the point of the prism, once you see it back off.

If you want to be sure at which direction the prism is inside the main camera's FOV before placing the camera on the scope adjust the prism until it crosses the main camera's FOV, then you can place the camera on the scope and see what image you have from the main camera so to avoid any misleading images.

When you use the OAG during imaging the stars at the FOV will be a bit elongated and they may show like they are out of focus even when you fine adjust the focus of the OAG, this is normal since the guide camera captures light form the edge of the optics were the image is a bit distorted. If the OAG's prism is not properly adjusted the stars will be too elongated and almost flattend. If you see the second type of image you need to set the prism again. I mention this since a too elongated star can have a great impact at guiding and locating stars even with a Lodestar will be difficult, with a corectly adjusted prism the Lodestar camera will have no problem showing stars even at light polluted areas.

The guide camera will not see the same image/target as the imaging camera. This is due to the fact the guide camera's FOV can be either at the egde of the imaging camera or outside the imaging camera's FOV. You can find the imaging camera's center FOV during night as follow: Adjust the main camera so left/right/up/down corespond to east/west/north/south. Locate a very bright star (Sirius would be a good example) and center it at the imaging camera (also sync the scope at the star's position so you can return if you encounter difficulties), write down its coordinates that EQMod shows. Move the star at the edge of the imaging camera's FOV. Start moving the star along the edges of the imaging camera's FOV until you see the star at phd, if you still can't see it move the star a bit ouside the imaging camera's FOV until you see it at phd. Once located, center it at phd and write down the coordinates of the star. The difference in those coordinates is the guide camera's center of FOV, the coordinates when the star is at the edges of phd is the guide camera's FOV. In my opinion the latter this doesn't matter a lot and you can calculate the guide camera's FOV as you would for the imaging camera, but the center is important to know if you want to use software that can suggest you guide stars for an OAG (the sky software is a good example). At CdC i have it showing me the FOV both of my cameras. A second way to do this is to place the guide camera so it corresponds to one of the mount's directions e/w/n/s, adjust the imaging camera to the mount's movements and move the scope in one direction until you see the star at phd

Hope these help, if you have any problems don't hesitate to post

edit: also make sure that the flat end of the prism points inside the scope and not at the camera

[riklaunim]

Quick question actually - to achieve perfect focus for both my imaging camera and the Lodestar, do both CCD sensors need to be at the same distance along the imaging plane?

Close to that. Filters, cover glasses, prism may shift the focus point a bit.

edit: also make sure that the flat end of the prism points inside the scope and not at the camera

That would be self-guiding?

[Gib007]

Thank you both so very much for your replies on this! It has been great reading it. I just came back home from taking the setup outside to test focusing the Lodestar on the OAG. I did some calculations at home and basically added T2 spacers between the OAG and the imaging camera. So, assuming that both CCD cameras required the same distance to the reducer, I set it out in this manner from the OAG's prism:

Imaging Camera = 4.5mm (to edge of OAG) + 22mm (T2 spacers) + 17.5mm (CCD sensor backfocus) = 44mm

Lodestar = 31mm (prism centre to terminal) + 13mm (CCD sensor backfocus) = 44mm

It still didn't focus quite right but after some fiddling, I found that in fact the Lodestar needed 3mm less than the imaging camera to get it focused. After moving the Lodestar closer to the prism (28mm away), I was sorted. This is easily repeatable too as the OAG has everything locked into place so all I do is thread the adapter for the Lodestar on to the OAG and slot in the Lodestar as far as it will go and I'm on focus. I will fiddle around with it a tad more though, to make perfectly sure, but that's work for another night. 

I have two points to raise though:

1. I did see elongated stars on PHD Guiding. I pointed at the M45 Pleiades Cluster and one of the brighter stars appeared on the top of my imaging camera and at the bottom of the Lodestar (expected as prism is further out) but stars that went further up (not captured by imaging camera at all) were elongated and grey-ish once the bright star was focused perfectly on the Lodestar. At this stage, you recommend I adjust the prism, right? By adjusting, do you mean I should move it inward (toward the imaging camera's CCD sensor)?

2. At the moment I do not own a filter wheel. I actually thread the filters I use for imaging at the very front of the optical train (on the reducer's front thread). There's an obvious issue here - the OAG and Lodestar are seeing the filtered image. I expect that if I want to do narrowband imaging, I'm going to find it very, very tough to guide as a result. I guess this isn't really a question, but more of a statement - I need to invest in a filter wheel!

Funny point to add too. Once I got a bright star focused on PHD Guiding, I decided to try my hand at guiding with it, just to check it out. After numerous failed calibrations, I just stopped trying and thought I'd just think about the settings at home for another night. Turns out I had not bothered connecting the autoguiding cable between the Lodestar and the mount OR even set the mount on ASCOM PulseGuiding either. There was literally NO CONNECTION between PHD Guiding and my mount. Well done me!

[kookoo_gr]

good to know you're setting up the oag. don't worry about the first bugs, once you fix them the guide setup will be permanent whitout any flexure

One small correction for setting up the prism since sleep deprivation made write some errors.

1. focus the imaging camera and move the prism inwards until you see its shadow at the imaging camera, then back it off

2. focus the guide camera and tilt the mirror until you see the clearest image, just to make sure for the images first tilt the the prism at both extremes. Once ypu have the best picture check the image camera so that the prism of the oag doesn't interere with the imaging camera's FOV. if it does just repeat the whole proccess. When you finish check again the focus space since it may have gone off a bit.

sorry if i made it difficult to setup the prism

Could you post an image of the elongated star? it may be alright for use. you need to not have almost flattened stars, once the wheather clears here, i will post some images from phd of my ed80 and GSO RC8'' in order to compare them with yours

yes you do need to buy a filter and place the oag before the FW since it will be too difficult to do narrowband imaging

[Gib007]

good to know you're setting up the oag. don't worry about the first bugs, once you fix them the guide setup will be permanent whitout any flexure

One small correction for setting up the prism since sleep deprivation made write some errors.

1. focus the imaging camera and move the prism inwards until you see its shadow at the imaging camera, then back it off

2. focus the guide camera and tilt the mirror until you see the clearest image, just to make sure for the images first tilt the the prism at both extremes. Once ypu have the best picture check the image camera so that the prism of the oag doesn't interere with the imaging camera's FOV. if it does just repeat the whole proccess. When you finish check again the focus space since it may have gone off a bit.

sorry if i made it difficult to setup the prism

Could you post an image of the elongated star? it may be alright for use. you need to not have almost flattened stars, once the wheather clears here, i will post some images from phd of my ed80 and GSO RC8'' in order to compare them with yours

yes you do need to buy a filter and place the oag before the FW since it will be too difficult to do narrowband imaging

You have my sincere thanks for all your help with this. I have to say though, that my OAG does not seem to be able to tilt the prism. The prism can be moved inwards or outwards (towards or away from the imaging camera's CCD sensor) and then in 120° steps (has a bit of leeway here but in 120° divisions) to capture a different segment of the sky (to find a guide star). The prism seems completely fixed on to the actual prism holder and perfectly perpendicular to the optical axis (what I assume should be the case, optically speaking). 

I have played around with the setup a little more, from my balcony pointing at very distant lights in Spain. I added T2 spacers between the OAG and the imaging camera to simulate the optical thickness of the filter wheel I like and a filter. Then, using what I learned last night, I measured the optical distance between the prism and the imaging camera's CCD sensor to be 53mm and estimated the Lodestar's CCD sensor should be 50mm away. Since the Lodestar has an optical backfocus of 13mm, that left 37mm to be accounted for by the OAG. I locked it into place at precisely 37mm from centre of prism and voila - focus achieved quickly with both cameras. 

Adding the filter wheel should replace the T2 spacers I have between the OAG and imaging camera so in theory it should already be sorted but otherwise the focal point will require very minor adjustments. 

Having said all this, the filter wheel I'm interested in is the Starlight Xpress USB Filter Wheel, which is £245 without VAT from First Light Optics and is entirely USB-powered. I find its price and lack of need for a 12V supply to be very much attractive over the ATIK EFW2 Filter Wheel I initially had in mind. I'm just worried that threading the TS Optics 9mm OAG on to the filter wheel will not be very possible if the guiding camera terminal collides with the filter wheel casing. I'd need to add a T2 spacer to separate the OAG from the filter wheel but I'd rather avoid that. Anybody have experience using this filter wheel with front and rear T2 (T-Thread) threads? Thanks!

[orion69]

Be careful choosing filter wheel. Calculate total back focus you will have after adding FW and all adapters and spacers. That could determine  which FW you can use.

[Gib007]

Hey, look what I found:

http://www.teleskop-express.de/shop/product_info.php/info/p2159_Filter-Quick-Changer-incl--1x-2--filter-drawer---low-profile.html

Fantastic - no need to spend on a filter wheel, can freely rotate OAG around it, adds very little size and weight, can have the filter easily exchanged, prevents putting too much weight on the filter itself and is cheaper. Oh and the optical path is shorter!

[Davey-T]

I get banana shaped stars with OAG and Loadstar, others have reported this and it doesn't seem to effect the guiding.

Dave

[Gib007]

I get banana shaped stars with OAG and Loadstar, others have reported this and it doesn't seem to effect the guiding.

Dave

I'm glad bananas are acceptable sources of guiding!

So I shouldn't worry too much if all the stars aren't nice, hard circles.

[kookoo_gr]

Last night i had a clear night, so here is the pic my Lodestar sees through my 8" RC scope. You ca see that at the top right corner the stars are defformed due being near at the edge of the primary mirror, the guide star i have selected is normal

[ollypenrice]

They're deformed because they're at the edge of the light cone, so with an OAG it's best to sink the pick-up prism as far into the cone as it will go without shadowing the image. Once you've done that there's not much more you can do I don't think. But if it guides it guides...

Olly

[Gina]

I get banana shaped guide stars sometimes with the Lodestar, OAG and MN190 particularly if the guide image is a bit out of focus - but guiding is still fine.

[Gib007]

They're deformed because they're at the edge of the light cone, so with an OAG it's best to sink the pick-up prism as far into the cone as it will go without shadowing the image. Once you've done that there's not much more you can do I don't think. But if it guides it guides...

Olly

I get banana shaped guide stars sometimes with the Lodestar, OAG and MN190 particularly if the guide image is a bit out of focus - but guiding is still fine.

Last night i had a clear night, so here is the pic my Lodestar sees through my 8" RC scope. You ca see that at the top right corner the stars are defformed due being near at the edge of the primary mirror, the guide star i have selected is normal

Good to know, thank you very much. I still haven't got round to testing everything. I received the filter drawer from Teleskop Service and it's great. Only 15mm optical thickness (+1/3 filter thickness, optically) and I strongly prefer it to a filter wheel, personally. Now just need the weather to clear up!