Advice on Filters + Exposure Length

[oisinandatelescope]

Hi folks,

Thanks for taking the time to read my post. I have been doing some astronomy on and off for the last few years - I would love to dedicate more time to it but unfortunately it wouldn't be sensible to bring any equipment to university with me. As my degree is coming to an end (graduating with my MPhys in May 2025) I am anticipating having more time on my hands to image and get to know astrophotography a lot more than I have had the opportunity to do previously.

Before I go further, let me preface what equipment I currently use and the sky I shoot from:

 

Skywatcher 130PDS + Baader MPCC Mk III Coma Corrector

Nikon D5600 (STOCK)

HEQ5 Pro (Rowan Belt Mod + EQMOD)

NINA / APT for imaging sessions

Bortle 4

 

One issue I have consistently had since purchasing my HEQ5 Pro is the issues with exposures over 90 seconds. My effective focal length with the combined crop fator of the D5600 (1.5x) and native focal length of the 130PDS (650mm) is 975mm I believe. Outside of polar alignment (which I feel I can do accurately enough?), does anyone have advice on how to increase my exposure time? Of course autoguiding is an option, but trying to keep on a budget makes forking out £100-200 for something I don't use often feels frivilous.

My next point is to ask about filters/modifications. I am thinking of astromodifying my camera myself. I have found a tutorial online which follows a very good step-by-step guide including for de-soldering the glass from in front of the sensor. Does anyone have any advice they can offer on whether or not this is worthwhile or whether a filter would be more beneficial? I have looked into plent of CLS and narrowband filters, and would be more leniant to this option.

 

I appreciate any advice greatly.

 

All the best,

Oisín

Edited November 12 by oisinandatelescope

[Cornelius Varley]

35 minutes ago, oisinandatelescope said:

My effective focal length with the combined crop fator of the D5600 (1.5x) and native focal length of the 130PDS (650mm) is 975mm I believe.

Hello and welcome to SGL. Ignore the focal length myth. The telescope still has a focal length of 650mm irrespective of the cameras sensor size.

[ONIKKINEN]

Autoguiding is basically not optional with affordable mounts, so you want to look into getting that going as soon as possible. The reason being that all mounts suffer from something called periodic error where the tracking speed very slightly changes over one rotation of the RA axis worm gear. Guiding solves this issue easily. The mounts that dont need autoguiding are all in the 4k+ price range (need to have high precision encoders - costs silly money).

You need not break the bank with guiding, a simple finderscope converted to a guider and an ASI120MM does the trick. With a reflector an OAG would be a much better choice, but up to you to decide where to draw the line in how much to spend.

Also echo the above, crop factor is not a real thing in astrophotography and should not be given any thought.

[Elp]

If you can image 90s without star trailing or autoguiding I don't think you have an issue. Total imaging time counts, you'd just get more instances of read noise in the stack using more subs. I found with my modded Canon 600D the background usually smoothed out after around 5-6 hours of imaging, I usually do 60-120s typically even in narrowband.

Bortle 4 is very dark, I don't think you really need a filter, definitely not a CLS or equivalent. What may help with emission nebulae rich in hydrogen alpha and oxygen iii is a dual narrowband filter, you'll get some result with your camera but the internal IR filter will block a lot if not all of the HA signal. Astro cameras even uncooled ones tend to be more quantum efficient than camera bodies hence why they're better, cooled ones also produce less noise, but you can still utilise what you have at hand, especially from bortle 4.

[Xilman]

1 hour ago, ONIKKINEN said:

Autoguiding is basically not optional with affordable mounts, so you want to look into getting that going as soon as possible. The reason being that all mounts suffer from something called periodic error where the tracking speed very slightly changes over one rotation of the RA axis worm gear. Guiding solves this issue easily. The mounts that dont need autoguiding are all in the 4k+ price range (need to have high precision encoders - costs silly money).

That last sentence is an excellent rule of thumb but not invariably true.

The mount of my 0.4m Dilworth is a massive fork which was built by the original owner of Tacande Observatory who is a very skilled mechanical and optical engineer. Most of the mass in the mount is cast aluminium. The cast was clearly made first from a wooden prototype because saw marks are still visible. The RA wheel is about half a metre in diameter and extremely well made. The mount, though not the RA drive wheel,  can be seen in this image: http://www.astropalma.com/equipment.html I have absolutely no idea how much it cost him to build it. Despite a complete lack of encoders unguided exposures of at least 15 minutes are (usually) possible.

The "usually" disclaimer is that the worm, wheel and the rollers on which the mount moves in RA are all open to collecting dust. Since the eruption 3km away and three years ago there has been rather a lot of very fine dust in the atmosphere. This dust is trapped in the silicon grease which lubricates everything and compacted into small solid lumps. Those lumps can, and often do, result in discrete jumps in the movement in RA. The attached mage shows a typical result. Some nights recently have seen as many as 25% of subs rendered useless.

When I return to La Palma in January everything is going to be very scrupulously cleaned and re-greased. It will be a big job which may well take most of a day.

Although my mount is unusual, there is a lesson here for everyone: keep the drive mechanisms clean and well lubricated to reduce the likelihood of guiding problems.

P.S. Apologies for the dust donut. The optics will be cleaned and new flats taken as part of the maintenance program.

 

Edited November 12 by Xilman
Add P.S.

[oisinandatelescope]

Folks thanks very much for the replies!!! I wasn't expecting to hear so much back already.

 

Having gone away and thought more, I'm realising the necessity of astro-modification. I had been considering a dual/triple bandpass filter (L-eXtreme/L-eNhance), but what good is improving the transmission of effectively non-existent wavenelenghts at 600nm +. I am torn mainly between buying a used astro-modded camera -or having a go at modding it myself. I have some experience with hands-on-fiddlieness but am still scared at the idea of wrecking my camera. My main reason for self-modifying it is that I have all the tools at home, so the money I would have spent on a used one could be spent on a auto-guiding setup/new filter.

31 minutes ago, Elp said:

 I found with my modded Canon 600D the background usually smoothed out after around 5-6 hours of imaging, I usually do 60-120s typically even in narrowband.

This was good to hear. I see so many videos of people taking 3-5min subs that I worry I could be doing better.

Another minor concern I have is with back-spacing, particularly to ensure no abberation and a flat field when using reflector telescopes. I understand the back-spacing for the Baader MPCC Mk 3 to be something like 58mm, which I think I have got. Unfortunately, I don't have access to my desktop right now so I can't attach any images I've taken for prrof, but I will do whenever I'm home.

 

Thanks,
Oisín

[Elp]

For backspacing you will get some sample variation due to manufacturing tolerances so the spec is normally a guide. You should add a third of filter thickness to that spec, typically people add 1mm if using a filter.

[oisinandatelescope]

Brill. Thanks very much

[oisinandatelescope]

15 hours ago, ONIKKINEN said:

You need not break the bank with guiding, a simple finderscope converted to a guider and an ASI120MM does the trick. With a reflector an OAG would be a much better choice, but up to you to decide where to draw the line in how much to spend.

Hi,

 

I just wanted to ask your opinion on setting up an OAG.

Will the thickness have an adverse affect on focusing? As I mentioned, Baader MPCC mk 3 requires ~55mm, so to include DSLR flange, T2/M48 adapter, and now an OAG... will that put me over 55mm?

[ONIKKINEN]

2 hours ago, oisinandatelescope said:

Hi,

 

I just wanted to ask your opinion on setting up an OAG.

Will the thickness have an adverse affect on focusing? As I mentioned, Baader MPCC mk 3 requires ~55mm, so to include DSLR flange, T2/M48 adapter, and now an OAG... will that put me over 55mm?

Ah, forgot the flange distance of DSLRs in this case. OAG wont work here, it would indeed go far beyond the 55mm required backspacing. In fact you might be at exactly 55mm just by having your t-ring in the camera, so you cant really fit anything else in the imaging train.

[oisinandatelescope]

1 hour ago, ONIKKINEN said:

Ah, forgot the flange distance of DSLRs in this case. OAG wont work here, it would indeed go far beyond the 55mm required backspacing. In fact you might be at exactly 55mm just by having your t-ring in the camera, so you cant really fit anything else in the imaging train.

Yeah I thought as much. I think I've about 44mm of flange from my DSLR, leaving only 11mm for the OAG and adapters. I've seen a couple 9mm OAGs online, but would need a 2mm T2 adapter which isn't really feasible right?

[ONIKKINEN]

30 minutes ago, oisinandatelescope said:

Yeah I thought as much. I think I've about 44mm of flange from my DSLR, leaving only 11mm for the OAG and adapters. I've seen a couple 9mm OAGs online, but would need a 2mm T2 adapter which isn't really feasible right?

There is a t-ring + OAG combo available, but i can only find a Canon model: https://www.teleskop-express.de/en/astrophotography-and-photography-15/everything-for-guiding-66/ts-optics-off-axis-guider-for-canon-eos-cameras-replaces-the-t-ring-2722

Not sure one exists for Nikon flanges. Couldnt find one with a quick search anyway, you could try making a new thread and asking if someone knows whether that exists or not (or if a low profile t-ring exists).

The OAG route seems like a bust with this in mind. The finder/guider route works as well as an OAG provided that its mounted sturdily and doesnt flex. I'd say cross that bridge when you get there and leave the small chance of troubleshooting guiding issues for another day.

[oisinandatelescope]

Its funny you mention that - I've seen a relatively cheap modified T3i on AstroBuyandSell. I understand the Canon models around the time of the T3i aren't exceptional with their readout noise, but still... for a good price along with some nice add-ons (SVBony UHC & CLS filters included) I'd be happy to take the noise hit and just try to expose for longer.

Ultimately, to my knowledge OAG is somewhat timeless - if I can get it set up and working now it saves upgrading in the future as the inherent benefits of using the same lens for guiding and imaging removes any error.

It may be the safer option to get a guidescope + cam but I would defo prefer to learn OAG now if it can last me longer.

[Paul_T]

The HEQ5 and 130PDS are a very good match.

The 130 PDS might be a "budget" imaging OTA but is well made and, if anything like my 150 PDS, there will be no noticeable flexure between it and a finder-guider. I would strongly recommend attaching it to a second dovetail to be sure though. You could also go full DIY like I did. Pictures are of it on a different scope, but you get the idea.

If your HEQ5 is in good condition, you'll be able to do 15 minute subs and longer if you so wish. The main difference that I've found between the HEQ5 and other (much more expensive!) mounts is the percentage of subs that will need to be rejected due to tracking errors as the exposure times increase but, at up to 5 minutes, I'd be surprised if that would be much more than about 10%.