Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

kz1bob

Focusing Issues when adding filter wheel

Recommended Posts

Hi All,

New challenge I am running into. When I add a filter wheel on my Bresser AR102s I run out of travel when trying to focus in. I have removed all the extensions and still can't focus in. Am I out of luck or is there something I can do? I am reading up on Focal Extenders am I heading down the right path?

Thanks for your help.

Share this post


Link to post
Share on other sites

Hi

You can measure approximately where the focal point of the objective is supposed to be just with a tape measure - 600mm from the centre of the objective lens. That might help you to see what's what. I see the scope comes with a diagonal. Without that fitted I imagine there would be something like 40mm of space - that should be enough for a filter wheel - depending on your camera requirements. Some filter wheels have a 'nosepiece' which can be removed/substituted. Maybe you could post a pic of your scope with everything attached. You probably would want a field flattener or reducer anyway. Reducers tend to have stringent backfocus requirements i.e. 55mm to sensor plane, whereas some field flatteners give you a lot more.

Louise

  • Like 1

Share this post


Link to post
Share on other sites

You could resort to using a binoviewer OCA to gain additional back focus.  Some folks shorten their OTA in this situation.

Share this post


Link to post
Share on other sites

If the thickness of the filter wheel matches or in this case exceeds the length of the extension tubes you have removed I cannot think of much, or at least not a lot that is easy. Mind you I am a bit surprised that the filter wheel is that thick but not overly familiar with the assorted options. Also those scopes are kind of aimed at AP so little chance of removing whatever items there are there. Bresser have already taken bits out to enable a DSLR to reach the focal plane. To Bresser it is easier to aim at AP+DSLR and add a 2" extension tube in for visual as that keeps more or less everyone happy.

I assume that you are not using the 1.25" adaptor as that takes up just under 20mm of length. If you are then it equally means your attachment is 1.25" so you would need a 2" adaptor of whatever sort in place of the present 1.25" item. I have a Bresser 102S also. But it would give another 18-20mm to you.

Will warn you that if you get it to JUST focus then filters will wipe up about another 1mm, refractive index and glass thickness. Just don't want you to barely squeeze in, then add filters and start all over again.

  • Like 1

Share this post


Link to post
Share on other sites
5 hours ago, kz1bob said:

Hi All,

New challenge I am running into. When I add a filter wheel on my Bresser AR102s I run out of travel when trying to focus in. I have removed all the extensions and still can't focus in. Am I out of luck or is there something I can do? I am reading up on Focal Extenders am I heading down the right path?

Thanks for your help.

There are some thin filter wheels available.

A Barlow might help

Dave

Edited by Davey-T

Share this post


Link to post
Share on other sites

Um, sorry Bob, thought you were setting up for imaging so assumed it was camera not able to focus. Quick-change filter drawers (teleskop express) are quite good and only use 15mm.

Louise

Share this post


Link to post
Share on other sites

I had presumed the same, that the travel was to allow a DSLR to be used. The 102S comes with 2 extension tubes, one at 25mm the other at 37mm.

As it is said "removed the extensions" that means at least the 25mm has gone and I would guess the focuser would have had some inward movement with the 25mm (even a little) so the filter wheel must be up around 30mm thick or more. Have to go look up filter wheel specifications as that or more might be normal.

If it is for AP then I guess that buying another filter wheell is not a great idea - more cost - also a barlow tends to increase image size and in AP the normal is to reduce it to get a brigher image.

Will say the initial post does not say AP and half implies visual but the use of a filter wheel is usually in relation to AP.

Any idea how close to focus you are getting? One bit mad idea if it is literally 1mm or 2mm

  • Like 2

Share this post


Link to post
Share on other sites

Ah looking at other topics by the op, it does look like this is indeed an imaging setup using an old Olympus dslr?

Louise

Share this post


Link to post
Share on other sites
3 hours ago, Thalestris24 said:

Ah looking at other topics by the op, it does look like this is indeed an imaging setup using an old Olympus dslr?

Louise

The OP might want to look into getting a dedicated astro camera with much less back focus requirements.

Share this post


Link to post
Share on other sites

Hi All,

Thank you for your comments and assistance.

Still fairly new to this. I am using my DSLR now for AP (will upgrade to a CCD/CMOS once I am more confident in what I am doing), however this latest came about when trying to get my new guide camera (Altair GPCAM 0130C) to actually 'see' anything. We have had a very bright moon the past few days so have managed to get it to show and focus on the moon, however, point it at something resembling a star and just get a black screen. I have read their forum and the recommendation is to keep playing with the software (adjusting gain and exposure time) and focusing until I get my "eureka moment". I had aligned the guide scope with my main tube during the day focusing on the door to a farm house about 3 miles away. The software (using Altair and SharpCap) worked fine during the day so was hoping I was at good starting point for when it got dark. Still haven't managed to see any stars with the guide cam.

The issue with the main focus came up when I had the guide scope focused on the moon, and the main scope focused. I then added the filter wheel and couldn't focus in. I removed the extension tube and was able to just about focus on the moon with the focuser dialled all the way in.

I am getting there slowly, just hope I get my eureka moment soon :-) 

Thanks for all your advice and help. It is greatly appreciated.

 

Share this post


Link to post
Share on other sites

Hi again

What guide scope do you have? Guide cams can be tricky to focus as the focus position is quite critical.

Louise

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By Dario
      Hi everyone!
      As I said few days ago in my presentation I recently got my first scope  (skywatcher 127) and over the last days I tried to test It.
      Due to impossibility (temporary) to bring the scope in a decent location to make observations, I had to test it from home through a window  (and under the sky of London - not the best at all). So i know that quality is of course affected.
      On the other hand, I am sure that the performances are worse than I could expect also under these conditions (maybe you will correct me) and would asks for opinions to improve.
      As a (mainly) planetary scope, I did not have a lot of choice: no Moon, Jupiter and Saturn out of my field of view. I pointed at Mars.
      First sight, I see the very big central obstruction. I played with the knob of the focus to get rid of the big black spot in the centre of my target and here you are the issues that I would like to talk about:
      1) the target became very small (ok) but still was not focus  (very blurry and not a lot of clues to get atleast the shape of the planet well defined - can imagine to find any detail on it )
      2) Also the colours did not convince me as no other colour than a light yellow was visible
      3) Changing the eyepiece  (10 instead of 25) for a magnification of 150x did not improve the situation: probably the image was even worse, more blurry with more irregular shape than before, no ability to focus at all. Differences from before : bigger (but still much smaller than expected) and very little variation in colours (I believe I saw a bit of orange).
      To complete, later, I tried the scope to stars: Orion costellation was visible from my place and tried to all the stars. Although still a bit blurry  (the brightest stars), I had impression of more sharp view, differentiation in colour and better shape  (stars almost like a dot). Moreover, to give you a final idea, Mars (at the best focus I could get) did not appear that much bigger than Betelguese.
      What do you think I can do to get better results? Could be a scope problem or some adjustments needed? Unfortunately I have no comparisons. 
      Thank you, Hope I was not too long 😅
    • By JimFR
      I’ve just acquired the above (birthday prezzie!).
      It’s a nice finder, and a definite boost from the 7x30 bundled with the scope!
      However, I’m struggling to get anywhere near focus with my Pentax K50 on the end of it... anyone know why? There’s no focus mechanism to speak of, other than the objective lens’ mounting thread...
    • By Padraic M
      I spent the full night out last night and got 6 hours of Ha lights on the Bubble and the Horsehead. Reasonably pleased with the results, but even though I followed my usual process and got good focus statistics in APT, I am slightly out of focus with roundy stars and some are even slightly donutty. Samples are attached below.
      Problem:
      - After getting close to spot-on focus, the APT Bahtinov Aid showed a focus distance oscillating from -0.02 to +0.02. Seeing seemed good to the inexpert eye. Not so sure about transparency as there was some thin, wispy cloud throughout the night. So, I started the night's imaging with focus 'Close' rather than 'On' focus.
      - Different subs show different quality stars, ranging from small donuts to circles.
      Background information:
      - HEQ5 Pro Rowan; SW Esprit 80 with field flattener, SW stock manual Crayford focuser; ZWO EFW Mini; Baader 1.25" 3.5nm Ha filter; ZWO ASI1600MM Pro binned 1x1 @ -20c.
      - AA Starwave 50mm guidescope with ZWO ASI290mm Mini guidecam binned 2x2.
      - All subs are 300s, gain 139, offset 10.
      - Polar alignment with Sharpcap to 17 arcsec ("Excellent"); capture with APT; guiding with Phd2. Focus with Bahtinov mask and APT Bahtinov Aid. Stacked in DSS with Darks, Flats and Dark Flats.
      - Mount is well balanced in RA, but is very camera-heavy in Dec.
      - PHD2 guiding was around 2"/px. Imaging pixel scale is 1.9"/px.
      Questions:
      - Do I put the round stars down to seeing, given that the Bahtinov Aid focus distance was bouncing equally above and below zero?
      - Can poor seeing cause the donut stars?
      - Would an electronic auto-focuser do any better in this situation?
      - Would the Seeing Monitor in Sharpcap give useful information? I didn't think to use it last night.
      - Could my guiding performance, and possibly the Dec balance, have affected the image quality in this way?
      - What are my options in future - abandon imaging for the night? Bin all images in software 2x2 or 4x4 to sharpen the stars at the expense of lower resolution?
      - Other suggestions?
      Sample 1: Detail from a single 5-min sub of Bubble nebula at 100% showing round stars, and a blurred bubble.

      Sample 2: Detail from a different sub of the Bubble nebula at 400% showing donuts

       
      Sample 3: Detail from a 5-min sub of the Horsehead nebula at 100%, showing both round and donut stars

       
      Finally, both images stacked, calibrated and stretched, scaled to 4x4 in Gimp. 28x300s Ha on bubble, 22*300s Ha on horsehead.


    • By Steenamaroo
      Hi all,
      I've been dabbling for about a year and am having a lot of fun.
      Nothing too serious and I don't have mega expectations but I'm starting to wonder if my images could be sharper and there's something I'm missing.
       
      I'm attaching a picture which is the best 10% of a 3 minute video with 500d 'bolted' straight to the 200p f5 scope. No filters...no barlows...just camera straight on.
      It's sharpened and saturated after the fact so in every way this is the sharpest I can get it.
      I've always had the same issue whether it's with a single shot, stacks of shots, or stacked video.
      Views through the eyepieces (circle-T 12.5/25) are mega sharp!
       
      I'm happy that collimation is very good and it was a remarkably clear night tonight.
      All in all I'd be pretty happy with this image (maybe over exposed a bit...) if the craters with shadows didn't make me want to rub my eyes.
       
      Thanks in advance for any thoughts or advice!

    • By SovietShibe
      Hey there! So if you have a telescope as mentioned in the title, you probably noticed that the focus wheel stops working after a bit.
      Well If you look under the focuser you can see a small metal piece with 4 screws on it. You need a Philips head screwdriver to unscrew these, WARNING there's a lot of superglue(which actually isn't needed at all) so wear gloves!after you've unscrewed
      the metal piece a small black 2 ended piece should fall out. If it didn't fall, take it out. Now place your focusing wheels back without that small piece that fell out and screw everything back in.
      The remaining black piece that fell out should probably be thrown away.
      Now just don't focus out to the max and everything should work just fine!
      I hope this helped(btw I'm new here so i might've butchered the terminology) *just let me know
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.